Understanding plant reproductive diversity

Flowering plants display spectacular floral diversity and a bewildering array of reproductive adaptations that promote mating, particularly outbreeding. A striking feature of this diversity is that related species often differ in pollination and mating systems, and intraspecific variation in sexual traits is not unusual, especially among herbaceous plants. This variation provides opportunities for evolutionary biologists to link micro-evolutionary processes to the macro-evolutionary patterns that are evident within lineages. Here, I provide some personal reflections on recent progress in our understanding of the ecology and evolution of plant reproductive diversity. I begin with a brief historical sketch of the major developments in this field and then focus on three of the most significant evolutionary transitions in the reproductive biology of flowering plants: the pathway from outcrossing to predominant self-fertilization, the origin of separate sexes (females and males) from hermaphroditism and the shift from animal pollination to wind pollination. For each evolutionary transition, I consider what we have discovered and some of the problems that still remain unsolved. I conclude by discussing how new approaches might influence future research in plant reproductive biology.     

High male reproductive success of hermaphrodites in the androdioecious Phillyrea angustifolia

Androdioecy, the coexistence of males and hermaphrodites within a population, is a rare breeding system, often considered as unlikely to evolve because of restrictive conditions for its maintenance. Phillyrea angustifolia, a wind-pollinated shrub, is one of the handful species reported to be androdioecious. Our previous studies have shown that natural populations of this species in southern France exhibit higher male frequencies (approximately 50%) than predicted on theoretical grounds. Thus, the male functionality of hermaphrodites is still debated. To assess the functional breeding system of this species in the wild, a paternity analysis was performed with two highly polymorphic microsatellite loci on 729 seeds collected on 10 maternal shrubs in a natural population of 24 mature individuals of P. angustifolia. A large proportion of seeds were found to have been sired by pollen from outside the population. Analysis of seeds sired by individuals within the study population revealed a high male fertility of hermaphrodites resulting in a low male advantage in fertility for male plants. Intermate distances were found to have a strong impact on male reproductive success, whereas sexual morph had no effect, with males and hermaphrodites performing equivalently. This study is the first to unequivocally document the occurrence of a male function of hermaphrodites in a natural population of an androdioecious species.     

How geitonogamous selfing affects sex allocation in hermaphrodite plants

Does the mode of self-pollination affect the evolutionarily stable allocation to male vs. female function? We distinguish the following scenarios. (1) An ‘autogamous’ species, in which selfing occurs within the flower prior to opening. The pollen used in selfing is a constant fraction of all pollen grains produced. (2) A species with ‘abiotic pollination’, in which selfing occurs when pollen dispersed in one flower lands on the stigma of a nearby flower on the same plant (geitonogamy). The selfing rate increases with male allocation but a higher selfing rate does not mean a reduced export of pollen. (3) An ‘animal-pollinated’ species with geitonogamous selfing. Here the selfing rate also increases with male allocation, but pollen export to other plants in the population is a decelerating function of the number of simultaneously open flowers. In all three models selfing selects for increased female allocation. For model 3 this contradicts the general opinion that geitonogamous selfing does not affect evolutionarily stable allocations. In all models, the parent benefits more from a female-biased allocation than any other individual in the population. In addition, in models 2 and 3, greater male allocation results in more local mate competition. In model 3 and in model 2 with low levels of inbreeding depression, hermaphroditism is evolutionarily stable. In model 2 with high inbreeding depression, the population converges to a fitness minimum for the relative allocation to male function. In this case the fitness set is bowed inwards, corresponding with accelerating fitness gain curves. If the selfing rate increases with plant size, this is a sufficient condition for size-dependent sex allocation (more allocation towards seeds in large plants) to evolve. We discuss our results in relation to size-dependent sex allocation in plants and in relation to the evolution of dioecy.     

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.     

