Gynodioecy to dioecy: are we there yet?

Paternal Inheritance of Mitochondrial DNA May Lead to Dioecy in Conifers

In angiosperms cytoplasmic DNA is typically passed on maternally through ovules. Genes in the mtDNA may cause male sterility. When male-sterile (female) cytotypes produce more seeds than cosexuals, they pass on more copies of their mtDNA and will co-occur with cosexuals with a neutral cytotype. Cytoplasmic gynodioecy is a well-known phenomenon in angiosperms, both in wild and crop plants. In some conifer families (e.g. Pinaceae) mitochondria are also maternally inherited. However in some other families (e.g. Taxaceae and Cupressaceae) mtDNA is paternally inherited through the pollen. With paternal mtDNA inheritance, male cytotypes that produce more pollen than cosexuals are expected to co-occur with cosexuals. This is uncharted territory. An ESS model shows that the presence of male cytotypes selects for more female allocation in the cosexual, i.e. for sexual specialisation. An allele that switches sex from male to female can then invade. This leads to rapid loss of the neutral cytotype of the cosexual, fixation of the male cytotype and dioecy with 50% males and 50% females. The models suggest that paternal inheritance of mtDNA facilitates the evolution dioecy. Consistent with this hypothesis the Pinaceae are 100% monoecious, while dioecy is common in the Taxaceae family and in the genus Juniperus (Cupressaceae). However, no reliable data are yet available on both mode of inheritance of mtDNA and gender variation of the same species. When cosexuals benefit from reproductive assurance (high selfing rate, low inbreeding depression, low fertilisation) they maintain themselves next to males and females. This predicted pattern with three sex types present in the same population is observed in conifers in nature.

Young evolutionary origins of dioecy in the genus Asparagus

PREMISE

Dioecy (separate sexes) has independently evolved numerous times across the angiosperm phylogeny and is recently derived in many lineages. However, our understanding is limited regarding the evolutionary mechanisms that drive the origins of dioecy in plants. The recent and repeated evolution of dioecy across angiosperms offers an opportunity to make strong inferences about the ecological, developmental, and molecular factors influencing the evolution of dioecy, and thus sex chromosomes. The genus Asparagus (Asparagaceae) is an emerging model taxon for studying dioecy and sex chromosome evolution, yet estimates for the age and origin of dioecy in the genus are lacking.

METHODS

We use plastome sequences and fossil time calibrations in phylogenetic analyses to investigate the age and origin of dioecy in the genus Asparagus. We also review the diversity of sexual systems present across the genus to address contradicting reports in the literature.

RESULTS

We estimate that dioecy evolved once or twice approximately 2.78-3.78 million years ago in Asparagus, of which roughly 27% of the species are dioecious and the remaining are hermaphroditic with monoclinous flowers.

CONCLUSIONS

Our findings support previous work implicating a young age and the possibility of two origins of dioecy in Asparagus, which appear to be associated with rapid radiations and range expansion out of Africa. Lastly, we speculate that paleoclimatic oscillations throughout northern Africa may have helped set the stage for the origin(s) of dioecy in Asparagus approximately 2.78-3.78 million years ago.

A unique C-terminal domain contributes to the molecular function of Restorer-of-fertility proteins in plant mitochondria

Restorer-of-fertility (Rf) genes encode pentatricopeptide repeat (PPR) proteins that are targeted to mitochondria where they specifically bind to transcripts that induce cytoplasmic male sterility and repress their expression. In searching for a molecular signature unique to this class of proteins, we found that a majority of known Rf proteins have a distinct domain, which we called RfCTD (Restorer-of-fertility C-terminal domain), and its presence correlates with the ability to induce cleavage of the mitochondrial RNA target. A screen of 219 angiosperm genomes from 123 genera using a sequence profile that can quickly and accurately identify RfCTD sequences revealed considerable variation in RFL/RfCTD gene numbers across flowering plants. We observed that plant genera with bisexual flowers have significantly higher numbers of RFL genes compared to those with unisexual flowers, consistent with a role of these proteins in restoration of male fertility. We show that removing the RfCTD from the RFL protein RNA PROCESSING FACTOR 2-nad6 prevented cleavage of its RNA target, the nad6 transcript, in Arabidopsis thaliana mitochondria. We provide a simple way of identifying putative Rf candidates in genome sequences, new insights into the molecular mode of action of Rf proteins and the evolution of fertility restoration in flowering plants.

Geranium sylvaticum increases pollination probability by sexually dimorphic flowers

Sexual dimorphism is expressed as different morphologies between the sexes of a species. Dimorphism is pronounced in gynodioecious populations which consist of female and hermaphrodite individuals. The small size of female flowers in gynodioecious species is often explained by resource re-allocation to seed production instead of large flowers. However, pollinator attraction is critical to female fitness, and factors other than resource savings are needed to explain the small size of female flowers. We hypothesized that the floral size dimorphism in the perennial gynodioecious Geranium sylvaticum (L.) is adaptive in terms of pollination. To test this "pollination hypothesis," we video recorded the small female and large hermaphrodite G. sylvaticum flowers. We parameterized floral visitor behavior when visiting a flower and calculated pollination probabilities by a floral visitor as the probability of touching anther and stigma with the same body part. Pollination probability differed in terms of flower sex and pollinator species. Bumblebees had the highest pollination probability. The small female flowers were more likely to receive pollen via several pollinator groups than the large hermaphrodite flowers. The pollen display of hermaphrodites matched poorly with the stigma display of hermaphrodites, but well with that of females. Although the small size of female flowers is commonly explained by resource re-allocation, we show that sexual dimorphism in flower size may increase the main reproductive functions of the females and hermaphrodites. Dimorphism increases pollination probability in females and fathering probability of the hermaphrodites likely driving G. sylvaticum populations towards dioecy.

Genetic mapping of sexually dimorphic volatile and non-volatile floral secondary chemistry of a dioecious willow

Secondary chemistry often differs between sexes in dioecious plant species, a pattern attributed to its possible role in the evolution and/or maintenance of dioecy. We used GC-MS to measure floral volatiles emitted from, and LC-MS to quantitate non-volatile secondary compounds contained in, female and male Salix purpurea willow catkins from an F2 family. Using the abundance of these chemicals, we then performed quantitative trait locus (QTL) mapping to locate them on the genome, identified biosynthetic candidate genes in the QTL intervals, and examined expression patterns of candidate genes using RNA-seq. Male flowers emitted more total terpenoids than females, but females produced more benzenoids. Male tissue contained greater amounts of phenolic glycosides, but females had more chalcones and flavonoids. A flavonoid pigment and a spermidine derivative were found only in males. Male catkins were almost twice the mass of females. Forty-two QTL were mapped for 25 chemical traits and catkin mass across 16 of the 19 S. purpurea chromosomes. Several candidate genes were identified, including a chalcone isomerase associated with seven compounds. A better understanding of the genetic basis of the sexually dimorphic chemistry of a dioecious species may shed light on how chemically mediated ecological interactions may have helped in the evolution and maintenance of dioecy.

