Phased Assembly of Neo-Sex Chromosomes Reveals Extensive Y Degeneration and Rapid Genome Evolution in Rumex hastatulus

Y chromosomes are thought to undergo progressive degeneration due to stepwise loss of recombination and subsequent reduction in selection efficiency. However, the timescales and evolutionary forces driving degeneration remain unclear. To investigate the evolution of sex chromosomes on multiple timescales, we generated a high-quality phased genome assembly of the massive older (<10 MYA) and neo (<200,000 yr) sex chromosomes in the XYY cytotype of the dioecious plant Rumex hastatulus and a hermaphroditic outgroup Rumex salicifolius. Our assemblies, supported by fluorescence in situ hybridization, confirmed that the neo-sex chromosomes were formed by two key events: an X-autosome fusion and a reciprocal translocation between the homologous autosome and the Y chromosome. The enormous sex-linked regions of the X (296 Mb) and two Y chromosomes (503 Mb) both evolved from large repeat-rich genomic regions with low recombination; however, the complete loss of recombination on the Y still led to over 30% gene loss and major rearrangements. In the older sex-linked region, there has been a significant increase in transposable element abundance, even into and near genes. In the neo-sex-linked regions, we observed evidence of extensive rearrangements without gene degeneration and loss. Overall, we inferred significant degeneration during the first 10 million years of Y chromosome evolution but not on very short timescales. Our results indicate that even when sex chromosomes emerge from repetitive regions of already-low recombination, the complete loss of recombination on the Y chromosome still leads to a substantial increase in repetitive element content and gene degeneration.

Parallel evolution of morphological and genomic selfing syndromes accompany the breakdown of heterostyly

Evolutionary transitions from outcrossing to selfing in flowering plants have convergent morphological and genomic signatures and can involve parallel evolution within related lineages. Adaptive evolution of morphological traits is often assumed to evolve faster than nonadaptive features of the genomic selfing syndrome. We investigated phenotypic and genomic changes associated with transitions from distyly to homostyly in the Primula oreodoxa complex. We determined whether the transition to selfing occurred more than once and investigated stages in the evolution of morphological and genomic selfing syndromes using 22 floral traits and both nuclear and plastid genomic data from 25 populations. Two independent transitions were detected representing an earlier and a more recently derived selfing lineage. The older lineage exhibited classic features of the morphological and genomic selfing syndrome. Although features of both selfing syndromes were less developed in the younger selfing lineage, they exhibited parallel development with the older selfing lineage. This finding contrasts with the prediction that some genomic changes should lag behind adaptive changes to morphological traits. Our findings highlight the value of comparative studies on the timing and extent of transitions from outcrossing to selfing between related lineages for investigating the tempo of morphological and molecular evolution.

Haplotype-resolved genome assembly provides insights into the evolution of S-locus supergene in distylous Nymphoides indica

Distyly has evolved independently in numerous animal-pollinated angiosperm lineages. Understanding of its molecular basis has been restricted to a few species, primarily Primula. Here, we investigate the genetic architecture of the single diallelic locus (S-locus) supergene, a linkage group of functionally associated genes, and explore how it may have evolved in distylous Nymphoides indica, a lineage of flowering plants not previously investigated. We assembled haplotype-resolved genomes, used read-coverage-based genome-wide association study (rb-GWAS) to locate the S-locus supergene, co-expression network analysis to explore gene networks underpinning the development of distyly, and comparative genomic analyses to investigate the origins of the S-locus supergene. We identified three linked candidate S-locus genes - NinBAS1, NinKHZ2, and NinS1 - that were only evident in the short-styled morph and were hemizygous. Co-expression network analysis suggested that brassinosteroids contribute to dimorphic sex organs in the short-styled morph. Comparative genomic analyses indicated that the S-locus supergene likely evolved via stepwise duplications and has been affected by transposable element activities. Our study provides novel insight into the structure, regulation, and evolution of the supergene governing distyly in N. indica. It also provides high-quality genomic resources for future research on the molecular mechanisms underlying the striking evolutionary convergence in form and function across heterostylous taxa.

A new type of cell related to organ movement for selfing in plants

Many plants employ osmotic and hydrostatic pressure to generate movement for survival, but little is known about the cellular mechanisms involved. Here, we report a new cell type in angiosperms termed 'contractile cells' in the stigmas of the flowering plant Chirita pumila with a much-expanded rough endoplasmic reticulum (RER). Cryo-scanning electron microscopy and transmission electron microscopy analyses revealed that the RER is continuously distributed throughout the entirety of cells, confirmed by endoplasmic reticulum (ER)-specific fluorescent labeling, and is distinct from the common feature of plant ER. The RER is water-sensitive and extremely elongated with water absorption. We show that the contractile cells drive circadian stigma closing-bending movements in response to day-to-night moisture changes. RNA-seq analyses demonstrated that contractile cells have distinct molecular components. Furthermore, multiple microstructural changes in stigma movements convert an anti-selfing structure into a device promoting selfing-a unique cellular mechanism of reproductive adaptation for uncertain pollination environments.

Genomic evidence supports the genetic convergence of a supergene controlling the distylous floral syndrome

Heterostyly, a plant sexual polymorphism controlled by the S-locus supergene, has evolved numerous times among angiosperm lineages and represents a classic example of convergent evolution in form and function. Determining whether underlying molecular convergence occurs could provide insights on constraints to floral evolution. Here, we investigated S-locus genes in distylous Gelsemium (Gelsemiaceae) to determine whether there is evidence of molecular convergence with unrelated distylous species. We used several approaches, including anatomical measurements of sex-organ development and transcriptome and whole-genome sequencing, to identify components of the S-locus supergene. We also performed evolutionary analysis with candidate S-locus genes and compared them with those reported in Primula and Turnera. The candidate S-locus supergene of Gelsemium contained four genes, of which three appear to have originated from gene duplication events within Gelsemiaceae. The style-length genes GeCYP in Gelsemium and CYP734A50 in Primula likely arose from duplication of the same gene, CYP734A1. Three out of four S-locus genes in Gelsemium elegans were hemizygous, as previously reported in Primula and Turnera. We provide genomic evidence on the genetic convergence of the supergene underlying distyly among distantly related angiosperm lineages and help to illuminate the genetic architecture involved in the evolution of heterostyly.