Ovule discounting in an outcrossing, cryptically dioecious tree

Ovule discounting denotes the reduction in the number of ovules available for cross-fertilization due to the interference of inferior pollen. Traditionally, ovule discounting has been discussed solely from the perspective of compromised outcrossing opportunities as a result of selfing, but the principle is more general. Here, we extend its applicability beyond the simple contrast between selfing and outcrossing by showing that, in the cryptically dioecious tree species Fraxinus ornus, ovule discounting through frequent outcrossing with inferior fathers also constitutes a substantial cost of mating. In F. ornus, hermaphrodites produce pollen capable of siring offspring, but these offspring are less viable than those sired by males and are inferred to produce few, if any, surviving progeny. In this paper, we used microsatellite markers to analyze the mating system and paternity in a wild population of F. ornus. We found that the effective number of sires per mother was low (N(ep) = 2.93 to 4.95), and that paternity was correlated among progeny sampled from the same mother, but not among progeny sampled from neighboring mothers. Despite the existence of a local spatial genetic structure (up to 30 m), we found no evidence of biparental inbreeding. There was negligible selfing by hermaphrodites, but they sired approximately one fourth of the seeds produced by other hermaphrodites. Given that these progeny are not inferred to reach reproductive maturity, this constitutes a substantial cost of ovule discounting in the broad sense. We discuss the possible reasons for why hermaphrodites invest resources into inferior pollen.     

The influence of demography and local mating environment on sex ratios in a wind‐pollinated dioecious plant

Negative frequency‐dependent selection should result in equal sex ratios in large populations of dioecious flowering plants, but deviations from equality are commonly reported. A variety of ecological and genetic factors can explain biased sex ratios, although the mechanisms involved are not well understood. Most dioecious species are long‐lived and/or clonal complicating efforts to identify stages during the life cycle when biases develop. We investigated the demographic correlates of sex‐ratio variation in two chromosome races of Rumex hastatulus, an annual, wind‐pollinated colonizer of open habitats from the southern USA. We examined sex ratios in 46 populations and evaluated the hypothesis that the proximity of males in the local mating environment, through its influence on gametophytic selection, is the primary cause of female‐biased sex ratios. Female‐biased sex ratios characterized most populations of R.  hastatulus (mean sex ratio = 0.62), with significant female bias in 89% of populations. Large, high‐density populations had the highest proportion of females, whereas smaller, low‐density populations had sex ratios closer to equality. Progeny sex ratios were more female biased when males were in closer proximity to females, a result consistent with the gametophytic selection hypothesis. Our results suggest that interactions between demographic and genetic factors are probably the main cause of female‐biased sex ratios in R. hastatulus. The annual life cycle of this species may limit the scope for selection against males and may account for the weaker degree of bias in comparison with perennial Rumex species.     

Invasion of unisexuals in hermaphrodite populations of animal-pollinated plants: effects of pollination ecology and floral size-number trade-offs

Abstract.— The most common sexual system in animal-pollinated plants is hermaphroditism, while some species are dioecious or gynodioecious and a very few are androdioecious. In this paper, I attempt to explain this pattern by extending previous models for the evolution of sexual systems to incorporate two main features: (1) a portion of investment in pollinator attraction contributes to only female or male function, because one sexual function of a flower is saturated with pollinator visitation earlier than the other sexual function; and (2) there are trade-offs between the size and number of flowers. The analysis was conducted to determine the conditions when females and males can increase in frequency in a hermaphroditic population, assuming either concave or convex pollinator gain curves (relation between investment to attractive structures of a flower and frequency of pollinator visits to the flower). The results suggest that both of the main factors play important roles in the evolution of plant sexual systems: uneven contribution of pollinator-attractive structures and nonlinear trade-offs between flower size and number can destabilize hermaph-roditism. When a convex pollinator gain curve was assumed, the effect of nonlinear trade-offs can produce accelerating compensation from the elimination of one sexual function, allowing males to increase for large regions of parameter space, where females could not increase. The last prediction obviously conflicts with the observed rarity of androdioecy in nature, indicating the necessity of exploring pollinator gain curves in more detail.     