A CLAVATA3-like Gene Acts as a Gynoecium Suppression Function in White Campion

How do separate sexes originate and evolve? Plants provide many opportunities to address this question as they have diverse mating systems and separate sexes (dioecy) that evolved many times independently. The classic "two-factor" model for evolution of separate sexes proposes that males and females can evolve from hermaphrodites via the spread of male and female sterility mutations that turn hermaphrodites into females and males, respectively. This widely accepted model was inspired by early genetic work in dioecious white campion (Silene latifolia) that revealed the presence of two sex-determining factors on the Y-chromosome, though the actual genes remained unknown. Here, we report identification and functional analysis of the putative sex-determining gene in S. latifolia, corresponding to the gynoecium suppression factor (GSF). We demonstrate that GSF likely corresponds to a Y-linked CLV3-like gene that is specifically expressed in early male flower buds and encodes the protein that suppresses gynoecium development in S. latifolia. Interestingly, GSFY has a dysfunctional X-linked homolog (GSFX) and their synonymous divergence (dS = 17.9%) is consistent with the age of sex chromosomes in this species. We propose that female development in S. latifolia is controlled via the WUSCHEL-CLAVATA feedback loop, with the X-linked WUSCHEL-like and Y-linked CLV3-like genes, respectively. Evolution of dioecy in the S. latifolia ancestor likely involved inclusion of ancestral GSFY into the nonrecombining region on the nascent Y-chromosome and GSFX loss of function, which resulted in disbalance of the WUSCHEL-CLAVATA feedback loop between the sexes and ensured gynoecium suppression in males.

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.

Labile sex expression in angiosperm species with sex chromosomes

Here, we review the literature on sexual lability in dioecious angiosperm species with well-studied sex chromosomes. We distinguish three types of departures from strict dioecy, concerning either a minority of flowers in some individuals (leakiness) or the entire individual, which can constantly be bisexual or change sex. We found that for only four of the 22 species studied, reports of lability are lacking. The occurrence of lability is only weakly related to sex chromosome characteristics (number of sex-linked genes, age of the non-recombining region). These results contradict the naive idea that lability is an indication of the absence or the recent evolution of sex chromosomes, and thereby contribute to a growing consensus that sex chromosomes do not necessarily fix sex determination once and for all. We discuss some implications of these findings for the evolution of sex chromosomes, and suggest that more species with well-characterized lability should be studied with genomic data and tools. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Genome and Transcriptome Sequencing of Populus × sibirica Identified Sex-Associated Allele-Specific Expression of the CLC Gene

Transcriptome sequencing of leaves, catkin axes, and flowers from male and female trees of Populus × sibirica and genome sequencing of the same plants were performed for the first time. The availability of both genome and transcriptome sequencing data enabled the identification of allele-specific expression. Such an analysis was performed for genes from the sex-determining region (SDR). P. × sibirica is an intersectional hybrid between species from sections Aigeiros (Populus nigra) and Tacamahaca (Populus laurifolia, Populus suaveolens, or Populus × moskoviensis); therefore, a significant number of heterozygous polymorphisms were identified in the SDR that allowed us to distinguish between alleles. In the SDR, both allelic variants of the TCP (T-complex protein 1 subunit gamma), CLC (Chloride channel protein CLC-c), and MET1 (DNA-methyltransferase 1) genes were expressed in females, while in males, two allelic variants were expressed for TCP and MET1 but only one allelic variant prevailed for CLC. Targeted sequencing of TCP, CLC, and MET1 regions on a representative set of trees confirmed the sex-associated allele-specific expression of the CLC gene in generative and vegetative tissues of P. × sibirica. Our study brings new knowledge on sex-associated differences in Populus species.

Impact of the female and hermaphrodite forms of Opuntia robusta on the plant defence hypothesis

The optimal defence hypothesis predicts that increased plant defence capabilities, lower levels of damage, and lower investment in vegetative biomass will occur more frequently in sexual forms with higher resource-demanding tissue production and/or younger plant parts. We aimed to examine the effects of sexual form, cladode, and flower age on growth rate, herbivore damage, and 4-hydroxybenzoic acid (4-HBA), chlorogenic acid, and quercetin (QUE) concentrations in Opuntia robusta plants in central Mexico. Our findings demonstrated that hermaphrodite flowers showed faster growth and lesser damage than female flowers. The effect of cladode sexual forms on 4-HBA and QUE occurrence was consistent with the predictions of the optimal defence hypothesis. However, chlorogenic acid occurrences were not significantly affected by sexual forms. Old cladodes exhibited higher QUE and 4-HBA occurrences than young cladodes, and hermaphrodites exhibited higher 4-HBA concentrations than females. Resource allocation for reproduction and secondary metabolite production, and growth was higher and lower, respectively, in females, compared to hermaphrodites, indicating a trade-off between investment in reproduction, growth, and secondary metabolite production. Secondary metabolite concentrations in O. robusta plants were not negatively correlated with herbivore damage, and the two traits were not accurate predictors of plant reproductive output.

A primary study of breeding system of Ziziphus jujuba var. spinosa

Ziziphus jujuba var. spinosa has been used as a windbreak and for soil conservation and water retention. Previous studies focused on pharmacological effects and extraction of chemical components in this species, and very few explored the breeding system. The present study combined the analysis of floral morphology, behavior of flower visitors, and artificial pollination to reveal reproductive characteristics of the species. Its flowers are characterized by dichogamy, herkogamy, and stamen movement, which are evolutionary adaptations to its breeding system. There were more than 40 species of visiting insects, mainly Hymenoptera and Diptera, and the characteristics of dichogamous and herkogamous flower adapted to the visiting insects. The breeding system is outcrossing, partially self-compatible, and demand for pollinators. The fruit setting rate after natural pollination was 2%. Geitonogamy and xenogamy did not significantly increase the fruit setting rate, indicating that the low fruit setting rate was not due to pollen limitation by likely caused by resource limitation or fruit consumption. The fruit setting rate of zero in emasculated and in naturally and hand self-pollinated individuals suggested the absence of apomixis and spontaneous self-pollination. The above results can be utilized in studies on evolution and cultivation of Z. jujuba var. spinosa.

Plant sex chromosomes defy evolutionary models of expanding recombination suppression and genetic degeneration

Hundreds of land plant lineages have independently evolved separate sexes in either gametophytes (dioicy) or sporophytes (dioecy), but 43% of all dioecious angiosperms are found in just 34 entirely dioecious clades, suggesting that their mode of sex determination evolved a long time ago. Here, we review recent insights on the molecular mechanisms that underlie the evolutionary change from individuals that each produce male and female gametes to individuals specializing in the production of just one type of gamete. The canonical model of sex chromosome evolution in plants predicts that two sex-determining genes will become linked in a sex-determining region (SDR), followed by expanding recombination suppression, chromosome differentiation and, ultimately, degeneration. Experimental work, however, is showing that single genes function as master regulators in model systems, such as the liverwort Marchantia and the angiosperms Diospyros and Populus. In Populus, this type of regulatory function has been demonstrated by genome editing. In other systems, including Actinidia, Asparagus and Vitis, two coinherited factors appear to independently regulate female and male function, yet sex chromosome differentiation has remained low. We discuss the best-understood systems and evolutionary pathways to dioecy, and present a meta-analysis of the sizes and ages of SDRs. We propose that limited sexual conflict explains why most SDRs are small and sex chromosomes remain homomorphic. It appears that models of increasing recombination suppression with age do not apply because selection favours mechanisms in which sex determination depends on minimal differences, keeping it surgically precise.

