Life history, floral development, and mating system in Clarkia xantiana (Onagraceae): do floral and whole-plant rates of development evolve independently?

Autogamously self-fertilizing taxa have evolved from outcrossing progenitors at least 12 times in the annual wildflower genus, Clarkia (Onagraceae). In C. xantiana, individuals of the selfing subspecies (ssp. parviflora) flower at an earlier age, produce successive flowers more rapidly, and produce flowers that complete their development more rapidly than their outcrossing counterparts (ssp. xantiana). Two hypotheses have been proposed to explain the joint evolution of these whole-plant and individual floral traits. The accelerated life cycle hypothesis proposes that selection favoring a short life cycle in environments with short growing seasons (such as those typically occupied by parviflora) has independently favored genotypes with early reproduction, synchronous flower production, and rapidly developing, self-fertilizing flowers. The correlated response to selection hypothesis similarly proposes that selection in environments with short growing seasons favors early reproduction, but that rapid floral development and increased selfing evolve as correlated responses to selection due to genetic linkage (or pleiotropy) affecting both whole-plant and floral development. We conducted a greenhouse experiment using maternal families from two field populations of each subspecies to examine covariation between floral and whole-plant traits within and among populations to seek support for either of these hypotheses. Our results are consistent with the accelerated life cycle hypothesis but not with the correlated response to selection hypothesis.     

Evolution of the self-pollinating flower in Clarkia xantiana (Onagraceae). I. Size and development of floral organs

Clarkia xantiana has two subspecies that differ in breeding system: ssp. xantiana, which is outcrossing, and ssp. parviflora, which is self-fertilizing. Outcrossing is the ancestral breeding system for the genus Clarkia. Flowers of ssp. parviflora have characteristics commonly associated with selfing taxa: they are smaller and have little temporal and spatial separation between mature anthers and stigma (dichogamy and herkogamy, respectively). Flower morphology and development were studied in four populations of each subspecies to establish the developmental changes that occurred in the evolution of selfing. In particular, we sought to evaluate the hypothesis that the selfing flower may have arisen as a byproduct of selection for rapid maturation in the arid environment occupied by ssp. parviflora. This hypothesis predicts that development time should be reduced in spp. parviflora relative to ssp. xantiana. We also sought to compare the pattern of covariation of flower morphology and development between subspecies to that within subspecies. Similar within vs. between patterns of covariation could be indicative of developmental or functional constraints on the independent evolution of floral parts. In spite of significant variation among populations within subspecies, the subspecies clearly differ in flower morphology and development. All floral organs, except ovaries, are smaller in ssp. parviflora than in ssp. xantiana. The flower plastochron, the duration of flower development from bud initiation to anthesis, and the duration of protandry are all shorter in ssp. parviflora than in ssp. xantiana. Maximum relative growth rates are higher for all organs in ssp. parviflora than in ssp. xantiana. Thus, progenesis (i.e., via a reduction in development time) is combined with growth acceleration in the evolution of the selfing flower. Since reduced development time and growth acceleration both allow selfing flowers to mature earlier than outcrossing ones, selection for early maturation may have contributed to the evolution of the selfing flower form. The pattern of trait covariation differs within spp. parviflora relative to the patterns within spp. xantiana and between the two subspecies, suggesting that floral parts can and have evolved independently of one another.     

Temporal variation in the pollen:ovule ratios of Clarkia (Onagraceae) taxa with contrasting mating systems: field populations

Among plants, pairs of selfing vs. outcrossing sister taxa provide interesting systems in which to test predictions concerning the magnitude and direction of temporal changes in sex allocation. Although resource availability typically declines towards the end of the growing season for annual taxa, temporal changes in mating opportunities depend on mating system and should change less in selfing taxa. Consequently, given that the pollen:ovule (P:O) ratio of flowers reflects the investment in (and potential fitness pay-off due to) male vs. female function, we predicted that the P:O ratio should also be less variable among and within selfers than in closely related outcrossers. To test these predictions, we measured temporal changes in sex allocation in multiple field populations of two pairs of sister taxa in the annual flowering plant genus Clarkia (Onagraceae). In the outcrossing Clarkia unguiculata and the selfing Clarkia exilis, ovule production declined similarly from early to late buds, whereas pollen production remained constant or increased in the outcrosser but remained constant or decreased in the selfer. Consequently, the P:O ratio increased within unguiculata populations but marginally increased or stayed constant in exilis populations. In all populations of the selfing Clarkia xantiana spp. parviflora and the outcrossing C. x. spp. xantiana, both ovule and pollen production per flower declined over time. The effects of these declines on the P:O ratio, however, differed between subspecies. In each xantiana population, the mean P:O ratio did not differ between early and late flowers, although individuals varied greatly in the direction and magnitude of phenotypic change. By contrast, parviflora populations differed in the mean direction of temporal change in the P:O ratio. We found little evidence to support our initial predictions that the P:O ratio of the selfing taxa will consistently vary less than in outcrossing taxa.     

