Uncovering genes involved in pollinator-driven mating system shifts and selfing syndrome evolution in Brassica rapa

Shifts in pollinator occurrence and their pollen transport effectiveness drive the evolution of mating systems in flowering plants. Understanding the genomic basis of these changes is essential for predicting the persistence of a species under environmental changes. We investigated the genomic changes in Brassica rapa over nine generations of pollination by hoverflies associated with rapid morphological evolution toward the selfing syndrome. We combined a genotyping-by-sequencing (GBS) approach with a genome-wide association study (GWAS) to identify candidate genes, and assessed their functional role in the observed morphological changes by studying mutations of orthologous genes in the model plant Arabidopsis thaliana. We found 31 candidate genes involved in a wide range of functions from DNA/RNA binding to transport. Our functional assessment of orthologous genes in A. thaliana revealed that two of the identified genes in B. rapa are involved in regulating the size of floral organs. We found a protein kinase superfamily protein involved in petal width, an important trait in plant attractiveness to pollinators. Moreover, we found a histone lysine methyltransferase (HKMT) associated with stamen length. Altogether, our study shows that hoverfly pollination leads to rapid evolution toward the selfing syndrome mediated by polygenic changes.

The timing of visits by large and small bees differentially affects pollination success in Mimulus ringens

PREMISE

Cross-fertilization in most flowering plants is facilitated by mobile animals that transport pollen while foraging for floral rewards. The contributions of different visitors can vary widely, depending on the amount of pollen transferred during a single visit and on the frequency and timing of the visits of each pollinator taxon.

METHODS

We used three approaches to measure the pollination value of bees that visit Mimulus ringens: pollinator interviews, field population observations, and caging studies.

RESULTS

The single-visit effectiveness of small bees (primarily Halictidae) was only half that of larger bees (primarily Bombus) for pollen delivery and removal. In five field populations, we found substantial temporal and spatial variation in visitation and pollination. In most sites big bees were active before 08:00 hours, and by 10:00-11:00 hours, stigmas were usually fully pollinated and closed, and little pollen remained in anthers. Small bees seldom visited before 10:00 hours. Excluding big bees from plants confirmed that pollination is reduced and delayed in this ecological context.

CONCLUSIONS

Big bees are the primary pollinators of M. ringens, accounting for at least 75% of seed production. Not only are they more effective per visit, in most situations they also visit before small bees become active. Although small bees are not usually important pollinators of M. ringens, they have the potential to partially replace them as a "fail-safe" pollinator in contexts where big bees are not abundant. In a world where pollinator abundance is declining, such backup pollinators may be important for maintaining plant reproduction.

Competition and drought affect cleistogamy in a non-additive way in the annual ruderal Lamium amplexicaule

Competition affects mixed-mating strategies by limiting available abiotic or biotic resources such as nutrients, water, space, or pollinators. Cleistogamous species produce closed (cleistogamous, CL), obligately selfed, simultaneously with open (chasmogamous, CH), potentially outcrossed flowers. The effects of intraspecific competition on fitness and cleistogamy variation can range from limiting the production of costly CH flowers because of resource limitation, to favouring CH production because of fitness advantages of outcrossed, CH offspring. Moreover, the effects of competition can be altered when it co-occurs with other environmental variations. We grew plants from seven populations of the ruderal Lamium amplexicaule, originating from different climates and habitats, in a common garden experiment combining drought, interspecific competition, and seasonal variation. All these parameters have been shown to influence the degree of cleistogamy in the species on their own. In spring, competition and drought negatively impacted fitness, but the CL proportion only increased when plants were exposed to both treatments combined. We did not observe the same results in autumn, which can be due to non-adaptive phenotypic variation, or to differences in soil compactness between seasons. The observed responses are largely due to phenotypic plasticity, but we also observed phenotypic differentiation between populations for morphological, phenological, and cleistogamy traits, pointing to the existence of different ecotypes. Our data do not support the hypothesis that CL proportion should decrease when resources are scarce, as plants with reduced growth had relatively low CL proportions. We propose that variation in cleistogamy could be an adaptation to pollinator abundance, or to environment-dependent fitness differences between offspring of selfed and outcrossed seeds, two hypotheses worth further investigation. This opens exciting new possibilities for the study of the maintenance of mixed-mating systems using cleistogamous species as models that combine the effects of inbreeding and reproductive costs.

Support for Baker's law: Facultative self-fertilization ability decreases pollen limitation in experimental colonization

PREMISE

The ability to self-fertilize is predicted to provide an advantage in colonization because a single individual can reproduce and establish a next generation in a new location regardless of the density of mates. While there is theoretical and correlative support for this idea, the strength of mate limitation as a selective agent has not yet been delineated from other factors that can also select for self-fertilization in colonization of new habitats. We used known mating-system variation in the American bellflower (Campanula americana) to explore how plants' ability to self-fertilize can mitigate density-dependent reproduction and impact colonization success.

METHODS

We created experimental populations of single individuals or a small number of plants to emulate isolated colonization events. These populations were composed of plants that differed in their ability to self-fertilize. We compared pollen limitation of the single individuals to that of small populations.

RESULTS

Experimental populations of plants that readily self-fertilize produced consistent seed numbers regardless of population size, whereas plants with lower ability to self-fertilize had density-dependent reproduction with greater seed production in small populations than in populations composed of a single individual.

CONCLUSIONS

We experimentally isolated the effect of mate limitation in colonization and found that it can select for increased self-fertilization. We show the benefit of self-fertilization in colonization, which helps to explain geographic patterns of self-fertilization and shows support for Baker's law, a long-held hypothesis in the field of mating-system evolution.

Introduced species shed friends as well as enemies

Many studies seeking to understand the success of biological invasions focus on species' escape from negative interactions, such as damage from herbivores, pathogens, or predators in their introduced range (enemy release). However, much less work has been done to assess the possibility that introduced species might shed mutualists such as pollinators, seed dispersers, and mycorrhizae when they are transported to a new range. We ran a cross-continental field study and found that plants were being visited by 2.6 times more potential pollinators with 1.8 times greater richness in their native range than in their introduced range. Understanding both the positive and negative consequences of introduction to a new range can help us predict, monitor, and manage future invasion events.

A novel type of neighbour perception elicits reproductive plasticity in an annual plant with a mixed mating system

Plants display various forms of phenotypic plasticity in anticipation of changing conditions, many of which are influenced by information obtained from neighbouring plants. Here, we tested the hypothesis that cleistogamic Lamium amplexicaule plants can adaptively modify production of chasmogamous (CH) and cleistogamous (CL) flowers based on the perception of conspecific neighbours. The production and proportion of CH and CL flowers was examined in individual L. amplexicaule grown at varying densities or treated with root leachates from plants grown at different densities. When growing at high density or treated with root leachates from high-density pots, L. amplexicaule increased production of more expensive, potentially outcrossing CH flowers. In contrast, single plants or plants treated with root leachates from empty pots or single-source plants predominantly developed cheaper, self-pollinated CL flowers. The results demonstrate a novel root-based neighbour-perception modality that enables plants to adaptively adjust production of CH and CL flowers in response to the presence of potential reproductive partners. Further research is needed to explore the broader ecological implications of this novel interplant cueing on reproductive bet-hedging and plasticity in natural settings, as well as to identify the involved cues and their mode of operation.

Phenotype-fitness relationships and pollen-transfer efficiency of five orchid species with different pollination strategies

PREMISE

Deceptive pollination, a fascinating mechanism that independently originated in several plant families for benefiting from pollinators without providing any reward, is particularly widespread among orchids. Pollination efficiency is crucial in orchids due to the aggregated pollen in a pollinarium, which facilitates pollen transfer and promotes cross-pollination as pollinators leave after being deceived.

METHODS

In this study, we compiled data on reproductive ecology from five orchid species with different pollination strategies: three deceptive-strategy species (shelter imitation, food deception, sexual deception), one nectar-rewarding species, and one shelter-imitation but spontaneously selfing species. We aimed to compare the reproductive success (female fitness: fruit set; male fitness: pollinarium removal) and pollination efficiency of species representing these strategies. We also investigated pollen limitation and inbreeding depression among the pollination strategies.