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.

Seed germination of gynodioecious species: theoretical considerations and a comparison of females and hermaphrodites

Better seed germination of females than of hermaphrodites is not a major contributor to the greater geometric lifetime fitness that females require to be maintained in a gynodioecious population. Gynodioecy is a sexually dimorphic breeding system in which females (F, male sterile) and hermaphrodites (H) coexist in the same population. For plants with nuclear (biparental) inheritance of male sterility, theory predicts that except when the product of selfing rate (s) and inbreeding depression (δ) in H is high (sδ > 0.50), F must compensate (female advantage) for the loss of gene transmission via pollen production by producing more or higher-quality offspring than H to be maintained in the population. For species with cytoplasmic (maternal) inheritance of male sterility, the female requires only a small compensation in seed production or some other offspring fitness trait to persist. Reallocation to seeds of resources saved by loss of pollen production is expected to increase the quantity (number) and/or quality (mass, germinability) of seeds produced by F, thus compensating for the lack of pollen production. The primary aim of our study was to compare seed germination of F and H via a literature review. Based on theoretical considerations, we hypothesized that seeds of F should germinate better or equally as well as those of H. We found that of 235 case studies for 47 species F_germ > H_germ in 48.1%, F_germ = H_germ in 38.3% and F_germ < H_germ in 13.6%. Our results are very similar to those of a previously published meta-analysis that included germination of F and H for 12 species. For 162 cases on seed size, F > H in 29.0%, F = H in 63.6% and F < H in 7.4%. Since [(F_germ > H_germ) < (F_germ ≤ H_germ)] and [(F_seedsize > H_seedsize) < (F_seedsize ≤ H_seedsize)], these results suggest that seed quality is not a major fitness component of female advantage.

Environmental stressors affect sex ratios in sexually dimorphic plant sexual systems

Revealing the environmental pressures determining the frequency of females amongst populations of sexually dimorphic plants is a key research question. Analyses of sex ratio variation have been mainly done in dioecious plants, which misses key plant sexual systems that might represent intermediate stages in the evolution of dioecy from hermaphroditism. We investigated female frequency across populations of sexually dimorphic plant species in relation to environmental stressors (temperature, precipitation), totaling 342 species, 2011 populations, representing 40 orders and three different sexual systems (dioecy, gynodioecy and subdioecy). We also included the biome where the population was located to test how female frequency may vary more broadly with climate conditions. After correcting for phylogeny, our results for gynodioecious systems showed a positive relationship between female frequency and increased environmental stress, with the main effects being temperature-related. Subdioecious systems also showed strong positive relationships with temperature, and positive and negative relationships related to precipitation, while no significant effects on sex ratio in dioecious plants were detected. Combined, we show that female frequencies in an intermediate sexual system on the pathway from hermaphroditism to dioecy respond strongly to environmental stressors and have different selective agents driving female frequency.

Gynodioecy in the common spindle tree (Euonymus europaeus L.) involves differences in the asymmetry of corolla shapes between sexually differentiated flowers

Gynodioecy is typically associated with a smaller perianth size in purely pistillate flowers than in hermaphrodite flowers. However, it is unclear whether this size differentiation is associated with any differences in flower shape between the two sexual groups. A geometric morphometric analysis of the symmetry of tetrameric corolla shapes was used in the study of Euonymus europaeus L., Darwin’s classical system of floral sexual differentiation. I investigated whether there are any shape differences between the female and bisexual flowers, with respect to both purely symmetric variation involving coordinated shape changes of the four petals and asymmetry among petals within flowers. The corolla shapes of the female and bisexual flowers and the variability among flowers within each sexual group were very similar in the purely symmetric components of shape variation. However, the female flowers were significantly more asymmetric with respect to both the lateral and transversal asymmetry of their corolla shapes. This is the first study to apply geometric morphometrics in the analysis of morphological patterns in a sexually differentiated gynodioecious plant system. The results showed that subtle shape differences in corolla asymmetry differ between the sexual groups and indicate diverging developmental or selection signals between the sexes.

The coexistence of hermaphroditic and dioecious plants is associated with polyploidy and gender dimorphism in Dasiphora fruticosa

Dasiphora fruticosa comprises male, female and hermaphrodite plants, which are distributed sympatrically in some populations on the Qinghai-Tibet Plateau. To explore what governs the coexistence of these three sexual phenotypes, we investigated the DNA contents, pollen and ovule production, pollen deposition, and performed hand-pollination in both hermaphroditic and dioecious individuals of D. fruticosa. Flow cytometry confirmed that the DNA content of males and females were almost twice as much as that of the hermaphrodites. Male and female flowers produced more pollen grains and ovules than hermaphroditic flowers. Hand-pollinated treatments showed that unisexual flowers were sterile in one sexual function and bisexual flowers were fertile for both functions, but no sterile seeds were produced between unisexual and bisexual flowers. Our findings imply that polyploidy is related to gender dimorphism, and both are likely to play a strong role in the coexistence of two cryptic biological species of D. fruticosa (low ploidy hermaphroditic species and high ploidy dioecious species) in the Qinghai-Tibet Plateau.

Colonization, Baker's law, and the evolution of gynodioecy in Hawaii: implications from a study of Lycium carolinianum

PREMISE

As Baker's law suggests, the successful colonization of oceanic islands is often associated with uniparental reproduction (self-fertility), but the high incidence of dimorphism (dioecy, gynodioecy) on islands complicates this idea. Lycium carolinianum is widespread, occurring on the North American mainland and the Hawaiian Islands. We examined Baker's ideas for mainland and island populations of L. carolinianum and examined inbreeding depression as a possible contributor to the evolution of gynodioecy on Maui.

METHODS

Controlled crosses were conducted in two mainland populations and two populations in Hawaii. Treatments included self and cross pollination, unmanipulated controls, and autogamy/agamospermy. Alleles from the self-incompatibility S-RNase gene were isolated and compared between mainland and island populations. Given self-compatibility in Hawaii, we germinated seeds from self- and cross- treatments and estimated inbreeding depression using seven traits and a measure of cumulative fitness.

RESULTS

Mainland populations of Lycium carolinianum are predominately self-incompatible with some polymorphism for self-fertility, whereas Hawaiian populations are self-compatible. Concordantly, S-RNase allelic diversity is reduced in Hawaii compared to the mainland. Hawaiian populations also exhibit significant inbreeding depression.