Genetic variation and clonal diversity in floating aquatic plants: Comparative genomic analysis of water hyacinth species in their native range

Many eukaryotic organisms reproduce by sexual and asexual reproduction. Genetic diversity in populations can be strongly dependent on the relative importance of these two reproductive modes. Here, we compare the amounts and patterns of genetic diversity in related water hyacinths that differ in their propensity for clonal propagation - highly clonal Eichhornia crassipes and moderately clonal E. azurea (Pontederiaceae). Our comparisons involved genotype-by-sequencing (GBS) of 137 E. crassipes ramets from 60 locations (193,495 nucleotide sites) and 118 E. azurea ramets from 53 locations (198,343 nucleotide sites) among six hydrological basins in central South America, the native range of both species. We predicted that because of more prolific clonal propagation, E. crassipes would exhibit lower clonal diversity than E. azurea. This prediction was supported by all measures of clonal diversity that we examined. Eichhornia crassipes also had a larger excess of heterozygotes at variant sites, another signature of clonality. However, genome-wide heterozygosity was not significantly different between the species. Eichhornia crassipes had weaker spatial genetic structure and lower levels of differentiation among hydrological basins than E. azurea, probably because of higher clonality and more extensive dispersal of its free-floating life form. Our findings for E. crassipes contrast with earlier studies from the invasive range which have reported very low levels of clonal diversity and extensive geographic areas of genetic uniformity.

Global analysis of floral longevity reveals latitudinal gradients and biotic and abiotic correlates

The length of time a flower remains open and functional - floral longevity - governs important reproductive processes influencing pollination and mating and varies considerably among angiosperm species. However, little is known about large-scale biogeographic patterns and the correlates of floral longevity. Using published data on floral longevity from 818 angiosperm species in 134 families and 472 locations world-wide, we present the first global quantification of the latitudinal pattern of floral longevity and the relationships between floral longevity and a range of biotic and abiotic factors. Floral longevity exhibited a significant phylogenetic signal and was longer at higher latitudes in both northern and southern hemispheres, even after accounting for elevation. This latitudinal variation was associated with several biotic and abiotic variables. The mean temperature of the flowering season had the highest predictive power for floral longevity, followed by pollen number per flower. Surprisingly, compatibility status, flower size, pollination mode, and growth form had no significant effects on flower longevity. Our results suggest that physiological processes associated with floral maintenance play a key role in explaining latitudinal variation in floral longevity across global ecosystems, with potential implications for floral longevity under global climate change and species distributions.

Contrasting patterns of genetic diversity and differentiation across the continental disjunct range of a sexually polymorphic aquatic plant

BACKGROUND AND AIMS

Reproductive systems enabling opportunities for self-fertilization influence population genetic structure and play a key role in colonization and genetic differentiation during range expansion. Because of their well-developed powers of dispersal, aquatic plants often have widespread disjunct geographical distributions, providing opportunities to investigate the role of reproductive systems in structuring genetic variation between parts of the range that differ in migration history and ecology.

METHODS

We compared reproductive systems and spatial genetic structure of the freshwater aquatic macrophyte Sagittaria latifolia between disjunct western and eastern ranges of North America (NA). Populations of this species are most commonly either monoecious or dioecious. We examined chloroplast DNA haplotype variation to test the hypothesis that the western range of this species represents a secondary colonization from the east, and evaluated the roles of reproductive system differences and geography in structuring contemporary patterns of genetic variation at 11 polymorphic SSR (simple sequence repeat) loci.

KEY RESULTS

Chloroplast haplotyping revealed a single haplotype in western NA compared to numerous haplotypes in eastern NA, consistent with a genetic bottleneck during westward migration. Estimates of genetic diversity in eastern NA populations differed significantly between reproductive systems, but this pattern was not evident in the western range. Eastern populations could be reliably assigned to genetic clusters based on their reproductive systems, whereas western populations clustered primarily by geographical location.

CONCLUSIONS

The sparser distribution of aquatic habitats in the drier western range of S. latifolia, combined with secondary colonization of this region, probably cause the lower genetic diversity and increased differentiation among populations, possibly overriding the effects of reproductive system evident in the eastern portion of the range. Our findings demonstrate that the complex interplay between migratory history, reproductive systems and habitat availability plays an important role in structuring spatial patterns of genetic variation in disjunct plant populations.

Effects of the neo-X chromosome on genomic signatures of hybridization in Rumex hastatulus

Natural hybrid zones provide opportunities for studies of the evolution of reproductive isolation in wild populations. Although recent investigations have found that the formation of neo-sex chromosomes is associated with reproductive isolation, the mechanisms remain unclear in most cases. Here, we assess the contemporary structure of gene flow in the contact zone between largely allopatric cytotypes of the dioecious plant Rumex hastatulus, a species with evidence of sex chromosome turn-over. Males to the west of the Mississippi river, USA, have an X and a single Y chromosome, whereas populations to the east of the river have undergone a chromosomal rearrangement giving rise to a larger X and two Y chromosomes. Using reduced-representation sequencing, we provide evidence that hybrids form readily and survive multiple backcross generations in the field, demonstrating the potential for ongoing gene flow between the cytotypes. Cline analysis of each chromosome separately captured no signals of difference in cline shape between chromosomes. However, principal component regression revealed a significant increase in the contribution of individual SNPs to inter-cytotype differentiation on the neo-X chromosome, but no correlation with recombination rate. Cline analysis revealed that the only SNPs with significantly steeper clines than the genome average were located on the neo-X. Our data are consistent with a role for neo-sex chromosomes in reproductive isolation between R. hastatulus cytotypes. Our investigation highlights the importance of studying plant hybrid zones for understanding the evolution of sex chromosomes.