Ecological context of the evolution of self-pollination in Clarkia xantiana: population size, plant communities, and reproductive assurance

The repeated evolutionary transition from outcrossing to self-pollination in flowering plants has been suggested to occur because selfing provides reproductive assurance. Reports from biogeographical and ecological surveys indicate that selfing taxa are often associated with stressful and ephemeral environments, situations in which plant abundance is low (e.g., Baker's law) and with novel plant communities, however experimental tests of ecological hypotheses are few. In this study, we examined the ecological context of selection on mating system traits (herkogamy and protandry) in a California annual, Clarkia xantiana, where natural selfing populations differ from outcrossing populations in that they are often of small size or low density and occur mainly outside the range of pollinator-sharing congeners. We constructed artificial populations of plants with broad genetic variation in floral traits and manipulated two ecological factors, plant population size, and the presence versus absence of pollinator-sharing congeners, in the center of the geographic range of outcrossing populations. We found evidence for context-dependent selection on herkogamy and protandry via female fitness in which reduced traits, which promote autonomous selfing, were favored in small populations isolated from congeners whereas selection was comparatively weak in large populations or when congeners were present. In small, isolated populations, the fertility of plants with low herkogamy or protandry was elevated by 66% and 58%, respectively, compared to those with high herkogamy or protandry. The presence of pollinator-sharing congeners augmented bee visitation rates to C. xantiana flowers by 47% for all bees and by 93% for pollen specialists. By facilitating pollinator visitation, congeners mitigated selection on mating system traits in small populations, where outcross mating success is often low (the Allee effect). We also found support for the hypothesis that pollinator availability directly influenced variation in the strength of selection on herkogamy among populations. The striking parallels between our experimental results and patterns of variation in ecological factors across the geographic range of outcrossing and selfing populations suggest that reproductive assurance may play a central role in directing mating system evolution in C. xantiana.     

Tempo and mode of mating system evolution between incipient Clarkia species

Mating systems are among the most labile characteristics of flowering plants, with transitions frequently occurring among populations or in association with speciation. The frequency of mating system shifts has made it difficult to reconstruct historical evolutionary dynamics unless transitions have been very recent. Here, we examine molecular and phenotypic variation to determine the polarity, timescale, and causes of a transition between outcrossing and self-fertilization in sister subspecies of Clarkia xantiana. Phylogenetic analyses and coalescent-based estimates of the time to most recent common ancestor indicated that outcrossing is ancestral to selfing and that there has been a single origin of selfing. Estimates of divergence time between outcrossing and selfing subspecies were 10,000 (95% CI [credible interval]: 3169-66,889) and 65,000 years ago (95% CI: 33,035-151,448) based on two different methods, suggesting a recent and rapid evolutionary transition. Population genetic data indicated that the transition to selfing was associated with a 80% reduction in molecular diversity, which is much greater than the 50% reduction expected under a shift from obligate outcrossing to obligate self-fertilization alone. Our data also suggest that this severe loss of diversity was caused by colonization bottlenecks. Together with previous studies, evidence for reproductive assurance in C. xantiana now connects variation in plant-pollinator interactions in the field to phenotypic and molecular evolution.     

Genetic analysis of traits distinguishing outcrossing and self-pollinating forms of currant tomato,Lycopersicon pimpinellifolium (Jusl.) Mill

The evolution of inbreeding is common throughout the angiosperms, although little is known about the developmental and genetic processes involved. Lycopersicon pimpinellifolium (currant tomato) is a self-compatible species with variation in outcrossing rate correlated with floral morphology. Mature flowers from inbreeding and outcrossing populations differ greatly in characters affecting mating behavior (petal, anther, and style lengths); other flower parts (sepals, ovaries) show minimal differences. Analysis of genetic behavior, including quantitative trait locus (QTL) mapping, was performed on representative selfing and outcrossing plants derived from two contrasting natural populations. Six morphological traits were analyzed: flowers per inflorescence; petal, anther, and style lengths; and lengths of the fertile and sterile portions of anthers. All traits were smaller in the selfing parent and had continuous patterns of segregation in the F(2). Phenotypic correlations among traits were all positive, but varied in strength. Quantitative trait locus mapping was done using 48 RFLP markers. Five QTL total were found involving four of the six traits: total anther length, anther sterile length, style length, and flowers per inflorescence. Each of these four traits had a QTL of major (>25%) effect on phenotypic variance.     