RESULTS

Male and female fitness were strongly correlated in all species but the spontaneously selfing species, which had high fruit set and low pollinarium removal. As expected, pollination efficiency was highest for the rewarding species and the sexually deceptive species. Rewarding species had no pollen limitation but did have high cumulative inbreeding depression; deceptive species had high pollen limitation and moderate inbreeding depression; and spontaneously selfing species did not have pollen limitation or inbreeding depression.

CONCLUSIONS

Pollinator response to deception is critical to maintain reproductive success and avoid inbreeding in orchid species with non-rewarding pollination strategies. Our findings contribute to a better understanding of the trade-offs associated with different pollination strategies in orchids and highlight the importance of pollination efficiency in orchids due to the pollinarium.

Genetic and demographic signatures accompanying the evolution of the selfing syndrome in Daphne kiusiana, an evergreen shrub

BACKGROUND AND AIMS

The evolution of mating systems from outcrossing to self-fertilization is a common transition in flowering plants. This shift is often associated with the 'selfing syndrome', which is characterized by less visible flowers with functional changes to control outcrossing. In most cases, the evolutionary history and demographic dynamics underlying the evolution of the selfing syndrome remain poorly understood.

METHODS

Here, we characterize differences in the demographic genetic consequences and associated floral-specific traits between two distinct geographical groups of a wild shrub, Daphne kiusiana, endemic to East Asia; plants in the eastern region (southeastern Korea and Kyushu, Japan) exhibit smaller and fewer flowers compared to those of plants in the western region (southwestern Korea). Genetic analyses were conducted using nuclear microsatellites and chloroplast DNA (multiplexed phylogenetic marker sequencing) datasets.

KEY RESULTS

A high selfing rate with significantly increased homozygosity characterized the eastern lineage, associated with lower levels of visibility and herkogamy in the floral traits. The two lineages harboured independent phylogeographical histories. In contrast to the western lineage, the eastern lineage showed a gradual reduction in the effective population size with no signs of a severe bottleneck despite its extreme range contraction during the last glacial period.

CONCLUSIONS

Our results suggest that the selfing-associated morphological changes in D. kiusiana are of relatively old origin (at least 100 000 years ago) and were driven by directional selection for efficient self-pollination. We provide evidence that the evolution of the selfing syndrome in D. kiusiana is not strongly associated with a severe population bottleneck.

Hermaphroditic origins of anisogamy

Anisogamy-the size dimorphism of gametes-is the defining difference between the male and female sexual strategies. Game-theoretic thinking led to the first convincing explanation for the evolutionary origins of anisogamy in the 1970s. Since then, formal game-theoretic models have continued to refine our understanding of when and why anisogamy should evolve. Such models typically presume that the earliest anisogamous organisms had separate sexes. However, in most taxa, there is no empirical evidence to support this assumption. Here, we present a model of the coevolution of gamete size and sex allocation, which allows for anisogamy to emerge alongside either hermaphroditism or separate sexes. We show that hermaphroditic anisogamy can evolve directly from isogamous ancestors when the average size of spawning groups is small and fertilization is relatively efficient. Sex allocation under hermaphroditism becomes increasingly female-biased as group size decreases and the degree of anisogamy increases. When spawning groups are very small, our model also predicts the existence of complex isogamous organisms in which individuals allocate resources equally to two large gamete types. We discuss common, but potentially unwarranted, assumptions in the literature that could be relaxed in future models. This article is part of the theme issue 'Half a century of evolutionary games: a synthesis of theory, application and future directions'.

EPFL peptide signalling ensures robust self-pollination success under cool temperature stress by aligning the length of the stamen and pistil

Successful sexual reproduction of plants requires temperature-sensitive processes, and temperature stress sometimes causes developmental asynchrony between male and female reproductive tissues. In Arabidopsis thaliana, self-pollination occurs when the stamen and pistil lengths are aligned in a single flower so that pollens at the stamen tip are delivered to the stigma at the pistil tip. Although intercellular signalling acts in several reproduction steps, how signalling molecules, including secreted peptides, contribute to the synchronous growth of reproductive tissues remains limited. Here, we show that the mutant of the secreted peptide EPIDERMAL PATTERNING FACTOR LIKE 6 (EPFL6), which shows no phenotypes at a moderate temperature, fails in fruit production at a cool temperature due to insufficient elongation of stamens. EPFL6 is expressed in stamen filaments and promotes filament elongation to achieve the alignment of stamen and pistil lengths at a cool temperature. We also found that, at a moderate temperature, all EPFL6-subfamily genes are required for stamen elongation. Furthermore, we showed that ERECTA (ER), known as a common receptor for EPFL-family peptides, mediates the stamen-pistil growth coordination. Lastly, we provided evidence that modulation of ER activity rescues the reproduction failure caused by insufficient stamen elongation by realigning the stamen and pistil lengths.

Interpopulation variation in inbreeding is primarily driven by tolerance of mating with relatives in a spermcasting invertebrate

The degree to which individuals inbreed is a fundamental aspect of population biology shaped by both passive and active processes. Yet, the relative influences of random and non-random mating on the overall magnitude of inbreeding are not well characterized for many taxa. We quantified variation in inbreeding among qualitatively accessible and isolated populations of a sessile marine invertebrate (the colonial ascidian Lissoclinum verrilli) in which hermaphroditic colonies cast sperm into the water column for subsequent uptake and internal fertilization. We compared estimates of inbreeding to simulations predicting random mating within sites to evaluate if levels of inbreeding were (1) less than expected because of active attempts to limit inbreeding, (2) as predicted by genetic subdivision and passive inbreeding tolerance, or (3) greater than simulations due to active attempts to promote inbreeding via self-fertilization or a preference for related mates. We found evidence of restricted gene flow and significant differences in the genetic diversity of L. verrilli colonies among sites, indicating that on average colonies were weakly related in accessible locations, but their levels of relatedness matched that of first cousins or half-siblings on isolated substrates. Irrespective of population size, progeny arrays revealed variation in the magnitude of inbreeding across sites that tracked with the mean relatedness of conspecifics. Biparental reproduction was confirmed in most offspring (86%) and estimates of total inbreeding largely overlapped with simulations of random mating, suggesting that interpopulation variation in mother-offspring resemblance was primarily due to genetic subdivision and passive tolerance of related mates. Our results highlight the influence of demographic isolation on the genetic composition of populations, and support theory predicting that tolerance of biparental inbreeding, even when mates are closely related, may be favoured under a broad set of ecological and evolutionary conditions.

Insect herbivores drive sex allocation in angiosperm flowers

Why sex has evolved and is maintained is an open question in evolutionary biology. The Red Queen hypothesis predicts that host lineages subjected to more intense parasite pressure should invest more in sexual reproduction to continuously create novel defences against their rapidly evolving natural enemies. In this comparative study across the angiosperms, we show that hermaphrodite plant species associated with higher species richness of insect herbivores evolved flowers with higher biomass allocation towards the male sex, an indication of their greater outcrossing effort. This pattern remained robust after controlling for key vegetative, reproductive and biogeographical traits, suggesting that long-term herbivory pressure is a key factor driving the selfing-outcrossing gradient of higher plants. Although flower evolution is frequently associated with mutualistic pollinators, our findings support the Red Queen hypothesis and suggest that insect herbivores drive the sexual strategies of flowering plants and their genetic diversity.

Phenotypic Selection in Halenia elliptica D. Don (Gentianaceae), an Alpine Biennial with Mixed Mating System

The transition from outcrossing to selfing is a common evolutionary trend in flowering plants, and floral traits change significantly with the evolution of selfing. Whether or not plant traits are subjected to selection remains an open question in species with mixed mating systems. We examined phenotypic selection in two populations of Halenia elliptica with different selfing rates. We found that the pollen-ovule ratio, seed size, plant height, spur length, and pollinator visitation rate in the population with the higher selfing rate were lower than those in the population with the lower selfing rate. Selfing provides reproductive assurance for populations when pollinator service is low, and the floral traits that are associated with selfing syndrome are evident in populations with a higher selfing rate but are subjected to weak selection in each of the two populations with different selfing rates. Directional selection for an early flowering time indicated that late blooming flowers could experience a risk of seed development in alpine environments, and for large plants, selection indicated that seed production could be limited by the available resources. The floral traits that are associated with pollinator attraction and specialization could be subjected to weak selection at the plant level as selfing evolves, and the selective pressures that are independent of pollinators might not change significantly; highlighting the selective biotic and abiotic pressures that shape the morphological traits of plant species and their independence from the mating system.

