Regional variation in sex ratios and sex allocation in androdioecious Mercurialis annua

Widespread male sterility and trioecy in androdioecious Mercurialis annua: Its distribution, genetic basis, and estimates of morph-specific fitness components

PREMISE

Angiosperms range in sexual system from hermaphroditism through gynodioecy and androdioecy to dioecy. Trioecy, where females and males coexist with hermaphrodites, is rare. Recently, trioecy was documented in hexaploid populations of the wind-pollinated herb Mercurialis annua in Spain.

METHODS

We surveyed the frequency of males, hermaphrodites, and females in M. annua across its distribution in the Iberian Peninsula, tracked sex-ratio variation in several populations over consecutive generations, and assessed evidence for pollen limitation. In a common garden, we estimated male, female, and hermaphroditic fitness. We used controlled crosses to infer the genetic basis of male sterility. Finally, we compared predictions of a deterministic model with the distribution of observed sex ratios in the field based on our fitness estimates and the inferred genetics of sex determination.

RESULTS

Trioecy is widespread in Spanish and Portuguese populations of M. annua. Males are determined by a dominant (Y-linked) allele, and female expression results from the interaction between cytoplasmic male sterility and multiple nuclear male sterility restorers partially linked to the male determiner. Male pollen production is approximately 12 times that of hermaphrodites, while female seed production is less than 1.12 times the observed hermaphroditic levels. The distribution of sex ratios in natural populations conforms with predictions of our deterministic simulations.

CONCLUSIONS

Our study documents and accounts for a clear case of trioecy in which sex is determined by both maternally and biparentally inherited genes.

The evolution and maintenance of trioecy with cytoplasmic male sterility

Trioecy, the co-existence of females, males and hermaphrodites, is a rare sexual system in plants that may be an intermediate state in transitions between hermaphroditism and dioecy. Previous models have identified pollen limitation as a necessary condition for the evolution of trioecy from hermaphroditism. In these models, the seed-production and pollen production of females and males relative to those of hermaphrodites, respectively, are compromised by self-fertilization by hermaphrodites under pollen- limitation. Here, we investigate the evolution of trioecy via the invasion of cytoplasmic male sterility (CMS) into androdioecious populations in which hermaphrodites co-occur with males and where the male determiner is linked to a (partial) fertility restorer. We show that the presence of males in a population renders invasion by CMS more difficult. However, the presence of males also facilitates the maintenance of trioecy even in the absence of pollen limitation by negative frequency-dependent selection, because males reduce the transmission of CMS by females by siring sons (which cannot transmit CMS). We discuss our results in light of empirical observations of trioecy in plants and its potential role in the evolution of dioecy.

Variable resource allocation pattern, biased sex-ratio, and extent of sexual dimorphism in subdioecious Hippophae rhamnoides

Evolutionary maintenance of dioecy is a complex phenomenon and varies by species and underlying pathways. Also, different sexes may exhibit variable resource allocation (RA) patterns among the vegetative and reproductive functions. Such differences are reflected in the extent of sexual dimorphism. Though rarely pursued, investigation on plant species harbouring intermediate sexual phenotypes may reveal useful information on the strategy pertaining to sex-ratios and evolutionary pathways. We studied H. rhamnoides ssp. turkestanica, a subdioecious species with polygamomonoecious (PGM) plants, in western Himalaya. The species naturally inhabits a wide range of habitats ranging from river deltas to hill slopes. These attributes of the species are conducive to test the influence of abiotic factors on sexual dimorphism, and RA strategy among different sexes. The study demonstrates sexual dimorphism in vegetative and reproductive traits. The sexual dimorphism index, aligned the traits like height, number of branches, flower production, and dry-weight of flowers with males while others including fresh-weight of leaves, number of thorns, fruit production were significantly associated with females. The difference in RA pattern is more pronounced in reproductive traits of the male and female plants, while in the PGM plants the traits overlap. In general, habitat conditions did not influence either the extent of sexual dimorphism or RA pattern. However, it seems to influence secondary sex-ratio as females show their significant association with soil moisture. Our findings on sexual dimorphism and RA pattern supports attributes of wind-pollination in the species. The observed extent of sexual dimorphism in the species reiterates limited genomic differences among the sexes and the ongoing evolution of dioecy via monoecy in the species. The dynamics of RA in the species appears to be independent of resource availability in the habitats as the species grows in a resource-limited and extreme environment.