CONCLUSIONS

Self-compatibility in Hawaii and individual variation in self-fertility in mainland populations suggests that a colonization filter promoting uniparental reproduction may be acting in this system. Comparison of S-RNase variation suggests a collapse of allelic diversity and heterozygosity at the S-RNase locus in Hawaii, which likely contributed to mate limitation upon arrival to the Pacific. Inbreeding depression coupled with autonomous self-fertilization may have led to the evolution of gynodioecy on Maui.

The role of infectious disease in the evolution of females: Evidence from anther-smut disease on a gynodioecious alpine carnation

In flowering plants, the evolution of females is widely hypothesized to be the first step in the evolutionary pathway to separate male and female sexes, or dioecy. Natural enemies have the potential to drive this evolution if they preferentially attack hermaphrodites over females. We studied sex-based differences in exposure to anther-smut (Microbotryum), a sterilizing pollinator-transmitted disease, in Dianthus pavonius, a gynodioecious perennial herb. We found that within a heavily diseased population, females consistently had lower levels of Microbotryum spore deposition relative to hermaphrodites and that this difference was driven by rapid floral closing in females following successful pollination. We further show that this protective closing behavior is frequency dependent; females close faster when they are rare. These results indicate that anther-smut disease is an important source of selection for females, especially since we found in a common garden experiment no evidence that females have any inherent fecundity advantages over hermaphrodites. Finally, we show that among populations, those where anther-smut is present have a significantly higher frequency of females than those where the disease is absent. Taken together our results indicate that anther-smut disease is likely an important biotic factor driving the evolution and maintenance of females in this gynodioecious species.

Development of an X-specific marker and identification of YY individuals in spinach

Spinach is a popular vegetable native to central and western Asia. It is dioecious with a pair of nascent sex chromosomes. The difficulties of working with the non-recombining sex determination region of XY individuals have hindered the progress toward sequencing sex chromosomes of most dioecious species. Here we present important advances toward characterizing the non-recombining sex chromosomes in spinach. Of nearly 400 spinach accessions screened, we identified a single accession of spinach in which androdioecious XY individuals segregate YY spinach. The male and female genomes of the spinach cultivar Shami and USDA accession PI 664497 were sequenced at 12-17 × coverage. X-specific sequences were identified by comparing the depth of coverage differences between male and female alignments to a female draft genome. YY individuals were used as a negative control to validate X-specific markers found by depth of coverage analysis. Of 19 possible X chromosome sequences found by depth of coverage analysis, one was verified to be X-specific by a PCR-based marker, SpoX, which amplified genomic DNA from XX and XY, but not YY templates. Androdioecious XY individuals of accession PI 217425 (Cornell #9) were used to develop inbred lines, and at S7 generation, all XY individuals were androdioecious and all YY individuals were pure male. The sex reversal of the XY mutant to hermaphrodite is strong evidence that the sex chromosomes in spinach have a two-gene sex determination system. These results are crucial towards sequencing the X and Y chromosomes to advance sex chromosome research in spinach.

On the possible role of nonreproductive traits for the evolution of unisexuality: Life-history variation among males, females, and hermaphrodites in Opuntia robusta (Cactaceae)

In angiosperms, dioecy has arisen in 871-5,000 independent events, distributed in approximately 43% of the flowering families. The reproductive superiority of unisexuals has been the favorite explanation for the evolution of separate sexes. However, in several instances, the observed reproductive performance of unisexuals, if any, does not seem to compensate for the loss of one of the sex functions. The involvement of fitness components not directly associated with reproduction is a plausible hypothesis that has received little attention. Life-history traits recently recognized as predictors of plant performance were compared among males, females, and hermaphrodites of a rare trioecious Opuntia robusta population in the field, using the cladode as the study unit. Cladode mortality by domestic herbivores was common and higher in females and hermaphrodites than in males. Males, females, or both displayed lower shrinkage and higher rates of survival, growth, and reproductive frequency than hermaphrodites. Unisexuals simultaneously outperformed hermaphrodites in demographic traits known to compete for common limiting resources, such as the acceleration of reproductive maturation (progenesis) and survival. A meta-analysis combining the outcomes of each of the analyzed life-history traits revealed a tendency of males (d++ = 1.03) and females (d++ = 0.93) to outperform hermaphrodites in presumably costly demographic options. Clonality is induced by human or domestic animal plant sectioning; and males and females highly exceeded hermaphrodites in their clonality potential by a factor of 8.3 and 5.3, respectively. The performances of unisexuals in the analyzed life-history traits may enhance their reproductive potential in the long run and their clonality potential and could explain the observed increase of unisexuality in the population. Life-history traits can be crucial for the evolution of unisexuality, but their impact appears to be habitat specific and may involve broad ontogenetic changes.

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.

Occurrence of subdioecy and scarcity of gender-specific markers reveal an ongoing transition to dioecy in Himalayan seabuckthorn (Hippophae rhamnoides ssp. turkestanica)

Dioecy and the dynamics of its evolution are intensely investigated aspects of plant reproduction. Seabuckthorn (Hippophae rhamnoides ssp. turkestanica) is an alpine shrub growing wild in certain parts of western Himalaya. The previous studies have reported heteromorphic sex chromosomes in the species and yet marker-based studies indicate high similarity between the male and female genomes. Lack of information on sexual system in the species has further complicated the situation. A systematic study was thus undertaken to understand the sexual system in seabuckthorn and to discern the extent of similarity/dissimilarity between the male and female genomes by generating a large number of markers using amplified fragment length polymorphism and representational difference analysis. Floral biology and regular monitoring of species revealed the presence of polygamomonoecious (PGM) plants in most populations at a low percentage (~2-4%). PGM plants showed low pollen production and overall low fertility, suggesting a monoecy-paradioecy pathway at function. The results of the marker study demonstrated that there are limited differences between male and female genomes and these differences were not uniform across the populations in the Leh-Ladakh region, especially when the geographical distance increases. Results also suggest that a dynamic partitioning of genomes is operational between the two genders of seabuckthorn and differences are not homogenized across the populations. Both reproductive biology-based and DNA marker-based studies indicate that genders have separated recently. The present study proposes seabuckthorn as a promising model system to study evolution of dioecy and sex determination.