Phenotypic selection on flowering phenology and size in two dioecious plant species with different pollen vectors

Dioecious plants may be pollinated biotically by animals or abiotically via wind or water currents. It has been hypothesized that these two types of pollen vectors might impose contrasting selective pressures on plant flowering phenology. In the present study we describe the flowering phenology of two sympatric dioecious species with contrasting pollination modes: Mercurialis perennis (wind‐pollinated) and Tamus communis (insect‐pollinated). We estimated selection differentials and gradients for flowering time and flowering synchrony. As flowering time might depend on the accumulation of enough internal resources, we also estimated direct and indirect selection on plant size. Both species have male‐biased sexual ratios, and males are bigger and produce larger flower displays than females, but only in T. communis do males bloom earlier and for longer than females. Selection gradients suggest that selection tends to favor early‐flowering females of T. communis. There is no evidence of direct current selection on the flowering phenology of M. perennis. Intersexual differences in phenology fit with sex allocation and sexual selection theories. As we hypothesized, phenology of the animal‐pollinated species is under stronger selection than that of the wind‐pollinated species and we discuss the potential role of pollen vectors in shaping the flowering phenologies of the study species.     

Pollination intensity influences sex ratios in dioecious Rumex nivalis, a wind-pollinated plant

Determining the mechanisms governing sex-ratio variation in dioecious organisms represents a central problem in evolutionary biology. It has been proposed that in plants with sex chromosomes competition between pollen tubes of female- versus male-determining microgametophytes (certation) causes female-biased primary sex ratios. Experimental support for this hypothesis is limited and recent workers have cast doubt on whether pollen-tube competition can modify sex ratios in dioecious plants. Here we investigate the influence of variation in pollination intensity on sex ratios in Rumex nivalis, a wind-pollinated alpine herb with strongly female-biased sex ratios. In a garden experiment, we experimentally manipulated pollination intensity using three concentric rings of female recipient plants at different distances from a central group of male pollen donors. This design enabled us to test the hypothesis that increasing pollen load size, by intensifying gametophyte competition, promotes female-biased sex ratios in R. nivalis. We detected a significant decline in pollen load at successive distance classes with concomitant reductions in seed set. Sex ratios of progeny were always female biased, but plants at the closest distance to male donors exhibited significantly greater female bias than more distant plants. The amount of female bias was positively correlated with the seed set of inflorescences. Hand pollination of stigmas resulted in approximately 100-fold higher stigmatic pollen loads than wind-pollinated stigmas and produced exceptionally female-biased progenies (female frequency = 0.96). Our results are the first to demonstrate a functional relation between stigmatic pollen capture, seed set, and sex ratio and suggest that certation can contribute towards female-biased sex ratios in dioecious plants.     

Simulated herbivory enhances leaky sex expression in the dioecious herb Mercurialis annua

Background and AimsPlant reproductive traits are widely understood to be responsive to the selective pressures exerted by pollinators, but there is also increasing evidence for an important role for antagonists such as herbivores in shaping these traits. Many dioecious species show leaky sex expression, with males and females occasionally producing flowers of the opposite sex. Here, we asked to what extent leakiness in sex expression in Mercurialis annua (Euphorbiaceae) might also be plastically responsive to simulated herbivory. This is important because enhanced leakiness in dioecious populations could lead to a shift in both the mating system and in the conditions for transitions between combined and separate sexes.MethodsWe examined the effect of simulated herbivory on the sexual expression of males and females of M. annua in two experiments in which different levels of simulated herbivory led to enhanced leakiness in both sexes.Key ResultsWe showed that leaky sex expression in both males and females of the wind-pollinated dioecious herb M. annua is enhanced in response to simulated herbivory, increasing the probability for and the degree of leakiness in both sexes. We also found that leakiness was greater in larger females but not in larger males.ConclusionsWe discuss hypotheses for a possible functional link between herbivory and leaky sex expression, and consider what simulated herbivory-induced leakiness might imply for the evolutionary ecology of plant reproductive systems, especially the breakdown of dioecy and the evolution of hermaphroditism.     