Changes in female function and autonomous selfing across floral lifespan interact to drive variation in the cost of selfing

PREMISE

Morphological and developmental changes as flowers age can impact patterns of mating. At the same time, direct or indirect costs of floral longevity can alter their fitness outcomes. This influence has been less appreciated, particularly with respect to the timing of selfing. We investigated changes in stigma events, autonomous selfing, outcross seed set capacity, and autofertility-a measure representing the potential for reproductive assurance-across floral lifespan in the mixed-mating biennial Sabatia angularis.

METHODS

We examined stigma morphology and receptivity, autonomous self-pollen deposition, and seed number and size under autonomous self-pollination and hand outcross-pollination for flowers of different ages, from 1 d of female phase until 14 d. We compared autonomous seed production to maximal outcross seed production at each flower age to calculate an index of autofertility.

RESULTS

The stigmatic lobes begin to untwist 1 d post anthesis. They progressively open, sextend, coil, and increase in receptivity, peaking or saturating at 8-11 d, depending on the measure. Autonomous seed production can occur early, but on average remains low until 6 d, when it doubles. In contrast, outcross seed number and size start out high, then decline precipitously. Consequently, autofertility increases steeply across floral lifespan.

CONCLUSIONS

Changes in stigma morphology and receptivity, timing of autonomous self-pollen deposition, and floral senescence can interact to influence the relative benefit of autonomous selfing across floral lifespan. Our work highlights the interplay between evolution of floral longevity and the mating system, with implications for the maintenance of mixed mating in S. angularis.

Mixed pollination system and floral signals of Paepalanthus (Eriocaulaceae): insects and geitonogamy ensure high reproductive success

BACKGROUND AND AIMS

Eriocaulaceae exhibit a great variety of floral traits associated with insect (e.g. nectariferous structures) and wind pollination (unisexual flowers, exposed sexual organs and small pollen grains), as well as the 'selfing syndrome' (small flowers, short distance between stigma and anthers, and temporal overlap of male and female phases). Paepalanthus bifidus, P. subtilis and P. tortilis are related species that differ in form, size and colour of floral structures. We aimed to investigate the pollination and reproductive biology of these three species.

METHODS

We analysed the floral biology, floral visitors, pollinator behaviour, and the contribution of insects, wind and spontaneous geitonogamy to fruit set. We also evaluated the floral colour and scent of the species. Colour reflectance of capitula of each species was measured and plotted in models of insect vision. Floral scent samples were extracted and the compounds were compared to vegetative scent samples.

KEY RESULTS

In all species, the staminate and pistillate flowers are arranged in alternating cycles with a temporal overlap between these phases. Ants were the most frequent floral visitors and were effective pollinators in P. bifidus and P. tortilis, while flies were occasional pollinators in P. tortilis. Floral visitors were not observed in P. subtilis. In all species, fruits were produced by spontaneous geitonogamy, with no evidence of wind pollination. According to the models of insect vision, the colours of the capitula of P. bifidus and P. subtilis are the most inconspicuous for ants and flies. We found no difference between the emission of volatiles of inflorescences and vegetative structures.

CONCLUSIONS

This study suggests that ant pollination might be more widespread in Eriocaulaceae than currently assumed. Furthermore, for small monocarpic plants, mixed mating strategies are most favourable, by ensuring reproduction either by outcrossing when pollinators are abundant or by spontaneous geitonogamy when pollinations are scarce/absent.

A flower with several secretions: anatomy, secretion composition, and functional aspects of the floral secretory structures of Chelonanthus viridiflorus (Helieae-Gentianaceae)

Floral secretory structures have been reported for Gentianaceae; however, morphoanatomical studies of these glands are rare. We described the development and secretory activity of the colleters and nectaries throughout the floral development of Chelonanthus viridiflorus. We collected flower buds, flowers at anthesis, and fruits to be investigated using light and scanning electron microscopy. We performed histochemical tests on the secretion of colleters and used glycophyte to confirm the presence of glucose in nectar. Colleters are located on the ventral surface of sepals and nectaries occur in four regions: (i) the dorsal and (ii) ventral surfaces of sepals; (iii) apex of petals; and (iv) base of ovary. The colleters have a short peduncle and a secretory portion with homogeneous cells. They are active in flower buds and secrete polysaccharides and proteins. In flowers at anthesis, they begin to senescence presenting protoplast retraction, cell collapse, and lignification; these characteristics are intensified in fruit. The nectaries of sepals and petals have two to five cells surrounding a central cell through which the secretion is released. Nectaries are numerous, forming a nectariferous area on the dorsal surface of sepals, like that observed on petals, and can form isolated units on the ventral surface of sepals. They are active from flower buds to fruits. A region with secretory activity was identified at the base of the ovary. The secretion of colleters acts in the protection of developing organs, while nectaries are related to defenses against herbivores and the supply of nectar to potential robbers or pollinators.

How early does the selfing syndrome arise? Associations between selfing ability and flower size within populations of the mixed-mater Collinsia verna

PREMISE

Widespread associations between selfing rate and floral size within and among taxa suggest that these traits may evolve in concert. Does this association develop immediately because of shared genetic and/or developmental control, or stepwise with selection shaping the evolution of one trait following the other? If the former, then association ought to appear within and across selfing populations. We explore this fundamental question in three populations of the mixed-mater Collinsia verna where autonomous selfing (AS) ability has been shown to be under selection by the pollination environment.

METHODS

We grew clonal replicates of C. verna in a controlled environment to characterize broad-sense genetic correlations among traits within populations and to assess whether divergence in mating system and floral traits among these populations is consistent with their previously observed selection pressures.

RESULTS

As predicted by their respective pollination environments, we demonstrate significant genetic divergence among populations in AS ability. However, patterns of divergence in floral traits (petal, stamen, and style size, stigmatic receptivity, and stigma-anther distance) were not as expected. Within populations, genetic variation in AS appeared largely independent from floral traits, except for a single weak negative association in one population between flower size and AS rate.

CONCLUSIONS

Together, these results suggest that associations between selfing rate and floral traits across Collinsia species are not reflected at microevolutionary scales. If C. verna were to continue evolving toward the selfing syndrome, floral trait evolution would likely follow stepwise from mating system evolution.

A test of the reproductive assurance hypothesis in Ipomoea hederacea: does inbreeding depression counteract the benefits of self-pollination?

PREMISE

Darwin proposed that self-pollination in allegedly outcrossing species might act as a reproductive assurance mechanism when pollinators or mates are scarce; however, in natural populations, the benefits of selfing may be opposed by seed discounting and inbreeding depression. While empirical studies show variation among species and populations in the magnitude of reproductive assurance, little is known about the counterbalancing effects of inbreeding depression.

METHODS

By comparing the female reproductive success of emasculated and open-pollinated flowers, we assessed the reproductive assurance hypothesis in two Mexican populations of Ipomoea hederacea. In one population, we assessed temporal variation in reproductive assurance for three years. We evaluated inbreeding depression on seed production, seedling germination, and dry plant mass by contrasting self- and cross-hand pollination treatments in one population for two years.

RESULT

The contribution of self-pollination to female reproductive success was high and consistent between populations, but there was variation in reproductive assurance across years. Inbreeding depression was absent in the early stages of progeny development, but there was a small negative effect of inbreeding in the probability of germination and the mass of adult progeny.

CONCLUSIONS

Self-pollination provided significant reproductive assurance in I. hederacea but this contribution was variable across time. The contribution of reproductive assurance is probably reduced by inbreeding depression in later stages of progeny development, but this counter effect was small in the study populations. This study supports the hypothesis that reproductive assurance with limited inbreeding depression is likely an important selective force in the evolution of self-pollination in the genus Ipomoea.