Seed germination and seedling allogamy in Rosmarinus officinalis: the costs of inbreeding

Self-pollination by geitonogamy is likely in self-compatible plants that simultaneously expose a large number of flowers to pollinators. However, progeny of these plants is often highly allogamous. Although mechanisms to increase cross-pollination have been identified and studied, their relative importance has rarely been addressed simultaneously in plant populations. We used Rosmarinus officinalis to explore factors that influence the probability of self-fertilisation due to geitonogamy or that purge its consequences, focusing on their effects on seed germination and allogamy rate. We experimentally tested the effect of geitonogamy on the proportion of filled seeds and how it influences germination rate. During two field seasons, we studied how life history and flowering traits of individuals influence seed germination and allogamy rates of their progeny in wild populations at the extremes of the altitudinal range. The traits considered were plant size, population density, duration of the flowering season, number of open flowers, flowering synchrony among individuals within populations and proportion of male-sterile flowers. We found that most seeds obtained experimentally from self-pollination were apparently healthy but empty, and that the proportion of filled seeds drove the differences in germination rate between self- and cross-pollination experiments. Plants from wild populations consistently had low germination rate and high rate of allogamy, as determined with microsatellites. Germination rate related positively to the length of the flowering season, flowering synchrony and the ratio of male-sterile flowers, whereas the rate of allogamous seedlings was positively related only to the ratio of male-sterile flowers. Rosemary plants purge most of the inbreeding caused by its pollination system by aborting the seeds. This study showed that the rates of seed germination and allogamy of the seedlings depend on a complex combination of factors that vary in space and time. Male sterility of flowers, length of the flowering season and flowering synchrony of individuals within populations all favour high rates of cross-pollination, therefore increasing germination and allogamy rates. Flowering traits appear to be highly plastic and respond to local and seasonal conditions.

Restricted female function of hermaphrodites in a gynodioecious shrub, Daphne jezoensis (Thymelaeaceae)

Gynodioecy is the coexistence of hermaphrodites and females in a population. It is supposed to be an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy in angiosperm. Hermaphrodites gain fitness through both seed and pollen production whereas females gain fitness only through seed production. As females spread in a gynodioecious population, sexual selection prompts hermaphrodites to invest in male function and male-biased hermaphrodites prevail. In the gynodioecious shrub Daphne jezoensis (Thymelaeaceae), female frequency is stably around 50% in most populations, and fruit-set rate of hermaphrodites is commonly low. Therefore, D. jezoensis is likely at a later stage in the evolutionary pathway. Female function of hermaphrodites (fruit-set rate, selfing rate, seed size, and germination rate) was assessed in three populations under natural conditions. In order to evaluate the potential seed fertility and inbreeding depression by selfing in hermaphrodites, hand pollination treatments were also performed. Over a 2-year period under natural conditions, 18-29% of hermaphrodites and 69-81% of females set fruit. Across all three populations, the mean fruit-set rate ranged 9.5-49.2% in females and only 3.9-10.2% in hermaphrodites. Even with artificial outcross-pollination, 59-91% of hermaphrodites failed to set any fruit. When self-pollination was performed in hermaphrodites, both of fruit-set and germination rates were decreased, indicating early-acting inbreeding depression. In addition, more than half of the hermaphrodite seeds were produced by selfing under natural pollination, but pollinator service was still required. Totally, hermaphrodites performed poorly as seed producers because of the intrinsically-low fruiting ability and a combination of autogamous selfing and strong inbreeding depression, indicating the absence of reproductive assurance. These results indicate that the mating system of D. jezoensis is functionally close to dioecy.

Small but attractive: female-biased nectar production and floral visitors in a dimorphic shrub

In sexually dimorphic species, hermaphrodite flowers in gynodioecious species or male flowers in dioecious species are often larger and produce more nectar than their conspecific female flowers. As a consequence, hermaphrodite or male flowers frequently receive more pollinator visits. Sex ratio, flower size, floral display, nectar production and floral visits were evaluated in two natural populations of Fuchsia thymifolia, a morphologically gynodioecious but functionally subdioecious insect-pollinated shrub. Sex ratio did not differ from the expected 1:1 in the two studied populations. As expected, hermaphrodite flowers were larger than female flowers, but in contrast to the general pattern, hermaphrodite flowers did not produce nectar or produced much less than female flowers. Flower visitors were flies (68%) and bumblebees (24%), both of which showed a preference for female flowers. No sex difference was detected in either flower longevity or floral display across the flowering season. Higher nectar production by females may attract more pollinators, and may be a strategy to enhance female reproductive success in this species. Finally, floral dimorphism and insect preferences did not seem to hamper the maintenance of sub-dioecy or prevent the evolution of dioecy in F. thymifolia.

Macroevolutionary synthesis of flowering plant sexual systems

Sexual system is a key determinant of genetic variation and reproductive success, affecting evolution within populations and within clades. Much research in plants has focused on evolutionary transitions away from the most common state of hermaphroditism and toward the rare state of dioecy (separate sexes). Rather than transitions predominantly toward greater sexual differentiation, however, evolution may proceed in the direction of lesser sexual differentiation. We analyzed the macroevolutionary dynamics of sexual system in angiosperm genera that contain both dioecious and nondioecious species. Our phylogenetic analyses encompass a total of 2145 species from 40 genera. Overall, we found little evidence that rates of sexual system transitions are greater in any direction. Counting the number of inferred state changes revealed a mild prevalence of transitions away from hermaphroditism and away from dioecy, toward states of intermediate sexual differentiation. We identify genera in which future studies of sexual system evolution might be especially productive, and we discuss how integrating genetic or population-level studies of sexual system could improve the power of phylogenetic comparative analyses. Our work adds to the evidence that different selective pressures and constraints act in different groups, helping maintain the variety of sexual systems observed among plants.

On the rarity of dioecy in flowering plants

Dioecy, the coexistence of separate male and female individuals in a population, is a rare but phylogenetically widespread sexual system in flowering plants. While research has concentrated on why and how dioecy evolves from hermaphroditism, the question of why dioecy is rare, despite repeated transitions to it, has received much less attention. Previous phylogenetic and theoretical studies have suggested that dioecy might be an evolutionary dead end. However, recent research indicates that the phylogenetic support for this hypothesis is attributable to a methodological bias and that there is no evidence for reduced diversification in dioecious angiosperms. The relative rarity of dioecy thus remains a puzzle. Here, we review evidence for the hypothesis that dioecy might be rare not because it is an evolutionary dead end, but rather because it easily reverts to hermaphroditism. We review what is known about transitions between hermaphroditism and dioecy, and conclude that there is an important need to consider more widely the possibility of transitions away from dioecy, both from an empirical and a theoretical point of view, and by combining tools from molecular evolution and insights from ecology.

The effect of disease on the evolution of females and the genetic basis of sex in populations with cytoplasmic male sterility

The evolution of separate males and females is an important evolutionary transition that has occurred multiple times in flowering plants. While empirical studies have stressed the potential importance of natural enemies and organismal interactions in the evolution of separate sexes, there has been no treatment of natural enemies in the theoretical literature. We investigated the effects of disease on the evolution of females in gynodioecious populations composed of females and hermaphrodites, where sex is determined by the interaction of cytoplasmic male sterility (CMS) and nuclear restorer genes. When females are significantly more resistant than hermaphrodites, disease drives an increase in the frequency of females and sex determination becomes nuclear, creating the pre-conditions for the evolution of separate males and females. However, when females are only moderately more resistant, disease drives changes in the frequency of CMS and restorer alleles, but has little effect on the frequency of females. We discuss our results in the context of the evolution of mating systems and cyto-nuclear epistasis.