Widespread functional android in Mercurialis annua L. (Euphorbiaceae)

The widespread coexistence of male and monoecious (cosexual) plants in Spanish, Portuguese and Moroccan populations of Mercurialis annua , an annual wind-pollinated ruderal, represents an important case of functional androdioecy, a rare breeding system in plants and animals. In M. annua , both males and cosexes disperse fully competent pollen. Quantitative gender varies discontinuously between males and cosexes, with males producing a mean of 6.09 times as much pollen as cosexes. It appears that gender is determined by a simple developmental switch, with male and cosexual inflorescences differing markedly in morphology: staminate flowers are borne on erect peduncles in males and in tight spiral clusters around a subsessile pistillate flower in cosexes. Males do not differ from cosexes in their biomass, but they are significantly taller, principally as a result of their greater internode lengths. The cosexual inflorescence is strongly protogynous so that outcrossing is favoured in dense stands, but seed-set is assured in cosexes isolated from prospective mates because of their ability to self-fertilize. Males typically occur at frequencies of less than about 30% in androdioecious populations, in accordance with theoretical predictions for functional androdioecy. In the genus Mercurialis , dioecy is the ancestral condition and monoecy and androdioecy, which occur in polyploid populations of M. annua , are derived. I argue here that androdioecy is most likely to evolve in plants (1) from dioecy, (2) in wind-pollinated species, and (3) in species with a colonizing habit. These predictions are also consistent with the limited published data available for other species.     

Condition dependence of male and female reproductive success: insights from a simultaneous hermaphrodite

Sexually selected traits are predicted to show condition dependence by capturing the genetic quality of its bearer. In separate‐sexed organisms, this will ultimately translate into condition dependence of reproductive success of the sex that experiences sexual selection, which is typically the male. Such condition dependence of reproductive success is predicted to be higher in males than females under conditions promoting intense sexual selection. For simultaneous hermaphrodites, however, sex allocation theory predicts that individuals in poor condition channel relatively more resources into the male sex function at the expense of the female function. Thus, male reproductive success is expected to be less condition dependent than female reproductive success. We subjected individuals of the simultaneously hermaphroditic snail Physa acuta to two feeding treatments to test for condition dependence of male and female reproductive success under varying levels of male–male competition. Condition dependence was found for female, but not for male, reproductive success, meaning that selection on condition is relatively stronger through the female sex function. This effect was consistent over both male–male competition treatments. Decomposition of male and female reproductive performance revealed that individuals in poor condition copulated more in their male role, indicating an increased male allocation to mate acquisition. These findings suggest that sex‐specific condition dependence of reproductive success is at least partially driven by condition‐dependent sex allocation. We discuss the implications of condition‐dependent sex allocation for the evolution of sexually selected traits in simultaneous hermaphrodites.     

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.

     

Intersexual mimicry and imperfect deceit of a threatened aquatic herb Ottelia acuminata

Mimicry of non-rewarding flowers to rewarding flowers has been accepted as a strategy to improve pollination success in angiosperms. It has been proposed that this mechanism depends on whether potential pollinators can discriminate between the flowers. In this study, the intersexual mimicry and deceit pollination were studied in a threatened dioecious aquatic herb, Ottelia acuminata. Its female flowers resemble male flowers in morphology and odor compounds, to avoid discrimination by pollinators and outcompete male flowers in attracting the pollinators using stronger scents and bigger flowers. However, an obvious visit bias of its pollinator (Apis cerana) to male flowers was detected, suggesting that bees can distinguish the rewarding males from non-rewarding females. Although the deceit was not successful, pollination was not seriously undermined because pollen limitation was found to be low in the sampled natural population. We speculate that, due to “accidental” visits on female flowers and “mistake” pollinations, pollen limitation could be mitigated by a high frequency of pollen donors, and is correlated with the size and sex ratio of a population. Ottelia acuminata is a threatened dioecious aquatic herb. We suggest that developing multi-stakeholder coalitions should be encouraged to save the threatened edible and ornamental plant species in China. We hope this study could provide new insights into understanding of the role of intersexual mimicry in other flowering plants.     