Species in lichen-forming fungi: balancing between conceptual and practical considerations, and between phenotype and phylogenomics

Lichens are symbiotic associations resulting from interactions among fungi (primary and secondary mycobionts), algae and/or cyanobacteria (primary and secondary photobionts), and specific elements of the bacterial microbiome associated with the lichen thallus. The question of what is a species, both concerning the lichen as a whole and its main fungal component, the primary mycobiont, has faced many challenges throughout history and has reached new dimensions with the advent of molecular phylogenetics and phylogenomics. In this paper, we briefly revise the definition of lichens and the scientific and vernacular naming conventions, concluding that the scientific, Latinized name usually associated with lichens invariably refers to the primary mycobiont, whereas the vernacular name encompasses the entire lichen. Although the same lichen mycobiont may produce different phenotypes when associating with different photobionts or growing in axenic culture, this discrete variation does not warrant the application of different scientific names, but must follow the principle "one fungus = one name". Instead, broadly agreed informal designations should be used for such discrete morphologies, such as chloromorph and cyanomorph for lichens formed by the same mycobiont but with either green algae or cyanobacteria. The taxonomic recognition of species in lichen-forming fungi is not different from other fungi and conceptual and nomenclatural approaches follow the same principles. We identify a number of current challenges and provide recommendations to address these. Species delimitation in lichen-forming fungi should not be tailored to particular species concepts but instead be derived from empirical evidence, applying one or several of the following principles in what we call the LPR approach: lineage (L) coherence vs. divergence (phylogenetic component), phenotype (P) coherence vs. divergence (morphological component), and/or reproductive (R) compatibility vs. isolation (biological component). Species hypotheses can be established based on either L or P, then using either P or L (plus R) to corroborate them. The reliability of species hypotheses depends not only on the nature and number of characters but also on the context: the closer the relationship and/or similarity between species, the higher the number of characters and/or specimens that should be analyzed to provide reliable delimitations. Alpha taxonomy should follow scientific evidence and an evolutionary framework but should also offer alternative practical solutions, as long as these are scientifically defendable. Taxa that are delimited phylogenetically but not readily identifiable in the field, or are genuinely cryptic, should not be rejected due to the inaccessibility of proper tools. Instead, they can be provisionally treated as undifferentiated complexes for purposes that do not require precise determinations. The application of infraspecific (gamma) taxonomy should be restricted to cases where there is a biological rationale, i.e., lineages of a species complex that show limited phylogenetic divergence but no evidence of reproductive isolation. Gamma taxonomy should not be used to denote discrete phenotypical variation or ecotypes not warranting the distinction at species level. We revise the species pair concept in lichen-forming fungi, which recognizes sexually and asexually reproducing morphs with the same underlying phenotype as different species. We conclude that in most cases this concept does not hold, but the actual situation is complex and not necessarily correlated with reproductive strategy. In cases where no molecular data are available or where single or multi-marker approaches do not provide resolution, we recommend maintaining species pairs until molecular or phylogenomic data are available. This recommendation is based on the example of the species pair Usnea aurantiacoatra vs. U. antarctica, which can only be resolved with phylogenomic approaches, such as microsatellites or RADseq. Overall, we consider that species delimitation in lichen-forming fungi has advanced dramatically over the past three decades, resulting in a solid framework, but that empirical evidence is still missing for many taxa. Therefore, while phylogenomic approaches focusing on particular examples will be increasingly employed to resolve difficult species complexes, broad screening using single barcoding markers will aid in placing as many taxa as possible into a molecular matrix. We provide a practical protocol how to assess and formally treat taxonomic novelties. While this paper focuses on lichen fungi, many of the aspects discussed herein apply generally to fungal taxonomy. The new combination Arthonia minor (Lücking) Lücking comb. et stat. nov. (Bas.: Arthonia cyanea f. minor Lücking) is proposed.

New genomic resources and comparative analyses reveal differences in floral gene expression in selfing and outcrossing Collinsia sister species

The evolutionary transition from outcross- to self-fertilization is one of the most common in angiosperms and is often associated with a parallel shift in floral morphological and developmental traits, such as reduced flower size and pollen to ovule ratios, known as the "selfing syndrome." How these convergent phenotypes arise, the extent to which they are shaped by selection, and the nature of their underlying genetic basis are unsettled questions in evolutionary biology. The genus Collinsia (Plantaginaceae) includes seven independent transitions from outcrossing or mixed mating to high selfing rates accompanied by selfing syndrome traits. Accordingly, Collinsia represents an ideal system for investigating this parallelism, but requires genomic resource development. We present a high quality de novo genome assembly for the highly selfing species Collinsia rattanii. To begin addressing the basis of selfing syndrome developmental shifts, we evaluate and contrast patterns of gene expression from floral transcriptomes across three stages of bud development for C. rattanii and its outcrossing sister species Collinsia linearis. Relative to C. linearis, total gene expression is less variable among individuals and bud stages in C. rattanii. In addition, there is a common pattern among differentially expressed genes: lower expression levels that are more constant across bud development in C. rattanii relative to C. linearis. Transcriptional regulation of enzymes involved in pollen formation specifically in early bud development may influence floral traits that distinguish selfing and outcrossing Collinsia species through pleiotropic functions. Future work will include additional Collinsia outcrossing-selfing species pairs to identify genomic signatures of parallel evolution.

The coevolution of flower longevity and self-fertilization in hermaphroditic plants

Self-fertilization, prevalent in plants, is typically divided into three modes - prior, competing, and delayed selfing - based on the timing in which it occurs. Flower longevity affects both the opportunity for pollination and the resources allocated for fertility, and thus may influence the selection on different modes of self-fertilization. Additionally, selfing causes fertilization to depend less on pollinators, which may also influence the evolution of flower longevity. Using game-theoretical models, I investigate how inbreeding depression and the pollination environment influences the coevolution of the three modes of self-fertilization with flower longevity. Invasion of prior selfing allows the subsequent evolution of shorter flower longevity, and thus is favored over competing selfing. Prior selfing can also invade even under high inbreeding depression when the pollen deposition rate is low, but is inhibited by a higher level of delayed selfing. In general, the evolution of selfing decreases flower longevity, and reveals asymmetric effects of pollen deposition and removal on flower longevity. This study suggests considering realization of selfing and outcrossing as concrete processes by incorporating flower reproductive strategies (e.g., flower longevity) and pollination ecology (e.g., accrual rate) may offer better understanding of the evolution of mating systems and flower reproductive traits.

Self-Fertilization, Inbreeding, and Yield in Alfalfa Seed Production

Selfing (self-pollination) is the ultimate form of inbreeding, or mating among close relatives. Selfing can create yield loss when inbreeding depression, defined as a lower survival and reproduction of inbred relative to outbred progeny, is present. To determine the impact of selfing in alfalfa (Medicago sativa L.), we quantified the selfing rate of 32 alfalfa seed production fields located in three regions, namely, the Pacific Northwest (PNW), the Central Valley of California (CEV), and the Imperial Valley of California (IMP). Selfing rates (the proportion of selfed seeds) varied between 5.3 and 30% with an average of 12.2% over the 32 seed production fields. In both the parents and their progeny, we observed an excess of heterozygotes relative to Hardy-Weinberg expectations. We detected notable levels of inbreeding in parents (0.231 ± 0.007 parental inbreeding coefficient) and progeny (0.229 ± 0.005). There were a 15% decrease in the number of seeds per stem (seed set) and a 13% decline in the number of seeds per pod in selfed relative to outcrossed stems, but negligible inbreeding depression for pods per raceme and seed weight. The number of racemes on selfed stems increased significantly in fields with greater selfing rates, supporting the presence of geitonogamous or among flower selfing. Despite the significant level of inbreeding depression, seed set did not decrease in fields with higher selfing rates, where the greater number of racemes on the selfed stems increased the seed set. The effects of the field selfing rate on the seed yield metrics were mostly indirect with direct effects of the number of racemes per stem. Available data indicate that the majority of selfing in alfalfa is pollinator-mediated, and thus, eliminating selfing in alfalfa seed production would require the selection of self-incompatible varieties, which, by eliminating inbreeding depression, would provide a 15% potential increase in seed yield and an increase in future hay yield.