Recombination landscape dimorphism and sex chromosome evolution in the dioecious plant Rumex hastatulus

There is growing evidence from diverse taxa for sex differences in the genomic landscape of recombination, but the causes and consequences of these differences remain poorly understood. Strong recombination landscape dimorphism between the sexes could have important implications for the dynamics of sex chromosome evolution because low recombination in the heterogametic sex can favour the spread of sexually antagonistic alleles. Here, we present a sex-specific linkage map and revised genome assembly of Rumex hastatulus and provide the first evidence and characterization of sex differences in recombination landscape in a dioecious plant. We present data on significant sex differences in recombination, with regions of very low recombination in males covering over half of the genome. This pattern is evident on both sex chromosomes and autosomes, suggesting that pre-existing differences in recombination may have contributed to sex chromosome formation and divergence. Our analysis of segregation distortion suggests that haploid selection due to pollen competition occurs disproportionately in regions with low male recombination. We hypothesize that sex differences in the recombination landscape have contributed to the formation of a large heteromorphic pair of sex chromosomes in R. hastatulus, but more comparative analyses of recombination will be important to investigate this hypothesis further. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Herbivore-Mediated Selection on Floral Display Covaries Nonlinearly With Plant-Antagonistic Interaction Intensity Among Primrose Populations

Quantifying the relations between plant-antagonistic interactions and natural selection among populations is important for predicting how spatial variation in ecological interactions drive adaptive differentiation. Here, we investigate the relations between the opportunity for selection, herbivore-mediated selection, and the intensity of plant-herbivore interaction among 11 populations of the insect-pollinated plant Primula florindae over 2 years. We experimentally quantified herbivore-mediated directional selection on three floral traits (two display and one phenological) within populations and found evidence for herbivore-mediated selection for a later flowering start date and a greater number of flowers per plant. The opportunity for selection and strength of herbivore-mediated selection on number of flowers varied nonlinearly with the intensity of herbivory among populations. These parameters increased and then decreased with increasing intensity of plant-herbivore interactions, defined as an increase in the ratio of herbivore-damaged flowers per individual. Our results provide novel insights into how plant-antagonistic interactions can shape spatial variation in selection on floral traits and contribute toward understanding the mechanistic basis of geographic variation in angiosperm flowers.

Do annual and perennial populations of an insect-pollinated plant species differ in mating system?

BACKGROUND AND AIMS

Theory predicts that outcrossing should be more prevalent among perennials than annuals, a pattern confirmed by comparative evidence from diverse angiosperm families. However, intraspecific comparisons between annual and perennial populations are few because such variation is uncommon among flowering plants. Here, we test the hypothesis that perennial populations outcross more than annual populations by investigating Incarvillea sinensis, a wide-ranging insect-pollinated herb native to China. The occurrence of both allopatric and sympatric populations allows us to examine the stability of mating system differences between life histories under varying ecological conditions.

METHODS

We estimated outcrossing rates and biparental inbreeding in 16 allopatric and five sympatric populations in which both life histories coexisted using 20 microsatellite loci. In each population we measured height, branch number, corolla size, tube length and herkogamy for ~30 individuals. In a sympatric population, we recorded daily flower number, pollinator visitation and the fruit and seed set of annual and perennial plants.

KEY RESULTS

As predicted, outcrossing rates (t) were considerably higher in perennial (mean = 0.76) than annual (mean = 0.09) populations. This difference in mating system was also maintained at sympatric sites where plants grew intermixed. In both allopatric and sympatric populations the degree of herkogamy was consistently larger in outcrossing than selfing plants. Perennials were more branched, with more and larger flowers than in annuals. In a sympatric population, annuals had a significantly higher fruit and seed set than perennials.

CONCLUSIONS

Genetically based differences in herkogamy between annuals and perennials appear to play a key role in governing outcrossing rates in populations, regardless of variation in local ecological conditions. The maintenance of mating system and life history trait differentiation between perennial and annual populations of I. sinensis probably results from correlated evolution in response to local environmental conditions.

Heteranthery

The stamens within a flower are usually very similar in appearance, although some species possess two or more structurally distinct types that often differ in reproductive function - a feature referred to as heteranthery. In this Quick guide, Spencer Barrett discusses the origins and functions of heteranthery.

The Genomic Selfing Syndrome Accompanies the Evolutionary Breakdown of Heterostyly

The evolutionary transition from outcrossing to selfing can have important genomic consequences. Decreased effective population size and the reduced efficacy of selection are predicted to play an important role in the molecular evolution of the genomes of selfing species. We investigated evidence for molecular signatures of the genomic selfing syndrome using 66 species of Primula including distylous (outcrossing) and derived homostylous (selfing) taxa. We complemented our comparative analysis with a microevolutionary study of P. chungensis, which is polymorphic for mating system and consists of both distylous and homostylous populations. We generated chloroplast and nuclear genomic data sets for distylous, homostylous, and distylous–homostylous species and identified patterns of nonsynonymous to synonymous divergence (d_N/d_S) and polymorphism (π_N/π_S) in species or lineages with contrasting mating systems. Our analysis of coding sequence divergence and polymorphism detected strongly reduced genetic diversity and heterozygosity, decreased efficacy of purifying selection, purging of large-effect deleterious mutations, and lower rates of adaptive evolution in samples from homostylous compared with distylous populations, consistent with theoretical expectations of the genomic selfing syndrome. Our results demonstrate that self-fertilization is a major driver of molecular evolutionary processes with genomic signatures of selfing evident in both old and relatively young homostylous populations.

The biomechanics of pollen release: new perspectives on the evolution of wind pollination in angiosperms

Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence-induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal- and wind-pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.

Plant sex: Best to be bisexual when mates are scarce

In flowering plants, transitions from bisexuality to unisexuality occur often, and have been considered irreversible. A new experimental evolution study demonstrates that this is unlikely to be true - 'leaky' sex expression can promote reversions to hermaphroditism when mates are scarce.