Dioecy and chromosomal sex determination are maintained through allopolyploid speciation in the plant genus Mercurialis

Polyploidization may precipitate dramatic changes to the genome, including chromosome rearrangements, gene loss, and changes in gene expression. In dioecious plants, the sex-determining mechanism may also be disrupted by polyploidization, with the potential evolution of hermaphroditism. However, while dioecy appears to have persisted through a ploidy transition in some species, it is unknown whether the newly formed polyploid maintained its sex-determining system uninterrupted, or whether dioecy re-evolved after a period of hermaphroditism. Here, we develop a bioinformatic pipeline using RNA-sequencing data from natural populations to demonstrate that the allopolyploid plant Mercurialis canariensis directly inherited its sex-determining region from one of its diploid progenitor species, M. annua, and likely remained dioecious through the transition. The sex-determining region of M. canariensis is smaller than that of its diploid progenitor, suggesting that the non-recombining region of M. annua expanded subsequent to the polyploid origin of M. canariensis. Homeologous pairs show partial sexual subfunctionalization. We discuss the possibility that gene duplicates created by polyploidization might contribute to resolving sexual antagonism.

Enhanced leaky sex expression in response to pollen limitation in the dioecious plant Mercurialis annua

In dioecious plants, males and females frequently show ‘leaky’ sex expression, with individuals occasionally producing flowers of the opposite sex. This leaky sex expression may have enabled the colonization of oceanic islands by dioecious plant species, and it is likely to represent the sort of variation upon which selection acts to bring about evolutionary transitions from dioecy to hermaphroditism. Although leakiness is commonly reported for dioecious species, it is not known whether it has plastic component. The question is interesting because males or females with an ability to enhance their leakiness plastically in the absence of mates would have an advantage of being able to produce progeny by self‐fertilization. Here, we demonstrate that leaky sex expression in the wind‐pollinated dioecious herb Mercurialis annua is plastically responsive to its mating context. We compared experimental populations of females growing either with or without males. Females growing in the absence of males were leakier in their sex expression than controls growing with males, producing more than twice as many male flowers. Our results thus provide a striking instance of plasticity in the reproductive behaviour of plants that is likely adaptive. We consider how females might sense their mating environment as a function of pollen availability, and we discuss possible constraints on the evolution of plasticity in sex expression when the environmental signals that individuals receive are unreliable.

Comparative development of staminate and pistillate flowers in the dioecious cactus Opuntia robusta

PCD role in unisexual flowers. The developmental processes underlying the transition from hermaphroditism to unisexuality are key to understanding variation and evolution of floral structure and function. A detailed examination of the cytological and histological patterns involved in pollen and ovule development of staminate and pistillate flowers in the dioecious Opuntia robusta was undertaken, and the potential involvement of programmed cell death in the abortion of the sex whorls was explored. Flowers initiated development as hermaphrodites and became functionally unisexual by anthesis. Female individuals have pistillate flowers with a conspicuous stigma, functional ovary, collapsed stamens and no pollen grains. Male individuals have staminate flowers, with large yellow anthers, abundant pollen grains, underdeveloped stigma, style and an ovary that rarely produced ovules. In pistillate flowers, anther abortion resulted from the premature degradation of the tapetum by PCD, followed by irregular deposition of callose wall around the microsporocytes, and finally by microspore degradation. In staminate flowers, the stigma could support pollen germination; however, the ovaries were reduced, with evidence of placental arrest and ovule abortion through PCD, when ovules were present. We demonstrate that PCD is recruited in both pistillate and staminate flower development; however, it occurs at different times of floral development. This study contributes to the understanding of the nature of the O. robusta breeding system and identifies developmental landmarks that contribute to sexual determination in Cactaceae.