Evidence of spontaneous selfing and disomic inheritance in Geranium robertianum

Knowing species' breeding system and mating processes occurring in populations is important not only for understanding population dynamics, gene flow processes, and species' response to climate change, but also for designing control plans of invasive species. Geranium robertianum, a widespread biennial herbaceous species showing high morphological variation and wide ecological amplitude, can become invasive outside its distribution range. A mixed-mating system may be expected given the species' floral traits. However, autonomous selfing is considered as a common feature. Genetic variation and structure, and so population mating processes, have not been investigated in wild populations. We developed 15 polymorphic microsatellite markers to quantify genetic variation and structure in G. robertianum. To investigate whether selfing might be the main mating process in natural conditions, we sampled three generations of plants (adult, F1, and F2) for populations from the UK, Spain, Belgium, Germany, and Sweden, and compared open-pollinated with outcrossed hand-pollinated F2 progeny. The highly positive Wright's inbreeding coefficient (F IS) values in adults, F1, and open-pollinated F2 progeny and the low F IS values in outcross F2 progeny supported autonomous selfing as the main mating process for G. robertianum in wild conditions, despite the presence of attractive signals for insect pollination. Genetic differentiation among samples was found, showing some western-eastern longitudinal trend. Long-distance seed dispersal might have contributed to the low geographic structure. Local genetic differentiation may have resulted not only from genetic drift effects favored by spontaneous selfing, but also from ecological adaptation. The presence of duplicate loci with disomic inheritance is consistent with the hypothesis of allotetraploid origin of G. robertianum. The fact that most microsatellite markers behave as diploid loci with no evidence of duplication supports the hypothesis of ancient polyploidization. The differences in locus duplication and the relatively high genetic diversity across G. robertianum range despite spontaneous autonomous selfing suggest multiple events of polyploidization.

Breeding systems of floral colour forms in the Drosera cistiflora species complex

Variation in plant breeding systems has implications for pollinator-mediated selection on floral traits and the ecology of populations. Here we evaluate pollinator contribution to seed production, self-compatibility and pollen limitation in different floral colour forms of Drosera cistiflora sensu lato (Droseraceae). These insectivorous perennial plants are endemic to fynbos and renosterveld vegetation in the Cape Floristic Region of South Africa, and the species complex includes five floral colour forms (pink, purple, red, white and yellow), some of which are known to be pollinated by beetles. Controlled hand-pollination experiments were conducted in 15 populations of D. cistiflora s.l. (two to four populations per floral colour form) to test whether the colour forms vary in their degree of self-compatibility and their ability to produce seeds through autonomous self-fertilization. Yellow-flowered forms were highly self-incompatible, while other floral colour forms exhibited partial self-compatibility. Seed set resulting from autonomous selfing was very low, and pollinator dependence indices were high in all populations. Since hand cross-pollination resulted in greater seed set than open pollination in 13 of the 15 populations, we inferred that seed production is generally pollen-limited. Drosera cistiflora s.l. typically exhibits high levels of pollinator dependence and pollen limitation. This is unusual among Drosera species worldwide and suggests that pollinators are likely to mediate strong selection on attractive traits such as floral colour and size in D. cistiflora s.l. These results also suggest that the floral colour forms of D. cistiflora s.l. which are rare and threatened are likely to be vulnerable to local extinction if mutualisms were to collapse indefinitely.

Self-Compatibility Not Associated with Morphological or Genetic Diversity Reduction in Oil-Rewarding Calceolaria Species

One of the most common evolutionary transitions in angiosperms is the reproductive change from outcrossing to selfing, commonly associated with changes in floral biology and genetic diversity. Here, we aim to test whether self-compatibility leads to a reduction of floral traits and genetic diversity. For this, we experimentally estimate levels of self-compatibility, measure three floral traits and estimate four genetic diversity parameters using nine microsatellites in nine Calceolaria species. Our analysis indicated that four of the study species were self-incompatible. In addition, we found that self-compatible species did not show a reduction in floral traits size, but rather displayed larger corolla and elaiophore areas. Our analyses of genetic diversity identified larger allele number and observed heterozygosity in selfers than in outcrossers, but did not find larger inbreeding in the self-compatible species. Even though our results contradict our expectations, in the case of Calceolaria, their high dependence on only two genera of oil-bees puts the genus in a vulnerable reproductive position, probably facilitating the evolution of reproductive assurance mechanisms in the absence of pollinators. As a result, plants maintain their pollinator attraction traits while evolving the ability to self, possibly in a delayed way.

Determinants of geographic range size in plants

Geographic range size has long fascinated ecologists and evolutionary biologists, yet our understanding of the factors that cause variation in range size among species and across space remains limited. Not only does geographic range size inform decisions about the conservation and management of rare and nonindigenous species due to its relationship with extinction risk, rarity, and invasiveness, but it also provides insights into fundamental processes such as dispersal and adaptation. There are several features unique to plants (e.g. polyploidy, mating system, sessile habit) that may lead to distinct mechanisms explaining variation in range size. Here, we highlight key studies testing intrinsic and extrinsic hypotheses about geographic range size under contrasting scenarios where species' ranges are static or change over time. We then present results from a meta-analysis of the relative importance of commonly hypothesized determinants of range size in plants. We show that our ability to infer the relative importance of these determinants is limited, particularly for dispersal ability, mating system, ploidy, and environmental heterogeneity. We highlight avenues for future research that merge approaches from macroecology and evolutionary ecology to better understand how adaptation and dispersal interact to facilitate niche evolution and range expansion.

Self-compatibility and autonomous selfing of plants in meadow communities

One of the most fundamental, although controversial, questions related to the evolution of plant mating systems is the distribution of outcrossing rates. Self-compatibility, and especially autonomous self-pollination, can become particularly beneficial in anthropogenically degraded habitats with impoverished pollinator assemblages and increased pollen limitation. In a hand-pollination experiment with 46 meadow plants from the Železné hory Mts., Czech Republic, we evaluated the species' ability to adopt different mating systems. For a subset of the species, we also tested seed germination for inbreeding depression. Subsequently, we analysed relationships between the species' mating systems and 12 floral and life-history traits. We found a relatively discrete distribution of the studied species into four groups. Fully and partially self-incompatible species formed the largest group, followed by self-compatible non-selfers and mixed mating species. The germination experiment showed an absence of inbreeding depression in 19 out of 22 examined species. Nectar sugar per flower, nectar sugar per shoot and dichogamy were significant associated with the mating system. Spontaneous selfing ability and self-incompatibility in species of the meadow communities had a discrete distribution, conforming to the general distribution of mating and breeding systems in angiosperms. The low frequency of spontaneous selfers and the lack of inbreeding depression at germination suggest the existence of a selection against selfing at the later ontogenetic stages. Some floral traits, such as the level of dichogamy and amount of nectar reward, may strongly impact the balance between selfing and outcrossing rates in the self-compatible species and thus shape the evolution of mating systems.

Fitness costs of delayed pollination in a mixed-mating plant

BACKGROUND AND AIMS

To predict the evolutionary consequences of pollinator declines, we need to understand the evolution of delayed autonomous self-pollination, which is expected to evolve as a mechanism of reproductive assurance when cross-pollination becomes unreliable. This involves estimating the costs of increased levels of selfing as well as those associated with floral senescence.

METHODS

We studied the mechanisms and costs of delayed self-pollination in the mixed-mating vine Dalechampia scandens (Euphorbiaceae) by first assessing among-population variation in herkogamy and dichogamy, which together determine the rate and timing of autonomous self-pollination. We then tested whether floral longevity responds plastically to delayed pollination. Finally, we assessed the costs of delayed self-pollination in terms of seed number and size, explicitly separating inbreeding depression from effects of floral senescence.

KEY RESULTS

Herkogamy varied extensively, while variation in dichogamy was more limited. Unpollinated blossoms increased their longevity, but seed quantity and quality decreased with increasing delays in pollination, independently of inbreeding depression.