Surviving in a Cosexual World: A Cost-Benefit Analysis of Dioecy in Tropical Trees

Dioecy has a demographic disadvantage compared with hermaphroditism: only about half of reproductive adults produce seeds. Dioecious species must therefore have fitness advantages to compensate for this cost through increased survival, growth, and/or reproduction. We used a full life cycle approach to quantify the demographic costs and benefits associated with dioecy while controlling for demographic differences between dioecious and hermaphroditic species related to other functional traits. The advantage of this novel approach is that we can focus on the effect of breeding system across a diverse tree community. We built a composite integral projection model for hermaphroditic and dioecious tree populations from Barro Colorado Island, Panama, using long-term demographic and newly collected reproductive data. Integration of all costs and benefits showed that compensation was realized through increased seed production, resulting in no net costs of dioecy. Compensation was also facilitated by the low contribution of reproduction to population growth. Estimated positive effects of dioecy on tree growth and survival were small and insignificant for population growth rates. Our model revealed that, for long-lived organisms, the cost of having males is smaller than generally expected. Hence, little compensation is required for dioecious species to maintain population growth rates similar to those of hermaphroditic species.

Size-dependent sex allocation and reproductive investment in a gynodioecious shrub

In sexually dimorphic plants, resource allocation to reproduction often differs between sex morphs. In gynodioecious species, i.e. coexisting hermaphrodite and female plants within a population, females often produce more fruits than hermaphrodites. Since fruit production is costlier than flower production, hermaphrodites and females may regulate flower and fruit production differently in response to resource availability. To clarify the gender-specific strategies of reproductive allocation, we assessed sexual dimorphism in reproductive traits, size-dependent resource allocation, morphological traits, and photosynthetic capacity in a natural population of a gynodioecious shrub, Daphne jezoensis Hermaphrodites had larger flowers and increased flower number with plant size at a rate greater than females, but showed consistently smaller fruit production. Although females did not increase flower production as much as hermaphrodites did as their size increased, they produced 3.7 times more fruits than did hermaphrodites. Despite a large sexual difference in fruiting ability based on hand-pollination, total resource investment in reproduction (the sum of flower and fruit mass) was similar between sex morphs across plant sizes, and there was little sexual difference in the cost of reproduction, i.e. the negative effect of current reproduction on future reproductive effort, in the natural population. In addition, there were no sexual differences in the resource allocation to vegetative organs (leaf and root mass) and photosynthetic capacity (light response photosynthetic rates). Under natural conditions, pollen limitation strongly restricted the fruit production of females, resulting in similar cost of reproduction between hermaphrodites and females.

High male fertility in males of a subdioecious shrub in hand-pollinated crosses

Female reproductive success in females versus hermaphrodites has been well documented. However, documenting a potential advantage in male fertility of male versus hermaphrodite individuals in subdioecious species is also essential for understanding the evolutionary pathway toward dioecy from hermaphroditism via gynodioecy. Siring success in terms of fruit set, fruit mass, number of seeds and mean seed mass was compared by hand-pollinated crosses in the subdioecious shrub Eurya japonica The pollen was from male and hermaphrodite individuals, and the pollen recipients were females and hermaphrodites. Seed quality was also evaluated in terms of seed germination rate, seed germination day and seedling survival. Overall, pollen from males sired more fruits of larger size and more seeds than did pollen from hermaphrodites. The male advantage was observed when pollen recipients were females, whereas no effect was found in hermaphrodite recipients. Pollen from males also produced better quality seeds with higher germination rate and sooner germination day. Although these results could also be explained by a higher pollen load for crosses with male pollen donors, we took care to saturate the stigma regardless of the pollen donor. Therefore, these results suggest that male individuals of E. japonica have advantages in male fertility in terms of both quantity and quality. Our previous studies indicated that females exhibit higher female reproductive success compared with hermaphrodites. Thus, both the female and male functions of hermaphrodites are outperformed by females and males, respectively, raising the possibility that the subdioecious E. japonica at this study site is entering the transitional phase to dioecy along the gynodioecy-dioecy pathway.

Male-biased hermaphrodites in a gynodioecious shrub, Daphne jezoensis

Gynodioecy, a state where female and hermaphrodite plants coexist in populations, has been widely proposed an intermediate stage in the evolutionary pathway from hermaphroditism to dioecy. In the gynodioecy-dioecy pathway, hermaphrodites may gain most of their fitness through male function once females invade populations. To test this prediction, comprehensive studies on sex ratio variation across populations and reproductive characteristics of hermaphrodite and female phenotypes are necessary. This study examined the variation in sex ratio, sex expression, flower and fruit production and sexual dimorphism of morphological traits in a gynodioecious shrub, Daphne jezoensis, over multiple populations and years. Population sex ratio (hermaphrodite:female) was close to 1:1 or slightly hermaphrodite-biased. Sex type of individual plants was largely fixed, but 15% of plants changed their sex during a 6-year census. Hermaphrodite plants produced larger flowers and invested 2.5 times more resources in flower production than female plants, but they exhibited remarkably low fruit set (proportion of flowers setting fruits). Female plants produced six times more fruits than hermaphrodite plants. Low fruiting ability of hermaphrodite plants was retained even when hand-pollination was performed. Fruit production of female plants was restricted by pollen limitation under natural conditions, irrespective of high potential fecundity, and this minimised the difference in resources allocated to reproduction between the sexes. Negative effects of previous flower and fruit production on current reproduction were not apparent in both sexes. This study suggests that gynodioecy in this species is functionally close to a dioecious mating system: smaller flower production with larger fruiting ability in female plants, and larger flower production with little fruiting ability in hermaphrodite plants.

Polyploidy and sexual system in angiosperms: Is there an association?

PREMISE OF THE STUDY

Flowering plants display a variety of sexual systems, ranging from complete cosexuality (hermaphroditism) to separate-sexed individuals (dioecy). While dioecy is relatively rare, it has evolved many times and is present in many plant families. Transitions in sexual systems are hypothesized to be affected by large genomic events such as whole-genome duplication, or polyploidy, and several models have been proposed to explain the observed patterns of association.

METHODS

In this study, we assessed the association between ploidy and sexual system (separate or combined sexes). To this end, we assembled a database of ploidy levels and sexual systems for ∼1000 species, spanning 18 genera and 15 families. We applied several phylogenetic comparative approaches, including Pagel's coevolutionary framework and sister clade analyses, for detecting correlations between ploidy level and sexual system.

KEY RESULTS

Our results indicate a broad association between polyploidy and sexual system dimorphism, with low evolutionary stability of the diploid-dioecious condition observed in several clades. A detailed examination of the clades exhibiting this correlation reveals that it is underlain by various patterns of transition rate asymmetry.

CONCLUSIONS

We conclude that the long-hypothesized connection between ploidy and sexual system holds in some clades, although it may well be affected by factors that differ from clade to clade. Our results further demonstrate that to better understand the evolutionary processes involved, more sophisticated methods and extensive and detailed data sets are required for both broad and focused inquiry.