A functional decomposition of sex inconstancy in the dioecious, colonizing plant Mercurialis annua

PREMISE

Plants with separate sexes often show "inconstant" or "leaky" sex expression, with females or males producing a few flowers of the opposite sex. The frequency and degree of such inconstancy may reflect residual hermaphroditic sex allocation after an evolutionary transition from combined to separate sexes. Sex inconstancy also represents a possible first step in the breakdown of dioecy back to hermaphroditism. In the Mercurialis annua (Euphorbiaceae) species complex, monoecy and androdioecy have evolved from dioecy in polyploid populations. Here, we characterize patterns of sex inconstancy in dioecious M. annua and discuss how sex inconstancy may have contributed to the breakdown of separate sexes in the genus.

METHODS

We measured sex inconstancy in three common gardens of M. annua over 2 years using a modification of Lloyd's phenotypic gender in terms of frequency and degree, with the degree calibrating inconstancy against the sex allocation of constant males and constant females, yielding a measure of gender that does not depend on the distribution of gender in the population.

RESULTS

Unusually for dioecious plants, the frequency of sex inconstancy in M. annua was greater in females, but its degree was greater for males in the 2 years of study. We suggest that this pattern is consistent with the maintenance of inconstancy in dioecious M. annua by selection for reproductive assurance under mate limitation.

CONCLUSIONS

Our study illustrates the utility of decomposing measures of sex inconstancy into its frequency and its degree and throws new light on the origin of variation in sexual systems in Mercurialis.

Low siring success of females with an acquired male function illustrates the legacy of sexual dimorphism in constraining the breakdown of dioecy

Dioecy has often broken down in flowering plants, yielding functional hermaphroditism. We reasoned that evolutionary transitions from dioecy to functional hermaphroditism must overcome an inertia of sexual dimorphism, because modified males or females will express the opposite sexual function for which their phenotypes have been optimised. We tested this prediction by assessing the siring success of monoecious individuals of the plant Mercurialis annua with an acquired male function but that are phenotypically still female‐like. We found that pollen dispersed by female‐like monoecious individuals was ~ 1/3 poorer at siring outcrossed offspring than pollen from monoecious individuals with an alternative male‐like inflorescence. We conclude that whereas dioecy might evolve from functional hermaphroditism by conferring upon individuals certain benefits of sexual specialisation, reversion from a strategy of separate sexes to one of combined sexes must overcome constraints imposed by the advantages of sexual dimorphism. The breakdown of dioecy must therefore often be limited to situations in which outcrossing cannot be maintained and where selection favours a capacity for inbreeding by functional hermaphrodites.

Size and Content of the Sex-Determining Region of the Y Chromosome in Dioecious Mercurialis annua, a Plant with Homomorphic Sex Chromosomes

Dioecious plants vary in whether their sex chromosomes are heteromorphic or homomorphic, but even homomorphic sex chromosomes may show divergence between homologues in the non-recombining, sex-determining region (SDR). Very little is known about the SDR of these species, which might represent particularly early stages of sex-chromosome evolution. Here, we assess the size and content of the SDR of the diploid dioecious herb Mercurialis annua, a species with homomorphic sex chromosomes and mild Y-chromosome degeneration. We used RNA sequencing (RNAseq) to identify new Y-linked markers for M. annua. Twelve of 24 transcripts showing male-specific expression in a previous experiment could be amplified by polymerase chain reaction (PCR) only from males, and are thus likely to be Y-linked. Analysis of genome-capture data from multiple populations of M. annua pointed to an additional six male-limited (and thus Y-linked) sequences. We used these markers to identify and sequence 17 sex-linked bacterial artificial chromosomes (BACs), which form 11 groups of non-overlapping sequences, covering a total sequence length of about 1.5 Mb. Content analysis of this region suggests that it is enriched for repeats, has low gene density, and contains few candidate sex-determining genes. The BACs map to a subset of the sex-linked region of the genetic map, which we estimate to be at least 14.5 Mb. This is substantially larger than estimates for other dioecious plants with homomorphic sex chromosomes, both in absolute terms and relative to their genome sizes. Our data provide a rare, high-resolution view of the homomorphic Y chromosome of a dioecious plant.