CONCLUSIONS

In D. scandens, earlier autonomous selfing is facilitated by reduced herkogamy rather than reduced protogyny, providing reproductive assurance while maintaining the possibility for outcrossing events. Effective early autonomous self-pollination may evolve under reduced cross-pollination reliability in response to costs associated with floral senescence.

Reproductive transitions in plants and animals: selfing syndrome, sexual selection and speciation

The evolution of predominant self-fertilisation frequently coincides with the evolution of a collection of phenotypes that comprise the 'selfing syndrome', in both plants and animals. Genomic features also display a selfing syndrome. Selfing syndrome traits often involve changes to male and female reproductive characters that were subject to sexual selection and sexual conflict in the obligatorily outcrossing ancestor, including the gametic phase for both plants and animals. Rapid evolution of reproductive traits, due to both relaxed selection and directional selection under the new status of predominant selfing, lays the genetic groundwork for reproductive isolation. Consequently, shifts in sexual selection pressures coupled to transitions to selfing provide a powerful paradigm for investigating the speciation process. Plant and animal studies, however, emphasise distinct selective forces influencing reproductive-mode transitions: genetic transmission advantage to selfing or reproductive assurance outweighing the costs of inbreeding depression vs the costs of males and meiosis. Here, I synthesise links between sexual selection, evolution of selfing and speciation, with particular focus on identifying commonalities and differences between plant and animal systems and pointing to areas warranting further synergy.

A novel case of autogamy and cleistogamy in Dendrobium wangliangii: A rare orchid distributed in the dry-hot valley

Dendrobium wangliangii is an epiphytic orchid distributed in the Jinshajiang dry-hot valley in Luquan County, Yunnan Province, China. Most Dendrobium spp. typically have a low fruit set, but this orchid shows a higher fruit set under natural conditions despite the lack of effective pollinators. The pollination biology of the critically endangered D. wangliangii was investigated in this study. A fruit set rate of 33.33 ± 4.71% was observed after bagging treatment in 2017 and a high fruit set rate (65.72 ± 4.44% in 2011; 50.79 ± 5.44% in 2017) was observed under natural conditions, indicating that D. wangliangii is characterized by spontaneous self-pollination. The anther cap blocked the growing pollinium; thus, the pollinium slid down and reached the stigmatic cavity, leading to autogamous self-pollination. Specifically, 51.50% of 162 unopened flowers (total 257 flowers) of this Dendrobium species under extreme water-deficit conditions developed into fruits, suggesting the presence of cleistogamy in D. wangliangii. Here, cleistogamy may represent the primary mode of pollination for this orchid. Spontaneous self-pollination and specific cleistogamous autogamy could represent major adaptions to the drought and pollinator-scarce habitat in the Jinshajiang dry-hot valley.

Bees may drive the reproduction of four sympatric cacti in a vanishing coastal mediterranean-type ecosystem

Background

Sympatric congeneric plants might share pollinators, or each species might avoid competition by evolving specialized traits that generate partitions in pollinator assemblages. In both cases, pollen limitation (a decrease in the quality and quantity of compatible reproductive pollen) can occur, driving the plant mating system to autogamy as a mechanism of reproductive assurance. We assessed the relationships between pollinator assemblages and mating systems in a group of sympatric congeneric plants. We attempted to answer the following questions: (i) How similar are pollinator assemblages among sympatric cactus species? (ii) Which mating systems do sympatric cactus species use?

Methods

We studied sympatric Eriosyce taxa that inhabit a threatened coastal strip in a mediterranean-type ecosystem in central Chile. We performed field observations on four taxa and characterized pollinators during the years 2016 and 2017. We estimated differences in the pollinator assemblages using the Bray-Curtis index. To elucidate the mating systems, we conducted hand-pollination experiments using three treatments: manual cross-pollination, automatic self-pollination, and control (unmanipulated individuals). We tested differences in seed production for statistical significance using Kruskal-Wallis analysis.

Results

Eriosyce subgibbosa showed a distinctive pollinator assemblage among the sympatric species that we studied (similarity ranged from 0% to 8%); it was visited by small bees and was the only species that was visited by the giant hummingbird Patagona gigas. Pollinator assemblages were similar between E. chilensis (year 2016 = 4 species; 2017 = 8) and E. chilensis var. albidiflora (2016 = 7; 2017 = 4); however, those of E. curvispina var. mutabilis (2016 = 7; 2017 = 6) were less similar to those of the aforementioned species. E. curvispina var. mutabilis showed the highest interannual variation in its pollinator assemblage (18% similarity). Reproduction in E. subgibbosa largely depends on pollinators, although it showed some degree of autogamy. Autonomous pollination was unfeasible in E. chilensis, which depended on flower visitors for its reproductive success. Both E. chilensis var. albidiflora and E. curvispina var. mutabilis showed some degree of autogamy.

Discussion

We observed differences in pollinator assemblages between E. subgibbosa and the remaining Eriosyce taxa, which depend on hymenopterans for pollen transfer. Pollinator assemblages showed considerable interannual variation, especially those of E. subgibbosa (ornithophilous syndrome) and E. curvispina var. mutabilis (melitophilous syndrome). Autogamous reproduction in these taxa may act as a reproductive assurance mechanism when pollinator availability is unpredictable. Our study contributes to improving our understanding of the reproductive systems of ecological interactions between threatened species in a Chilean mediterranean-type ecosystem.

Pollen limitation and autonomous selfing ability interact to shape variation in outcrossing rate across a species range

PREMISE

Hermaphroditic plants commonly reproduce through a mixture of selfing and outcrossing. The degree to which outcrossing rates reflect the availability of outcross pollen, genetic differentiation in the ability to autonomously self-fertilize, or both is often unclear. Despite the potential for autonomy and the pollination environment to jointly influence outcrossing, this interaction is rarely studied.

METHODS

We reviewed studies from the literature that tested whether the pollination environment or floral traits that cause autonomous selfing predict variation in outcrossing rate among populations. We also measured outcrossing rates in 23 populations of Campanula americana and examined associations with the pollination environment, autonomy, and their interaction.

RESULTS

Our review revealed that traits that facilitate selfing were often negatively associated with outcrossing rates whereas most aspects of the pollination environment poorly predicted outcrossing. Populations of C. americana varied from mixed mating to highly outcrossing, but variation was unrelated to population size, density, pollen limitation, or autonomous selfing ability. Outcrossing rate was significantly influenced by an interaction between autonomous selfing ability and pollen limitation. Across highly autonomous populations, elevated pollen limitation was associated with reduced outcrossing, while there was no relationship for less autonomous populations.

CONCLUSIONS

Both the ability to self autonomously and pollen limitation interact to shape outcrossing rates in C. americana. This work suggests that autonomy affords mating-system flexibility, though it is not ubiquitous in all populations across the species range. Interactions between traits influencing autonomy and pollen limitation are likely to explain variation in outcrossing rates among populations of flowering plants.

Reproductive assurance weakens pollinator-mediated selection on flower size in an annual mixed-mating species

BACKGROUND AND AIMS

In animal-pollinated plants, direct and indirect selection for large and small flowers in predominantly outcrossing and selfing species, respectively, is a common consequence of pollen limitation (PL). However, many hermaphroditic species show a mixed-mating system known as delayed selfing, which provides reproductive assurance (RA) only when outcrossing is not realized. Although RA is expected to reduce pollinator-mediated selection towards larger flowers, the consequences of delayed selfing for selection on flower size in mixed-mating species remain overlooked. We investigated whether RA weakens selection on flower size in Tuberaria guttata, a mixed-mating annual herb.

METHODS

We related pollinator visitation rates to flower size and measured seed production in emasculated, hand cross-pollinated and intact (control) flowers in three natural populations. For each population, we estimated variation in PL and RA across individuals differing in flower size and phenotypic selection on this trait.

KEY RESULTS

Pollinator visitation increased and RA decreased with flower size in all populations. Increasing RA diminished but did not fully alleviate PL, because of early-acting inbreeding depression. In the least-visited and most pollen-limited population, RA increased seed production by >200 %, intensely counteracting the strong pollinator-mediated selection for larger corollas. In the most-visited population, however, RA increased seed production by an average of only 9 %. This population exhibited the largest fraction of individuals that showed a decrease in seed production due to selfing and the weakest pollinator-mediated selection on flower size.