Multilocus Sex Determination Revealed in Two Populations of Gynodioecious Wild Strawberry, Fragaria vesca subsp. bracteata

Gynodioecy, the coexistence of females and hermaphrodites, occurs in 20% of angiosperm families and often enables transitions between hermaphroditism and dioecy. Clarifying mechanisms of sex determination in gynodioecious species can thus illuminate sexual system evolution. Genetic determination of gynodioecy, however, can be complex and is not fully characterized in any wild species. We used targeted sequence capture to genetically map a novel nuclear contributor to male sterility in a self-pollinated hermaphrodite of Fragaria vesca subsp. bracteata from the southern portion of its range. To understand its interaction with another identified locus and possibly additional loci, we performed crosses within and between two populations separated by 2000 km, phenotyped the progeny and sequenced candidate markers at both sex-determining loci. The newly mapped locus contains a high density of pentatricopeptide repeat genes, a class commonly involved in restoration of fertility caused by cytoplasmic male sterility. Examination of all crosses revealed three unlinked epistatically interacting loci that determine sexual phenotype and vary in frequency between populations. Fragaria vesca subsp. bracteata represents the first wild gynodioecious species with genomic evidence of both cytoplasmic and nuclear genes in sex determination. We propose a model for the interactions between these loci and new hypotheses for the evolution of sex determining chromosomes in the subdioecious and dioecious Fragaria.

Absence of sex differential plasticity to light availability during seed maturation in Geranium sylvaticum

Sex-differential plasticity (SDP) hypothesis suggests that since hermaphrodites gain fitness through both pollen and seed production they may have evolved a higher degree of plasticity in their reproductive strategy compared to females which achieve fitness only through seed production. SDP may explain the difference in seed production observed between sexes in gynodioecious species in response to resource (nutrients or water) availability. In harsh environments, hermaphrodites decrease seed production whereas females keep it relatively similar regardless of the environmental conditions. Light availability can be also a limiting resource and thus could theoretically affect differently female and hermaphrodite seed output even though this ecological factor has been largely overlooked. We tested whether the two sexes in the gynodioecious species Geranium sylvaticum differ in their tolerance to light limitation during seed maturation in the field. We used a fully factorial block experiment exposing female and hermaphrodite plants to two different light environments (control and shade) after their peak flowering period. Specifically, we measured fruit and seed production in response to decreased light availability and compared it between the sexes. Shading reduced the number of fruits and seeds produced, but the decrease was similar between the sexes. Furthermore, shading delayed seed production by three days in both sexes, but did not affect seed mass, seed P content, or the probability of re-flowering the following year. Our results give no evidence for reproductive SDP in response to light during seed maturation.

Selfish male-determining element favors the transition from hermaphroditism to androdioecy

According to the current, widely accepted paradigm, the evolutionary transition from hermaphroditism toward separate sexes occurs in two successive steps: an initial, intermediate step in which unisexual individuals, male or female, sterility mutants coexist with hermaphrodites and a final step that definitively establishes dioecy. Two nonexclusive processes can drive this transition: inbreeding avoidance and reallocation of resources from one sexual function to the other. Here, we report results of controlled crosses between males and hermaphrodites in Phillyrea angustifolia, an androdioecious species with two mutually intercompatible, but intraincompatible groups of hermaphrodites. We observed different segregation patterns that can be explained by: (1) epistatic interactions between two unlinked diallelic loci, determining sex and mating compatibility, and (2) a mutation with pleiotropic effects: female sterility, full compatibility of males with both hermaphrodite incompatibility groups, and complete male-biased sex-ratio distortion in one of the two groups. Modeling shows that these mechanisms can explain the high frequency of males in populations of P. angustifolia and can promote the maintenance of androdioecy without requiring inbreeding depression or resource reallocation. We thus argue that segregation distortion establishes the right conditions for the evolution of cryptic dioecy and potentially initiates the evolution toward separate sexes.

Differences in female reproductive success between female and hermaphrodite individuals in the subdioecious shrub Eurya japonica (Theaceae)

Subdioecy is thought to occupy a transitional position in the gynodioecy-dioecy pathway, explaining one of the evolutionary routes from hermaphroditism to dioecy. Quantifying any female reproductive advantage of females versus hermaphrodites is fundamental to examining the spectrum between subdioecy and dioecy; however, this is challenging, as multiple interacting factors, such as pollen limitation and resource availability, affect plant reproduction. We compared the female reproductive success of females and hermaphrodites via a field experiment in which we hand-pollinated individuals of the subdioecious shrub Eurya japonica of similar size growing under similar light conditions. Effects of pollen limitation and seed quality were also evaluated through comparing the results of hand- and natural-pollination treatments and performing additional laboratory and greenhouse experiments. Overall, females had higher fruit set and produced heavier fruit and more seeds than hermaphrodites, and these results were more pronounced for hand-pollinated than for natural-pollinated plants of both sexes. We also found that seeds naturally produced by females had a higher mean germination rate. These results indicate that females had a pronounced advantage in female reproductive success under conditions of no pollen limitation. The sexual difference in the degree of pollen limitation suggests a pollinator-mediated interaction, whereas the higher female reproductive success of females even under natural conditions implies that E. japonica is a good model species for elucidating the later stages of the gynodioecy-dioecy pathway.

Intraspecific variation in gender strategies in Lycium (Solanaceae): associations with ploidy and changes in floral form following the evolution of gender dimorphism

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PREMISE OF THE STUDY

An association between polyploidy and gender dimorphism has been noted in several plant lineages. Whereas the majority of Lycium species are diploid and have hermaphroditic flowers in cosexual populations, gender dimorphism (gynodioecy, dioecy) has been shown to be uniformly associated with polyploidy in previous studies. Preliminary field observations suggested that some populations of Lycium carolinianum were dimorphic, providing a test of this association.•

METHODS

We assessed sexual systems and cytotype variation (to infer ploidy) across 17 populations of L. carolinianum. Comparison of flowers in cosexual and dimorphic populations were used to infer changes in reproductive morphology associated with the evolution of gynodioecy.•

KEY RESULTS

The majority of populations were cosexual in gender expression, but dimorphism was present in the Yucatán and in some populations in Hawaii. Populations varied in ploidy and were either diploid or tetraploid. Floral sexual dimorphism was present in all gynodioecious populations, though the magnitude differed and was cryptic in some cases. Our results are consistent with the hypothesis that following the evolution of gynodioecy, flowers on hermaphrodites increased in size.•

CONCLUSIONS

Dimorphic sexual systems have likely evolved convergently in L. carolinianum. In contrast to previous studies, dimorphism is not perfectly associated with polyploidy. Although our sample from the Yucatán was both tetraploid and dimorphic, all populations in Hawaii were diploid regardless of sexual system. Ongoing phylogeographic and mating system studies will contribute to our understanding of reproductive evolution in this widespread, polymorphic species.