Widespread Recombination Suppression Facilitates Plant Sex Chromosome Evolution

Classical models suggest that recombination rates on sex chromosomes evolve in a stepwise manner to localize sexually antagonistic variants in the sex in which they are beneficial, thereby lowering rates of recombination between X and Y chromosomes. However, it is also possible that sex chromosome formation occurs in regions with preexisting recombination suppression. To evaluate these possibilities, we constructed linkage maps and a chromosome-scale genome assembly for the dioecious plant Rumex hastatulus. This species has a polymorphic karyotype with a young neo-sex chromosome, resulting from a Robertsonian fusion between the X chromosome and an autosome, in part of its geographic range. We identified the shared and neo-sex chromosomes using comparative genetic maps of the two cytotypes. We found that sex-linked regions of both the ancestral and the neo-sex chromosomes are embedded in large regions of low recombination. Furthermore, our comparison of the recombination landscape of the neo-sex chromosome to its autosomal homolog indicates that low recombination rates mainly preceded sex linkage. These patterns are not unique to the sex chromosomes; all chromosomes were characterized by massive regions of suppressed recombination spanning most of each chromosome. This represents an extreme case of the periphery-biased recombination seen in other systems with large chromosomes. Across all chromosomes, gene and repetitive sequence density correlated with recombination rate, with patterns of variation differing by repetitive element type. Our findings suggest that ancestrally low rates of recombination may facilitate the formation and subsequent evolution of heteromorphic sex chromosomes.

Characterization of 30 microsatellite markers for distylous Primula denticulata (Primulaceae) using HiSeq sequencing

Primula denticulata exhibits considerable variation in floral morphology and flowering phenology along elevational gradients in SW China. We isolated 30 microsatellite markers from P. denticulata to facilitate further investigation of population genetics and floral evolution in this species. We used the HiSeq X-Ten sequencing system to develop a set of markers, and measured polymorphism and genetic diversity in a sample of 72 individuals from three natural populations of P. denticulata subsp. denticulata. The markers displayed relatively high polymorphism, with the number of alleles ranging from two to seven (mean = 3.567). The observed and expected heterozygosity ranged from 0 to 1.000 and 0.041 to 0.702, respectively. Twenty-eight of the loci were also successfully amplified in P. denticulata subsp. sinodenticulata. The microsatellite markers we have identified will provide valuable tools for investigations of the population genetic structure, mating systems and phylogeography of the P. denticulata complex, and will help to address questions concerning the ecological and genetic mechanisms responsible for the evolution of reproductive traits in the species.

Sexual conflict in protandrous flowers and the evolution of gynodioecy

Sexual interference between male and female function in hermaphrodite plants is reduced by protandry. In environments with insufficient pollinator service, prolongation of male function owing to limited pollen removal could restrict the duration of female function and lower seed production. We provide evidence that this form of sexual conflict has played a role in the spread of females in gynodioecious populations of Cyananthus delavayi in the pollen-limited environments in which this subalpine species occurs. Using field experiments involving artificial pollen removal from the strongly protandrous flowers of hermaphrodites, we demonstrated a trade-off between male- and female-phase duration with no influence on overall floral longevity. Pollen removal at the beginning of anthesis resulted in hermaphrodite seed production matching that of females. In contrast, restricted pollen removal increased the duration of male function at the expense of female function lowering maternal fertility compared to females. This pattern was evident in five populations with females experiencing a twofold average seed fertility advantage compared to hermaphrodites. Gynodioecy often appears to evolve from protandrous ancestors and pollen limitation is widespread in flowering plants suggesting that sexual conflict may play an unappreciated role in the evolution of this form of sexual dimorphism.

Influence of local density and sex ratio on pollination in an ambophilous flowering plant

PREMISE

Variation in local density and sex ratio in dioecious plants can affect mating success through the actions of pollen vectors, principally generalist insects or wind. Increased density and male-biased sex ratios should promote pollen transfer and seed production, but their combined effects have not been investigated for ambophilous species, which exhibit both insect and wind pollination.

METHODS

We manipulated density (low vs. high) and sex ratio (1:1 vs. 3:1 male-biased) in arrays of dioecious ambophilous Thalictrum pubescens. We quantified visitation rates and foraging times to examine whether pollinators exhibited sex-specific preferences and determined the seed set of arrays.

RESULTS

Pollinators visited more plants per foraging bout at high than low density. Visitation rates and foraging times of visitors were greater for male than for female plants but did not depend on the density or sex ratio of arrays. However, whereas solitary bees displayed a strong preference for males, hover flies were indifferent to plant sex phenotype. Solitary bees also visited significantly more plants per foraging bout than hover flies. There was a significant interaction between density and sex ratio on seed set. At low density, seed set was greater for 3:1 than for 1:1 arrays, but at high density the opposite pattern occurred.

CONCLUSIONS

The demographic factors we investigated had complex influences on pollinator foraging behavior and patterns of seed set. Several factors may explain our results, including the influence of density and sex ratio on pollen export from arrays, grooming by pollinators, and the contribution of wind pollination.

Ancestral and neo-sex chromosomes contribute to population divergence in a dioecious plant

Empirical evidence from several animal groups suggests sex chromosomes disproportionately contribute to reproductive isolation. This effect may be enhanced when sex chromosomes are associated with turnover of sex determination systems resulting from structural rearrangements to the chromosomes. We investigated these predictions in the dioecious plant Rumex hastatulus, which is composed of populations of two different sex chromosome cytotypes caused by an X-autosome fusion. Using population genomic analyses, we investigated the demographic history of R. hastatulus and explored the contributions of ancestral and neo-sex chromosomes to population genetic divergence. Our study revealed that the cytotypes represent genetically divergent populations with evidence for historical but not contemporary gene flow between them. In agreement with classical predictions, we found that the ancestral X chromosome was disproportionately divergent compared with the rest of the genome. Excess differentiation was also observed on the Y chromosome, even when we used measures of differentiation that control for differences in effective population size. Our estimates of the timing of the origin of neo-sex chromosomes in R. hastatulus are coincident with cessation of gene flow, suggesting that the chromosomal fusion event that gave rise to the origin of the XYY cytotype may have also contributed to reproductive isolation.