Sex-specific strategies of resource allocation in response to competition for light in a dioecious plant

The differential plasticity hypothesis suggests that sexual dimorphism in dioecious plants could evolve in response to sex-specific resource requirements for reproduction (i.e., high carbon requirements for ovules and high nitrogen demands for pollen). When resources become limiting during growth, males and females should, therefore, adjust their allocation to resource-harvesting organs differently. To investigate the potential for plants to respond to resource limitation late in life and to test the differential plasticity hypothesis, we grew male and female individuals of the annual wind-pollinated plant Mercurialis annua in a common garden. Late in the growth season, we simulated a change in competition by decreasing plant density in half of the replicates. We measured both allocation to vegetative and reproductive traits and analyzed the relative allocation to reproduction vs. growth. Males and females differentially adjusted their resource allocation in response to varying plant densities, despite the fact that they were reproductively mature. Males maintained the same relative allocation of resource to reproductive vs. vegetative tissues at both densities. In contrast, females reduced vegetative growth proportionally less than seed production at the higher density. Our results highlight the dynamic nature of allocation decisions taken by plants, which respond quickly and in a sexually dimorphic way to changes in their competitive circumstances. The existence of resource 'currencies' limiting male and female functions differently have potentially led to the evolution of sex-specific strategies of resource acquisition and deployment, with females conserving resources for vegetative organs to ensure their future carbon-rich reproduction.

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 scope of Baker's law

Baker's law refers to the tendency for species that establish on islands by long-distance dispersal to show an increased capacity for self-fertilization because of the advantage of self-compatibility when colonizing new habitat. Despite its intuitive appeal and broad empirical support, it has received substantial criticism over the years since it was proclaimed in the 1950s, not least because it seemed to be contradicted by the high frequency of dioecy on islands. Recent theoretical work has again questioned the generality and scope of Baker's law. Here, we attempt to discern where the idea is useful to apply and where it is not. We conclude that several of the perceived problems with Baker's law fall away when a narrower perspective is adopted on how it should be circumscribed. We emphasize that Baker's law should be read in terms of an enrichment of a capacity for uniparental reproduction in colonizing situations, rather than of high selfing rates. We suggest that Baker's law might be tested in four different contexts, which set the breadth of its scope: the colonization of oceanic islands, metapopulation dynamics with recurrent colonization, range expansions with recurrent colonization, and colonization through species invasions.

Sex determination in dioecious Mercurialis annua and its close diploid and polyploid relatives

Separate sexes have evolved on numerous independent occasions from hermaphroditic ancestors in flowering plants. The mechanisms of sex determination is known for only a handful of such species, but, in those that have been investigated, it usually involves alleles segregating at a single locus, sometimes on heteromorphic sex chromosomes. In the genus Mercurialis, transitions between combined (hermaphroditism) and separate sexes (dioecy or androdioecy, where males co-occur with hermaphrodites rather than females) have occurred more than once in association with hybridisation and shifts in ploidy. Previous work has pointed to an unusual 3-locus system of sex determination in dioecious populations. Here, we use crosses and genotyping for a sex-linked marker to reject this model: sex in diploid dioecious M. annua is determined at a single locus with a dominant male-determining allele (an XY system). We also crossed individuals among lineages of Mercurialis that differ in their ploidy and sexual system to ascertain the extent to which the same sex-determination system has been conserved following genome duplication, hybridisation and transitions between dioecy and hermaphroditism. Our results indicate that the male-determining element is fully capable of determining gender in the progeny of hybrids between different lineages. Specifically, males crossed with females or hermaphrodites always generate 1:1 male:female or male:hermaphrodite sex ratios, respectively, regardless of the ploidy levels involved (diploid, tetraploid or hexaploid). Our results throw further light on the genetics of the remarkable variation in sexual systems in the genus Mercurialis. They also illustrate the almost identical expression of sex-determining alleles in terms of sexual phenotypes across multiple divergent backgrounds, including those that have lost separate sexes altogether.