Fragaria: a genus with deep historical roots and ripe for evolutionary and ecological insights

The cultivated strawberry, Fragaria ×ananassa, is one of the youngest domesticated plants. Its 18th century origin via hybridization in Europe between the North American F. virginiana and the South American F. chiloensis was documented by the botanist Antoine Nicolas Duchesne. His 1766 "Natural History of Strawberries" is an extraordinary work that integrates fundamental discoveries on the biology, ecology, and phylogeny of Fragaria with applied information on cultivation and ethnobotanical uses, serving as an inspiration for current research in the genus. Fragaria species exhibit the full range of sexual systems in the gynodioecy pathway from hermaphroditism to dioecy (and back again), as well as variation in self-compatibility, and evidence of sex chromosomes with female heterogamety. The genus is also characterized by interspecific hybridization and polyploidy, with a natural range of ploidy levels from diploids to decaploids. This biological diversity, combined with the availability of genomic resources and the ease of growing and experimenting with the plants, makes Fragaria a very attractive system for ecological and evolutionary genomics. The goal of this review is to introduce Fragaria as a model genus and to provide a roadmap for future integrative research. These research directions will deepen our understanding of the ecological and evolutionary context that shaped the ancestors of the cultivated strawberry, not only providing information that can be applied to efforts to shape the future of this important fruit crop but also our understanding of key transitions in plant evolution.

The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database

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PREMISE OF THE STUDY

Separating sexual function between different individuals carries risks, especially for sedentary organisms. Nevertheless, many land plants have unisexual gametophytes or sporophytes. This study brings together data and theoretical insights from research over the past 20 yr on the occurrence and frequency of plant sexual systems, focusing on the flowering plants.•

METHODS

A list of genera with dioecious species, along with other information, is made available (http://www.umsl.edu/∼renners/). Frequencies of other sexual systems are tabulated, and data on the genetic regulation, ecological context, and theoretical benefits of dioecy reviewed.•

KEY RESULTS

There are 15600 dioecious angiosperms in 987 genera and 175 families, or 5-6% of the total species (7% of genera, 43% of families), with somewhere between 871 to 5000 independent origins of dioecy. Some 43% of all dioecious angiosperms are in just 34 entirely dioecious clades, arguing against a consistent negative influence of dioecy on diversification. About 31.6% of the dioecious species are wind-pollinated, compared with 5.5-6.4% of nondioecious angiosperms. Also, 1.4% of all angiosperm genera contain dioecious and monoecious species, while 0.4% contain dioecious and gynodioecious species. All remaining angiosperm sexual systems are rare. Chromosomal sex determination is known from 40 species; environmentally modulated sex allocation is common. Few phylogenetic studies have focused on the evolution of dioecy.•

CONCLUSIONS

The current focus is on the genetic mechanisms underlying unisexual flowers and individuals. Mixed strategies of sexual and vegetative dispersal, together with plants' sedentary life style, may often favor polygamous systems in which sexually inconstant individuals can persist. Nevertheless, there are huge entirely dioecious clades of tropical woody plants.

An angiosperm-wide analysis of the gynodioecy-dioecy pathway

BACKGROUND AND AIMS

About 6 % of an estimated total of 240 000 species of angiosperms are dioecious. The main precursors of this sexual system are thought to be monoecy and gynodioecy. A previous angiosperm-wide study revealed that many dioecious species have evolved through the monoecy pathway; some case studies and a large body of theoretical research also provide evidence in support of the gynodioecy pathway. If plants have evolved through the gynodioecy pathway, gynodioecious and dioecious species should co-occur in the same genera. However, to date, no large-scale analysis has been conducted to determine the prevalence of the gynodioecy pathway in angiosperms. In this study, this gap in knowledge was addressed by performing an angiosperm-wide survey in order to test for co-occurrence as evidence of the gynodioecy pathway.

METHODS

Data from different sources were compiled to obtain (to our knowledge) the largest dataset on gynodioecy available, with 275 genera that include at least one gynodioecious species. This dataset was combined with a dioecy dataset from the literature, and a study was made of how often dioecious and gynodioecious species could be found in the same genera using a contingency table framework.

KEY RESULTS

It was found that, overall, angiosperm genera with both gynodioecious and dioecious species occur more frequently than expected, in agreement with the gynodioecy pathway. Importantly, this trend holds when studying different classes separately (or sub-classes, orders and families), suggesting that the gynodioecy pathway is not restricted to a few taxa but may instead be widespread in angiosperms.

CONCLUSIONS

This work complements that previously carried out on the monoecy pathway and suggests that gynodioecy is also a common pathway in angiosperms. The results also identify angiosperm families where some (or all) dioecious species may have evolved from gynodioecious precursors. These families could be the targets of future small-scale studies on transitions to dioecy taking phylogeny explicitly into account.

Constraints imposed by pollinator behaviour on the ecology and evolution of plant mating systems

Most flowering plants rely on pollinators for their reproduction. Plant-pollinator interactions, although mutualistic, involve an inherent conflict of interest between both partners and may constrain plant mating systems at multiple levels: the immediate ecological plant selfing rates, their distribution in and contribution to pollination networks, and their evolution. Here, we review experimental evidence that pollinator behaviour influences plant selfing rates in pairs of interacting species, and that plants can modify pollinator behaviour through plastic and evolutionary changes in floral traits. We also examine how theoretical studies include pollinators, implicitly or explicitly, to investigate the role of their foraging behaviour in plant mating system evolution. In doing so, we call for more evolutionary models combining ecological and genetic factors, and additional experimental data, particularly to describe pollinator foraging behaviour. Finally, we show that recent developments in ecological network theory help clarify the impact of community-level interactions on plant selfing rates and their evolution and suggest new research avenues to expand the study of mating systems of animal-pollinated plant species to the level of the plant-pollinator networks.

Segregation of male-sterility alleles across a species boundary

Hybrid zones may serve as bridges permitting gene flow between species, including alleles influencing the evolution of breeding systems. Using greenhouse crosses, we assessed the likelihood that a hybrid zone could serve as a conduit for transfer of nuclear male-sterility alleles between a gynodioecious species and a hermaphroditic species with very rare females in some populations. Segregation patterns in progeny of crosses between rare females of hermaphroditic Schiedea menziesii and hermaphroditic plants of gynodioecious Schiedea salicaria heterozygous at the male-sterility locus, and between female S. salicaria and hermaphroditic plants from the hybrid zone, were used to determine whether male-sterility was controlled at the same locus in the parental species and the hybrid zone. Segregations of females and hermaphrodites in approximately equal ratios from many of the crosses indicate that the same nuclear male-sterility allele occurs in the parent species and the hybrid zone. These rare male-sterility alleles in S. menziesii may result from gene flow from S. salicaria through the hybrid zone, presumably facilitated by wind pollination in S. salicaria. Alternatively, rare male-sterility alleles might result from a reversal from gynodioecy to hermaphroditism in S. menziesii, or possibly de novo evolution of male sterility. Phylogenetic analysis indicates that some species of Schiedea have probably evolved separate sexes independently, but not in the lineage containing S. salicaria and S. menziesii. High levels of selfing and expression of strong inbreeding depression in S. menziesii, which together should favour females in populations, argue against a reversal from gynodioecy to hermaphroditism in S. menziesii.