Comparative analysis of pollen release biomechanics in Thalictrum: implications for evolutionary transitions between animal and wind pollination

Transitions from animal to wind pollination have occurred repeatedly in flowering plants, driven by structural and biomechanical modifications to flowers. But the initial changes promoting wind pollination are poorly understood, especially those required to release pollen into airflows - the critical first stage of wind pollination. Using a wind tunnel, we performed a comparative study of pollen release biomechanics in 36 species of animal- and wind-pollinated Thalictrum. We quantified pollination syndromes and stamen natural frequency (f_n ), a key vibration parameter, to determine if floral traits reliably predicted pollen release probability. We then investigated if pollen release was caused by wind-induced resonance vibration of stamens. We detected wind-induced stamen resonance in 91% of species and a strong effect of stamen acceleration on pollen release, inversely driven by f_n . However, unlike f_n , pollination syndromes did not reliably predict the probability of pollen release among species. Our results directly link f_n to the capacity of stamens to release pollen by wind and suggest that structural mechanisms reducing f_n are likely to be important for initiating transitions from animal to wind pollination. Our inability to predict the probability of pollen release based on pollination syndromes suggests diverse phenotypic trajectories from animal to wind pollination.

Variation in sexual dimorphism in a wind-pollinated plant: the influence of geographical context and life-cycle dynamics

Understanding the mechanisms causing phenotypic differences between females and males has long fascinated evolutionary biologists. An extensive literature exists on animal sexual dimorphism but less information is known about sex differences in plants, particularly the extent of geographical variation in sexual dimorphism and its life-cycle dynamics. Here, we investigated patterns of genetically based sexual dimorphism in vegetative and reproductive traits of a wind-pollinated dioecious plant, Rumex hastatulus, across three life-cycle stages using open-pollinated families from 30 populations spanning the geographic range and chromosomal variation (XY and XY1 Y2 ) of the species. The direction and degree of sexual dimorphism was highly variable among populations and life-cycle stages. Sex-specific differences in reproductive function explained a significant amount of temporal change in sexual dimorphism. For several traits, geographical variation in sexual dimorphism was associated with bioclimatic parameters, likely due to the differential responses of the sexes to climate. We found no systematic differences in sexual dimorphism between chromosome races. Sex-specific trait differences in dioecious plants largely result from a balance between sexual and natural selection on resource allocation. Our results indicate that abiotic factors associated with geographical context also play a role in modifying sexual dimorphism during the plant life-cycle.

'A most complex marriage arrangement': recent advances on heterostyly and unresolved questions

Heterostylous genetic polymorphisms provide paradigmatic systems for investigating adaptation and natural selection. Populations are usually comprised of two (distyly) or three (tristyly) mating types, maintained by negative frequency-dependent selection resulting from disassortative mating. Theory predicts this mating system should result in equal style-morph ratios (isoplethy) at equilibrium. Here, I review recent advances on heterostyly, focusing on examples challenging stereotypical depictions of the polymorphism and unresolved questions. Comparative analyses indicate multiple origins of heterostyly, often within lineages. Ecological studies demonstrate that structural components of heterostyly are adaptations improving the proficiency of animal-mediated cross-pollination and reducing pollen wastage. Both neutral and selective processes cause deviations from isoplethy in heterostylous populations, and, under some ecological and demographic conditions, cause breakdown of the polymorphism, resulting in either the evolution of autogamy and mixed mating, or transitions to alternative outcrossing systems, including dioecy. Earlier ideas on the genetic architecture of the S-locus supergene governing distyly have recently been overturned by discovery that the dominant S-haplotype is a hemizygous region absent from the s-haplotype. Ecological, phylogenetic and molecular genetic data have validated some features of theoretical models on the selection of the polymorphism. Although heterostyly is the best-understood floral polymorphism in angiosperms, many unanswered questions remain.

Water mediates fertilization in a terrestrial flowering plant

Water-mediated fertilization is ubiquitous in early land plants. This ancestral mode of fertilization has, however, generally been considered to have been lost during the evolutionary history of terrestrial flowering plants. We investigated reproductive mechanisms in the subtropical ginger Cautleya gracilis (Zingiberaceae), which has two pollen conditions - granular and filiform masses - depending on external conditions. We tested whether rain transformed granular pollen into filiform masses and whether this then promoted pollen-tube growth and fertilization of ovules. Using experimental manipulations in the field we investigated the contribution of water-mediated fertilization to seed production. Rain caused granular pollen to form filiform masses of germinating pollen tubes, which transported sperm to ovules, resulting in fertilization and seed set. Flowers exposed to rain produced significantly more seeds than those protected from the rain, which retained granular pollen. Insect pollination made only a limited contribution to seed set because rainy conditions limited pollinator service. Our results reveal a previously undescribed fertilization mechanism in flowering plants involving water-mediated fertilization stimulated by rain. Water-mediated fertilization is likely to be adaptive in the subtropical monsoon environments in which C. gracilis occurs by ensuring reproductive assurance when persistent rain prevents insect-mediated pollination.