CONCLUSIONS

The results suggest that the balance between the extent of RA and outcrossing contributes to determine flower size in mixed-mating systems. Pollinator-mediated selection favours larger flowers by increasing outcrossed seeds, but the benefits of RA greatly lessen this effect, especially under severe conditions of pollen limitation. Our findings also indicate that a mixed-mating system can represent an 'evolutionary trap' under an adequate pollinator supply.

Rapid loss of self-incompatibility in experimental populations of the perennial outcrossing plant Linaria cavanillesii

Transitions from self-incompatibility to self-compatibility in angiosperms may be frequently driven by selection for reproductive assurance when mates or pollinators are rare, and are often succeeded by loss of inbreeding depression by purging. Here, we use experimental evolution to investigate the spread of self-compatibility from one such population of the perennial plant Linaria cavanillesii into self-incompatible (SI) populations that still have high inbreeding depression. We introduced self-compatible (SC) individuals at different frequencies into replicate experimental populations of L. cavanillesii that varied in access to pollinators. Our experiment revealed a rapid shift to self-compatibility in all replicates, driven by both greater seed set and greater outcross siring success of SC individuals. We discuss our results in the light of computer simulations that confirm the tendency of self-compatibility to spread into SI populations under the observed conditions. Our study illustrates the ease with which self-compatibility can spread among populations, a requisite for species-wide transitions from self-incompatibility to self-compatibility.

Genome of Crucihimalaya himalaica, a close relative of Arabidopsis, shows ecological adaptation to high altitude

Crucihimalaya himalaica, a close relative of Arabidopsis and Capsella, grows on the Qinghai-Tibet Plateau (QTP) about 4,000 m above sea level and represents an attractive model system for studying speciation and ecological adaptation in extreme environments. We assembled a draft genome sequence of 234.72 Mb encoding 27,019 genes and investigated its origin and adaptive evolutionary mechanisms. Phylogenomic analyses based on 4,586 single-copy genes revealed that C. himalaica is most closely related to Capsella (estimated divergence 8.8 to 12.2 Mya), whereas both species form a sister clade to Arabidopsis thaliana and Arabidopsis lyrata, from which they diverged between 12.7 and 17.2 Mya. LTR retrotransposons in C. himalaica proliferated shortly after the dramatic uplift and climatic change of the Himalayas from the Late Pliocene to Pleistocene. Compared with closely related species, C. himalaica showed significant contraction and pseudogenization in gene families associated with disease resistance and also significant expansion in gene families associated with ubiquitin-mediated proteolysis and DNA repair. We identified hundreds of genes involved in DNA repair, ubiquitin-mediated proteolysis, and reproductive processes with signs of positive selection. Gene families showing dramatic changes in size and genes showing signs of positive selection are likely candidates for C. himalaica's adaptation to intense radiation, low temperature, and pathogen-depauperate environments in the QTP. Loss of function at the S-locus, the reason for the transition to self-fertilization of C. himalaica, might have enabled its QTP occupation. Overall, the genome sequence of C. himalaica provides insights into the mechanisms of plant adaptation to extreme environments.

Can obligate apomixis and more stable reproductive assurance explain the distributional successes of asexual triploids in Hieracium alpinum (Asteraceae)?

Although reproductive assurance has been suggested to be one of the most important factors shaping the differential distributional patterns between sexuals and asexuals (geographic parthenogenesis), it has only rarely been studied in natural populations of vascular plants with autonomous apomixis. Moreover, there are almost no data concerning the putative relationship between the level of apomictic versus sexual plant reproduction on one hand, and reproductive assurance on the other. We assessed the level of sexual versus apomictic reproduction in diploid and triploid plants of Hieracium alpinum across its distributional range using flow cytometric analyses of seeds, and compared the level of potential and realized seed set, i.e. reproductive assurance, between the two cytotypes under field and greenhouse conditions. Flow cytometric screening of embryos and endosperms of more than 4,100 seeds showed that diploids produced solely diploid progeny sexually, while triploids produced triploid progeny by obligate apomixis. Potential fruit set was much the same in diploids and triploids from the field and the greenhouse experiment. While in the pollination-limited environment in the greenhouse apomictic triploids had considerably higher realized fruit set than sexual diploids, there was no significant difference between cytotypes under natural conditions. In addition, sexuals varied to a significantly larger extent in realized fruit set than asexuals under both natural and greenhouse conditions. Our results indicate that triploid plants reproduce by obligate apomixis, assuring more stable and predictable fruit reproduction when compared to sexual diploids. This advantage could provide apomictic triploids with a superior colonisation ability, mirrored in a strong geographic parthenogenesis pattern observed in this species.

Autonomous self-fertilization in Linum sulcatum, a native annual with a previously unknown mating system

The mating systems of wild plant populations have profound effects on their genetic structure and evolution, yet remain unknown or incompletely described for many species. One such species, the herbaceous native annual Linum sulcatum Riddell (Linaceae), is thought to be self-compatible, but there has been no experimental evidence to date to support this claim. To assess the breeding system of this species, seeds were collected from wild populations and reared in a controlled environment. Floral manipulations and controlled pollinations were conducted to determine the degree of self-compatibility of this species and to distinguish between autonomous and facilitated modes of selfing. Additional controlled within- and between-population crosses were conducted to determine the relative degree to which this species can outcross. This study showed that self-fertilization was highly successful and can occur autonomously. In contrast, outcrossing success, both within and between populations, was very limited, suggesting this species may exhibit an extreme degree of cross-incompatibility. A pollen tube growth experiment showed that self-pollination resulted in the formation of more pollen tubes relative to cross-pollination and that complete pollen tube growth can occur less than 2 h following self-pollination. This information is relevant to the future persistence of this species, as much of its remaining habitat is distributed among small, highly fragmented patches subjected to current and future environmental stressors.

Breeding systems in Balsaminaceae in relation to pollen/ovule ratio, pollination syndromes, life history and climate zone

Pollen/ovule (P/O) ratios are often used as proxy for breeding systems. Here, we investigate the relations between breeding systems and P/O ratios, pollination syndromes, life history and climate zone in Balsaminaceae. We conducted controlled breeding system experiments (autonomous and active self-pollination and outcrossing tests) for 65 Balsaminaceae species, analysed pollen grain and ovule numbers and evaluated the results in combination with data on pollination syndrome, life history and climate zone on a phylogenetic basis. Based on fruit set, we assigned three breeding systems: autogamy, self-compatibility and self-incompatibility. Self-pollination led to lower fruit set than outcrossing. We neither found significant P/O differences between breeding systems nor between pollination syndromes. However, the numbers of pollen grains and ovules per flower were significantly lower in autogamous species, but pollen grain and ovule numbers did not differ between most pollination syndromes. Finally, we found no relation between breeding system and climate zone, but a relation between climate zone and life history. In Balsaminaceae reproductive traits can change under resource or pollinator limitation, leading to the evolution of autogamy, but are evolutionary rather constant and not under strong selection pressure by pollinator guild and geographic range changes. Colonisation of temperate regions, however, is correlated with transitions towards annual life history. Pollen/ovule-ratios, commonly accepted as good indicators of breeding system, have a low predictive value in Balsaminaceae. In the absence of experimental data on breeding system, additional floral traits (overall pollen grain and ovule number, traits of floral morphology) may be used as proxies.