Sex-determining and floating translocation complexes in Viscutn cruciatum Sieber ex Boiss.(Viscaceae) in southern Spain. Some evolutionary and ecological comments

As in other dioecious species of Viscum, males of V.cruciatum always produce large rings of chromosomes during meiosis due to the existence of permanent translocation heterozygosity. Only rings often (and very rarely 12) chromosomes have been found in this species, probably a relative of V.album in the Mediterranean area. Females form bivalentS. Another two floating translocations–not sex-associated and affecting different chromosomes–have been observed in both male and female plants in some populations. These produce meiotic configurations previously unrecorded in Viscum. This floating heterozygosity seems to be correlated with the aptitude of V.cruciatum to develop on different hosts, especially Prunus. All studied populations showed an excess of female plants, perhaps partly due to facultative agamospermy.     

濒危植物夏蜡梅(Sinocalycanthus chinensis)的开花物候与传粉成功

野外定点观测了夏蜡梅在3个群体的开花物候进程,分别计算了结实率和结籽率,并分析了始花日和开花同步性等开花物候指数对其传粉成功的影响,还进一步从个体水平分析了开花时间对传粉成功的影响.结果如下:夏蜡梅的花期在5月上旬到6月下旬,开花物候参数在不同群体间存在较明显的差异,而各群体的开花进程均呈明显的"单峰型",且开花同步性高,为"大量开花模式".群体内各开花物候参数与传粉成功相关性不明显;群体间开花物候参数中的始花日和开花中值日与结实率呈极显著的负相关,终花日与结实率和结籽率分别呈极显著和显著的负相关.个体水平不同开花时间对结实率有着显著影响(F=15.960,df=3,P＜0.01),而对结籽率影响不大(F=2.358,df=3,P=0.073).相同海拔高度小生境不同对夏蜡梅的传粉成功影响不大,但不同海拔高度对夏蜡梅的传粉成功影响显著.作为一种濒危物种,夏蜡梅这种"集中开花模式"可以吸引更多的传粉昆虫,有助于其获得更多的传粉成功,但增加了其花粉在个体内及邻近个体间的传递,不利于花粉在群体间的扩散,而导致了一定程度的自交和近交衰退,这可能是夏蜡梅群体遗传变异性低、遗传分化明显及濒危的一个原因.     

Lifetime patterns of pairing success in male Ortolan Buntings Emberiza hortulana

Analyses of lifetime fitness in birds are typically based on estimates of breeding success, in particular the number of offspring fledged. Small and isolated bird populations often have a male‐skewed adult sex ratio, so that male lifetime productivity depends to a large degree on pairing success, but few studies have focused on patterns of lifetime pairing success. The Norwegian population of Ortolan Buntings Emberiza hortulana is strongly male‐skewed, such that in any year about half of all males are unpaired. Pairing success of first‐year males (16–44%) was significantly lower than for older males (52–89%). Lifetime pairing success was correlated with lifespan and was strongly skewed, with a majority of males being paired only once or never, and only 11% paired three or more times despite a stable lifetime annual survival rate of 63%. Males that were paired in one year were more likely to be paired the next year than males that were unpaired in the previous year. The shortage of females caused even the older males to have a substantial probability of becoming unpaired, and 49% of long‐lived males (known as adults for at least 4 years) were unpaired after years in which they were paired. Pairing success in the Ortolan Bunting therefore follows similar age‐related and lifetime patterns in breeding success documented in other species. However, even the older males ran a high risk of not being paired, contrasting with earlier distinctions between pre‐breeding and breeding lifespans. I discuss the importance of knowledge of pairing success for the management of endangered and declining populations.     

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.