Phylogenomic analysis reveals multiple evolutionary origins of selfing from outcrossing in a lineage of heterostylous plants

Evolutionary transitions from outcrossing to selfing often occur in heterostylous plants. Selfing homostyles originate within distylous populations and frequently evolve to become reproductively isolated species. We investigated this process in 10 species of Primula section Obconicolisteri using phylogenomic approaches and inferred how often homostyly originated from distyly and its consequences for population genetic diversity and floral trait evolution. We estimated phylogenetic relationships and reconstructed character evolution using the whole plastome comprised of 76 protein-coding genes. To investigate mating patterns and genetic diversity we screened 15 microsatellite loci in 40 populations. We compared floral traits among distylous and homostylous populations to determine how phenotypically differentiated homostyles were from their distylous ancestors. Section Obconicolisteri was monophyletic and we estimated multiple independent transitions from distyly to homostyly. High selfing rates characterised homostylous populations and this was associated with reduced genetic diversity. Flower size and pollen production were reduced in homostylous populations, but pollen size was significantly larger in some homostyles than in distylous morphs. Repeated transitions to selfing in section Obconicolisteri are likely to have been fostered by the complex montane environments that species occupy. Unsatisfactory pollinator service is likely to have promoted reproductive assurance in homostyles leading to subsequent population divergence through isolation.

Evolutionary history of the buildup and breakdown of the heterostylous syndrome in Plumbaginaceae

The evolutionary pathways leading to the heterostylous syndrome are not well understood, and models concerning the origins of distyly differ in the order in which reciprocal herkogamy and self-incompatibility evolve. We investigated the evolution and breakdown of distyly in Plumbaginaceae, a family with considerable diversity of floral traits and reproductive systems. Using Bayesian Markov chain Monte Carlo analyses and stochastic character mapping, we examined the evolutionary assembly and breakdown of the heterostylous syndrome based on a well-resolved phylogeny of 121 species of Plumbaginaceae and six outgroup taxa using five nuclear and plastid gene regions. We used the distribution of reproductive traits and reconstructed ancestral characters across phylogenies to evaluate competing models for the evolution of distyly. The most likely common ancestor of Plumbaginaceae was self-incompatible and monomorphic for sex-organ arrangement and pollen-stigma characters. Character state reconstructions indicated that reciprocal herkogamy evolved at least three times and that shifts to selfing and apomixis occurred on multiple occasions. Our results provide comparative support for the early ideas of H. G. Baker on evolutionary pathways in Plumbaginaceae, and the more recent selfing avoidance model by D. & B. Charlesworth in which distyly evolves from self-incompatible ancestors.

Geographic variation of reproductive traits and competition for pollinators in a bird-pollinated plant

Geographic variation in the reproductive traits of animal-pollinated plants can be shaped by spatially variable selection imposed by differences in the local pollination environment. We investigated this process in Babiana ringens (Iridaceae), an enigmatic species from the Western Cape region of South Africa. B. ringens has evolved a specialized perch facilitating cross-pollination by sunbirds and displays striking geographic variation in perch size and floral traits. Here, we investigate whether this variation can be explained by geographic differences in the pollinator communities. We measured floral and inflorescence traits, and abiotic variables (N, P, C, and rainfall) and made observations of sunbirds in populations spanning the range of B. ringens. In each population, we recorded sunbird species identity and measured visitation rates, interfloral pollen transfer, and whether the seed set of flowers was pollen limited. To evaluate whether competition from co-occurring sunbird-pollinated species might reduce visitation, we quantified nectar rewards in B. ringens and of other co-flowering bird-pollinated species in local communities in which populations occurred. Variation in abiotic variables was not associated with geographical variation of traits in B. ringens. Malachite sunbirds were the dominant visitor (97% of visits) and populations with larger-sized traits exhibited higher visitation rates, more between-flower pollen transfer and set more seed. No sunbirds were observed in four populations, all with smaller-sized traits. Sunbird visitation to B. ringens was not associated with local sunbird activity in communities, but sunbird visitation was negatively associated with the amount of B. ringens sugar relative to the availability of alternative nectar sources. Our study provides evidence that B. ringens populations with larger floral traits are visited more frequently by sunbirds, and we propose that visitation rates to B. ringens may be influenced, in part, by competition with other sunbird-pollinated species.

Characterization of 30 microsatellite markers in distylous Primula sinolisteri (Primulaceae) using HiSeq sequencing

PREMISE OF THE STUDY

Microsatellite markers were developed for Primula sinolisteri, a perennial distylous herb belonging to section Obconicolisteri (Primulaceae), to facilitate future investigations of the population genetics and mating patterns of populations in this species.

METHODS AND RESULTS

We developed 30 microsatellite markers for P. sinolisteri using HiSeq X-Ten sequencing and measured polymorphism and genetic diversity in a sample of 36 individuals from three natural populations. The markers displayed relatively high polymorphism, with the number of observed alleles per locus ranging from one to 19 (mean = 4.42). The observed and expected heterozygosity ranged from 0-1.000 and 0.083-0.882, respectively. Twenty-nine of the loci were also successfully amplified in homostylous P. sinolisteri var. aspera.

CONCLUSIONS

The microsatellite markers we have identified in P. sinolisteri provide powerful tools for investigating patterns of population genetic diversity and the evolutionary relationships between heterostyly and homostyly in this species.

Sex-specific plasticity of reproductive allocation in response to water depth in a clonal, dioecious macrophyte

PREMISE OF THE STUDY

Sex-specific differences in reproductive investment contribute to sexual dimorphism in dioecious plants. Along environmental gradients, males and females may plastically adjust reproductive allocation differently because of contrasting reproductive costs. In dioecious macrophytes, variation in water depth is likely to influence reproductive allocation but has not been investigated in detail.

METHODS

Vallisneria spinulosa was grown in aquatic mesocosms at water depths of 50, 100 and 150 cm for 14 weeks. Plasticity in allocation was measured to investigate whether sexual dimorphism in reproductive allocation and vegetative growth changed in response to varying water depths.

KEY RESULTS

Females invested a higher fraction of resources to sexual reproduction than males across all water depths and decreased proportional allocation to sexual structures in shallow and deep water compared to intermediate water depth. In contrast, males maintained similar sexual allocation across all water depths. Females displayed larger vegetative size than males, despite greater sexual investment, but decreased vegetative biomass more than males in shallow or deep water. The sexes invested similarly in clonal propagation by tubers at all water depths, but a trade-off with sexual reproduction was only evident in females.