Delayed selfing ensures reproductive assurance in Utricularia praeterita and Utricularia babui in Western Ghats

Numerous bladderwort (Utricularia) species are distributed worldwide, but their reproductive biology is rarely investigated. Bladderworts are known to depend on tiny organisms to meet a significant proportion of their energy requirement by trapping them in bladders. However, information on the extent of their reliance on insects for pollination success is limited. We examined the reproductive strategy of two Utricularia species viz. Utricularia praeterita and U. babui, endemic to Western Ghats, India. The main aspects of the investigation involved floral biology, breeding system, pollination mechanism, and reproductive success. Flowers of both the species are structured for outbreeding through entomophilous floral suites, herkogamy, protandrous dichogamy and sensitive lobes of the stigma. With nearly 65% natural fruit-set, both the species appeared to be sufficiently open-pollinated. However, pollinators failed to show in plants of U. praeterita while in U. babui there was an apparent mismatch between the extent of fruit-set and pollinator visits. The study demonstrated that in the absence/insufficient visits of pollinators, the two species resort to autonomous selfing. In U. babui, denser patches of plants appeared to be crucial for attracting the pollinators. Both species are self-compatible, and reproductive success is predominantly achieved by delayed autonomous selfing. The sensitive stigma in the species fails to prevent selfing due to diminished herkogamy during the late anthetic stages. It is inferred that in the pollinator-limited environment, delayed selfing contributes to absolute natural fecundity in U. praeterita, while it produces a mixed progeny in U. babui.

Shifts to earlier selfing in sympatry may reduce costs of pollinator sharing

Coexisting plant congeners often experience strong competition for resources. Competition for pollinators can result in direct fitness costs via reduced seed set or indirect costs via heterospecific pollen transfer (HPT), causing subsequent gamete loss and unfit hybrid offspring production. Autonomous selfing may alleviate these costs, but to preempt HPT, selfing should occur early, before opportunities for HPT occur (i.e., "preemptive selfing hypothesis"). We evaluated conditions for this hypothesis in Collinsia sister species, C. linearis and C. rattanii. In field studies, we found virtually identical flowering times and pollinator sharing between congeners in sympatric populations. Compared to allopatric populations, sympatric C. linearis populations enjoyed higher pollinator visitation rates, whereas visitation to C. rattanii did not differ in sympatry. Importantly, the risk of HPT to each species in sympatry was strongly asymmetrical; interspecies visits comprised 40% of all flower-to-flower visits involving C. rattanii compared to just 4% involving C. linearis. Additionally, our greenhouse experiment demonstrated a strong cost of hybridization when C. rattanii was the pollen donor. Together, these results suggest that C. rattanii pays the greatest cost of pollinator sharing. Matching predictions of the preemptive selfing hypothesis, C. rattanii exhibit significantly earlier selfing in sympatric relative to allopatric populations.

The best of both worlds? A review of delayed selfing in flowering plants

PREMISE OF STUDY

In a seminal body of theory, Lloyd showed that the fitness consequences of selfing will depend on its timing in anthesis. Selfing that occurs after opportunities for outcrossing or pollen dispersal can provide reproductive assurance when pollinators are limited and is expected to incur little cost, even when inbreeding depression is high. As a result, delayed selfing is often interpreted as a "best-of-both-worlds" mating system that combines the advantages of selfing and outcrossing.

METHODS

We surveyed 65 empirical studies of delayed selfing, recording floral mechanisms and examining information on inbreeding depression, autofertility, and other parameters to test the support for delayed selfing as a best-of-both-worlds strategy.

KEY RESULTS

Phylogenetic distribution of the diverse floral mechanisms suggests that some basic floral structures may predispose plant taxa to evolve delayed selfing. Delayed selfing appears to serve as a best-of-both-worlds strategy in some but not all species. While the capacity for autonomous selfing is often high, it is lower, in some cases, than in related species with earlier modes of selfing. In other delayed-selfers, low inbreeding depression and reduced investment in corollas and pollen suggest limited benefits from outcrossing.

CONCLUSIONS

Despite a growing literature on the subject, experimental evidence for delayed selfing is limited and major gaps in knowledge remain, particularly with respect to the stability of delayed selfing and the conditions that may favor transitions between delayed and earlier selfing. Finally, we suggest a potential role of delayed selfing in facilitating transitions from self-incompatibility to selfing.

Small and surrounded: population size and land use intensity interact to determine reliance on autonomous selfing in a monocarpic plant

BACKGROUND AND AIMS

Habitat fragmentation has transformed landscapes globally, leaving remnants embedded within a complex matrix that is rapidly becoming more developed. For many plant populations, the associated factors of decreased size and intensification of land use surrounding them are expected to increase pollen limitation ('PL'), unless autonomous self-pollination provides reproductive assurance ('RA'). Decreased pollinator visitation is often assumed to drive these patterns, but other, less studied mechanisms might include increased heterospecific pollen transfer or decreased conspecific pollen availability via florivory. I investigate how PL and RA and their potential underlying mechanisms vary with population size and land use intensity surrounding populations in the biennial Sabatia angularis (Gentianaceae).

METHODS

I estimated the capacity for seed production via autonomous self-pollination (i.e. autofertility). Over 2 years in 22 S. angularis populations across a fragmented landscape, I performed emasculation and pollen supplementation experiments measuring RA and PL, and quantified visitation rates of potential pollinators and a pollen consumer, conspecific pollen loads and rates of heterospecific pollen deposition.

KEY RESULTS

Autofertility based on fruit mass was 93 % under PL but only 51.6 % relative to maximal conditions. PL and RA were significant on average across populations in the first year of study. Variation in RA was significantly influenced by the interaction between population size and land use intensity, which in turn rendered PL independent of these factors. Visitation and heterospecific pollen deposition rates were greatest in small populations and declined with population size, while conspecific pollen loads were greatest in intermediate sized populations.

CONCLUSIONS

Increased reliance on RA is predicted in small S. angularis populations surrounded by intense development, which can explain elevated selfing rates in fragmented populations of plant species more generally. Results from this study point toward forces such as heterospecific pollen transfer, self-pollen limitation or resource availability influencing the need and ability to rely on RA.

Does stigma curvature promote delayed selfing? An experimental investigation in Triodanis perfoliata (Campanulaceae)

Self-fertilisation that is delayed until after opportunities for outcrossing have ceased has been argued to provide both the reproductive assurance benefits of selfing and the genetic advantages of outcrossing. In the Campanulaceae, presentation of pollen on stylar hairs and progressive stigma curvature have been hypothesised to facilitate delayed selfing, but experimental tests are lacking. Stigma curvature is common in Campanula, a genus largely characterised by self-incompatibility, and therefore is unlikely to have initially evolved to promote self-fertilisation. In derived self-compatible species, however, stigma curvature might serve the secondary function of delayed selfing. We investigated delayed selfing in Triodanis perfoliata, a self-compatible relative of Campanula. Using floral manipulation experiments and pollen tube observations, we quantified the extent and timing of self-pollination. Further, we hypothesised that, if stigma curvature provides the benefit of delayed selfing in Triodanis, selection should have favoured retention of self-pollen through the loss of a stylar hair retraction mechanism. Results of a stigma removal experiment indicated that autonomous selfing produces partial seed set, but only some selfing was delayed. Pollen tube observations and a flower senescence assay also supported the finding of partial delayed selfing. Scanning electron microscopy revealed that pollen-collecting hairs retract during anthesis, which may limit the extent of delayed selfing. Delayed selfing appeared to be only partially effective in T. perfoliata. The stylar hair retraction in this species would seem to contradict selection for selfing. We suggest that caution and rigour are needed in interpreting floral traits as adaptive mechanisms for delayed selfing.

Correlation between the timing of autonomous selfing and floral traits: a comparative study from three selfing Gentianopsis species (Gentianaceae)

About 20% of angiosperms employ self-fertilization as their main mating strategy. In this study, we aimed to examine how the selfing timing correlated with floral traits in three Gentianopsis species in which autonomous selfing is achieved through filament elongation. Although the three Gentianopsis species exhibit no significant variation in their capacity for autonomous selfing, flowers of G. grandis last longer, are larger and have a higher corolla biomass, P/O ratios and male biomass allocation than those of G. paludosa, and especially those of G. contorta. Autonomous selfing occurs in the early floral life of G. paludosa and G. contorta and in the later floral life of G. grandis. Seed production mainly results from autonomous selfing in G. paludosa and G. contorta; however, G. grandis could be more described as having a mixed mating system. We suggest that autonomous selfing in later floral life increases the chance of cross-pollination prior to this, while autonomous selfing in early floral life offers a selective advantage to plants by reducing the resource investment in traits that may increase pollinator attraction and visitation.