CONCLUSIONS

Our results suggest that females of V. spinulosa have mechanisms to compensate for the costs of sexual reproduction in heterogeneous environments. Compared to males, females expressed greater plasticity in biomass allocated to sexual reproduction and vegetative growth in response to water depth variation. Environmental variation in underwater light availability probably caused the sex-specific allocation strategies found in V. spinulosa.

Divergent selection on the biomechanical properties of stamens under wind and insect pollination

Wind pollination has evolved from insect pollination in numerous angiosperm lineages and is associated with a characteristic syndrome of morphological traits. The traits initiating transitions to wind pollination and the ecological drivers involved are poorly understood. Here, we examine this problem in Thalictrum pubescens, an ambophilous (insect and wind pollination) species that probably represents a transitional state in the evolution of wind pollination. We investigated wind-induced pollen release by forced harmonic motion by measuring stamen natural frequency ( fn), a key vibration parameter, and its variability among nine populations. We assessed the repeatability of fn over consecutive growing seasons, the effect of this parameter on pollen release in a wind tunnel, and male reproductive success in the field using experimental manipulation of the presence or absence of pollinators. We found significant differences among populations and high repeatability within genotypes in fn. The wind tunnel assay revealed a strong negative correlation between fn and pollen release. Siring success was greatest for plants with lower fn when pollinators were absent, but this advantage diminished when pollinators were present. Our biomechanical analysis of the wind-flower interface has identified fn as a key trait for understanding early stages in the transition from insect to wind pollination.

The effects of haploid selection on Y chromosome evolution in two closely related dioecious plants

The evolution of sex chromosomes is usually considered to be driven by sexually antagonistic selection in the diploid phase. However, selection during the haploid gametic phase of the lifecycle has recently received theoretical attention as possibly playing a central role in sex chromosome evolution, especially in plants where gene expression in the haploid phase is extensive. In particular, male‐specific haploid selection might favor the linkage of pollen beneficial alleles to male sex determining regions on incipient Y chromosomes. This linkage might then allow such alleles to further specialize for the haploid phase. Purifying haploid selection is also expected to slow the degeneration of Y‐linked genes expressed in the haploid phase. Here, we examine the evolution of gene expression in flower buds and pollen of two species of Rumex to test for signatures of haploid selection acting during plant sex chromosome evolution. We find that genes with high ancestral pollen expression bias occur more often on sex chromosomes than autosomes and that genes on the Y chromosome are more likely to become enriched for pollen expression bias. We also find that genes with low expression in pollen are more likely to be lost from the Y chromosome. Our results suggest that sex‐specific haploid selection during the gametophytic stage of the lifecycle may be a major contributor to several features of plant sex chromosome evolution.

The effects of plant sexual system and latitude on resistance to herbivores

PREMISE OF THE STUDY

The strength of plant-herbivore interactions varies in space and time, but the factors that explain this variation are poorly understood. Several lines of research suggest that variation in plant reproductive systems and latitude may explain resistance against herbivores, but how these factors jointly affect plant-herbivore interactions has not been investigated in detail. We examined the effects of latitude, sexual system, and plant gender on herbivory in Sagittaria latifolia, an aquatic plant in which populations are typically monoecious (separate female and male flowers) or dioecious (separate female and male plants).

METHODS

We surveyed 43 populations of S. latifolia between 42 and 48° N in Ontario, Canada. In each population, we recorded the sexual system and obtained estimates of herbivore damage to ramets of known gender (i.e. female, male, or hermaphrodite) by the weevil Listronotus appendiculatus, the principal herbivore of S. latifolia. Herbivore damage was quantified as the percent leaf area removed by adult L. appendiculatus weevils, and the abundance of larvae feeding within flowering stalks, which was correlated with the amount of damage by herbivores to the inflorescence.

KEY RESULTS

Leaf herbivory significantly decreased with increasing latitude but did not vary with sexual system or plant gender. By contrast, larvae were more abundant in dioecious populations and on female plants, corresponding to increased stem damage, providing evidence for sex-biased larval abundance in S. latifolia. These effects of sexual system and gender on larval abundance were strongest at lower latitudes.

CONCLUSIONS

Our study found latitudinal variation in leaf herbivory and sex-biased resistance to weevil larvae that feed on the reproductive tissues of S. latifolia, which is predicted to be a necessary condition for herbivory to influence the evolution of dioecy.

Genetics of distyly and homostyly in a self-compatible Primula

The transition from outcrossing to selfing through the breakdown of distyly to homostyly has occurred repeatedly among families of flowering plants. Homostyles can originate by major gene changes at the S-locus linkage group, or by unlinked polygenic modifiers. Here, we investigate the inheritance of distyly and homostyly in Primula oreodoxa, a subalpine herb endemic to Sichuan, China. Controlled self- and cross-pollinations confirmed that P. oreodoxa unlike most heterostylous species is fully self-compatible. Segregation patterns indicated that the inheritance of distyly is governed by a single Mendelian locus with the short-styled morph carrying at least one dominant S-allele (S-) and long-styled plants homozygous recessive (ss). Crossing data were consistent with a model in which homostyly results from genetic changes at the distylous linkage group, with the homostylous allele (S^h) dominant to the long-styled allele (s), but recessive to the short-styled allele (S). Progeny tests of open-pollinated seed families revealed high rates of intermorph mating in the L-morph but considerable selfing and possibly intramorph mating in the S-morph and in homostyles. S-morph plants homozygous at the S-locus (SS) occurred in several populations but may experience viability selection. The crossing data from distylous and homostylous plants are consistent with either recombination at the S-locus governing distyly, or mutation at gene(s) controlling sex-organ height; both models predict the same patterns of segregation. Recent studies on the molecular genetics of distyly in Primula demonstrating the hemizygous nature of genes at the S-locus make it more likely that homostyles have resulted from mutation rather than recombination.

Genomic Loss and Silencing on the Y Chromosomes of Rumex

Across many unrelated lineages of plants and animals, Y chromosomes show a recurrent pattern of gene degeneration and loss, but the relative importance of inefficient selection, adaptive gene silencing, and neutral genetic drift in causing degeneration remain poorly understood. Here, we use next-generation genome and transcriptome sequencing to investigate patterns of ongoing Y chromosome degeneration in two annual plant species of Rumex (Polygonaceae) differing in their degree of degeneration and sex chromosome heteromorphism. We find evidence for both gene loss as well as silencing in these young plant sex chromosomes. Our analyses revealed significantly more gene deletion relative to silencing in R. rothschildianus, which has had a larger nonrecombining region for a longer period than R. hastatulus, consistent with this system being at a more advanced stage of degeneration. Intra- and interspecific comparisons of genomic coverage and heterozygosity indicated that loss of expression precedes gene deletion, implying that the final stages of mutation accumulation and gene loss may often occur neutrally. We found no evidence for adaptive silencing of genes that have lost expression. Our results suggest that the initial spread of deleterious regulatory variants and/or epigenetic silencing may be an important driver of early degeneration of Y chromosomes.

Ecological correlates and genetic consequences of evolutionary transitions from distyly to homostyly

BACKGROUND AND AIMS

The outbreeding floral polymorphism heterostyly frequently breaks down, resulting in the evolution of self-fertilization as a result of homostyle formation. Here, the loss of floral polymorphism in distylous Primula oreodoxa, a sub-alpine species restricted to western Sichuan, China, was examined by investigating the ecological correlates and genetic consequences of mating system transitions. Several key questions were addressed. (1) What are the frequencies, geographical distribution and reproductive characteristics of floral morphs in distylous and homostylous populations? (2) Does increased elevation influence pollinator service and the likelihood of inbreeding in populations? (3) How often has homostyly originated and what are the consequences of the breakdown of distyly for the amounts and distribution of genetic diversity in populations?

METHODS

Fourteen populations throughout the range of P. oreodoxa were sampled, and morph frequencies and floral characteristics were recorded. Polymorphism at microsatellite loci and chloroplast DNA (cpDNA) variation were used to quantify population genetic structure and genetic relationships among populations. Controlled pollinations and studies of pollen tube growth and fertility were conducted to determine the compatibility status of populations and their facility for autonomous self-pollination. Finally, visitation rates of long- and short-tongued pollinators to distylous and homostylous populations at different elevations were compared to determine if increased elevation was associated with deterioration in pollinator service.

KEY RESULTS

In contrast to most heterostylous species, both distylous and homostylous morphs of P. oreodoxa are highly self-compatible, but only homostyles have the facility for autonomous self-pollination. Homostyles set significantly more fruit and seeds following open pollination than the distylous morphs. Visitation by long-tongued pollinators was significantly lower in homostylous populations, and overall rates of insect visitation decreased with elevation. Genetic diversity was significantly lower in homostylous populations, with evidence of increased inbreeding at higher elevation. Patterns of cpDNA variation were consistent with multiple transitions from distyly to homostyly and limited gene flow among populations.

CONCLUSIONS

The results of this study support the hypothesis that the multiple loss of floral polymorphism in distylous P. oreodoxa is associated with unsatisfactory pollinator service, with homostyles benefiting from reproductive assurance as a result of autonomous self-pollination.

Purifying and Positive Selection Influence Patterns of Gene Loss and Gene Expression in the Evolution of a Plant Sex Chromosome System

Sex chromosomes are unique regions of the genome, with a host of properties that distinguish them from autosomes and from each other. Although there is extensive theory describing sex chromosome formation and subsequent degeneration of the Y chromosome, the relative importance of processes governing degeneration is poorly understood. In particular, it is not known whether degeneration occurs solely as a direct result of inefficient selection due to loss of recombination, or whether adaptive gene silencing on the Y chromosome results in most degeneration occurring neutrally. We used comparative transcriptome data from two related annual plants with highly heteromorphic sex chromosomes, Rumex rothschildianus and Rumex hastatulus, to investigate the patterns and processes underlying Y chromosome degeneration. The rate of degeneration varied greatly between the two species. In R. rothschildianus, we infer widespread gene loss, higher than previously reported for any plant. Gene loss was not random: genes with lower constraint and those not expressed during the haploid phase were more likely to be lost. There was indirect evidence of adaptive evolution on the Y chromosome from the over-expression of Y alleles in certain genes with sex-biased gene expression. There was no complete dosage compensation, but there was evidence for targeted dosage compensation occurring in more selectively constrained genes. Overall, our results are consistent with selective interference playing the dominant role in the degeneration of the Y chromosome, rather than adaptive gene silencing.

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE): POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken the harmful effects of inbreeding by exposing genetic load to selection. This study examines the maintenance of inbreeding depression in partially self-fertilizing populations of the long-lived, herbaceous wetland plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through self-fertilization. Population-genetic estimates of inbreeding depression (δ = 1 - relative mean fitness of selfed progeny) based on changes in the inbreeding coefficient for the same ten populations were uniformly high, ranging from 0.49 to 1.79 and averaging 1.11 ± 0.29 SE. Although confidence intervals of individual population estimates were large, estimates were significantly greater than 0 in six populations and greater than 0.5 in four. Inbreeding depression was also estimated by comparing growth, survival, and flowering of experimentally selfed and outcrossed offspring from two of these populations in a 1-yr glasshouse experiment involving three density regimes; after which offspring were transplanted into garden arrays and two field sites and monitored for two consecutive growing seasons. Overall δ^ for survival averaged 0.27 ± 0.01 in the glasshouse, 0.33 ± 0.04 in the garden, and 0.46 ± 0.04 in the field. The glasshouse experiment also revealed strong inbreeding depression for growth variables, especially above-soil dry weight (δ^ = 0.42 ± 0.03). The fitness consequences of inbreeding depression for these growth variables approximately doubles if survival to maturity is determined by severe truncation selection. Despite substantial selfing, inbreeding depression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.