Modification of flower architecture during early stages in the evolution of self-fertilization

Background and Aims

The evolution of selfing from outcrossing is characterized by a series of morphological changes to flowers culminating in the selfing syndrome. However, which morphological traits initiate increased self-pollination and which are accumulated after self-fertilization establishes is poorly understood. Because the expression of floral traits may depend on the conditions experienced by an individual during flower development, investigation of changes in mating system should also account for environmental and developmental factors. Here, early stages in the evolution of self-pollination are investigated by comparing floral traits among Brazilian populations of Eichhornia paniculata (Pontederiaceae), an annual aquatic that displays variation in selfing rates associated with the breakdown of tristyly to semi-homostyly.

Methods

Thirty-one Brazilian populations under uniform glasshouse conditions were compared to investigate genetic and environmental influences on flower size and stigma–anther separation (herkogamy), two traits that commonly vary in association with transitions to selfing. Within-plant variation in herkogamy was also examined and plants grown under contrasting environmental conditions were compared to examine to what extent this trait exhibits phenotypic plasticity.

Key Results

In E. paniculata a reduction in herkogamy is the principal modification initiating the evolution of selfing. Significantly, reduced herkogamy was restricted to the mid-styled morph and occurred independently of flower size. Significant genetic variation for herkogamy was detected among populations and families, including genotypes exhibiting developmental instability of stamen position with bimodal distributions of herkogamy values. Cloned genets exposed to contrasting growth conditions demonstrated environmental control of herkogamy and genotypic differences in plasticity of this trait.

Conclusions

The ability to modify herkogamy independently of other floral traits, genetic variation in the environmental sensitivity of herkogamy, and the production of modified and unmodified flowers within some individuals, reveal the potential for dynamic control of the mating system in a species that commonly confronts heterogeneous aquatic environments.Key words: Eichhornia paniculata, expressivity, flower morphology, herkogamy, phenotypic plasticity, pleiotropy, population variation, self-fertilization, stigma–anther separation, outcrossing, tristyly     

Experimental studies of adaptation in Clarkia xantiana. II. Fitness variation across a subspecies border

Because the range boundary is the locale beyond which a taxon fails to persist, it provides a unique opportunity for studying the limits on adaptive evolution. Adaptive constraints on range expansion are perplexing in view of widespread ecotypic differentiation by habitat and region within a species' range (regional adaptation) and rapid evolutionary response to novel environments. In this study of two parapatric subspecies, Clarkia xantiana ssp. xantiana and C. x. ssp. parviflora, we compared the fitness of population transplants within their native region, in a non-native region within the native range, and in the non-native range to assess whether range expansion might be limited by a greater intensity of selection on colonists of a new range versus a new region within the range. The combined range of the two subspecies spans a west-to-east gradient of declining precipitation in the Sierra Nevada of California, with ssp. xantiana in the west being replaced by ssp. parviflora in the east. Both subspecies had significantly higher fitness in the native range (range adaptation), whereas regional adaptation was weak and was found only in the predominantly outcrossing ssp. xantiana but was absent in the inbreeding ssp. parvifilora. Because selection intensity on transplants was much stronger in the non-native range relative to non-native regions, there is a larger adaptive barrier to range versus regional expansion. Three of five sequential fitness components accounted for regional and range adaptation, but only one of them, survivorship from germination to flowering, contributed to both. Flower number contributed to regional adaptation in ssp. xantiana and fruit set (number of fruits per flower) to range adaptation. Differential survivorship of the two taxa or regional populations of ssp. xantiana in non-native environments was attributable, in part, to biotic interactions, including competition, herbivory, and pollination. For example, low fruit set in ssp. xantiana in the east was likely due to the absence of its principal specialist bee pollinators in ssp. parviflora's range. Thus, convergence on self-fertilization may be necessary for ssp. xantiana to invade ssp. parviflora's range, but the evolution of outcrossing would not be required for ssp. parviflora to invade ssp. xantiana's range.     

Comparative analysis of late floral development and mating-system evolution in tribe Collinsieae (Scrophulariaceae s.l.)

Species of Collinsia and Tonella, the two sister genera of self-compatible annuals that constitute tribe Collinsieae, show extensive variation in floral size and morphology and in patterns of stamen and style elongation during the life of the flower (anthesis). We used a nuclear ribosomal ITS phylogeny, independent contrasts, and phylogenetically corrected path analysis to explore the patterns of covariance of the developmental and morphological traits potentially influencing mating system. Large-flowered taxa maintain herkogamy (spatial separation of anthers and stigmas) early in anthesis by differential elongation of staminal filaments, which positions each of the four anthers at the tip of the "keel" upon dehiscence. Small-flowered taxa do not show this pattern of filament elongation. The styles of large-flowered taxa elongate late in the 2-5 d of anthesis, resulting in late anther-stigma contact and delayed self-pollination. Anther-stigma contact and self-pollination occur early in anthesis in small-flowered species/populations. Thus, we found complex covariation of morphological and developmental traits that can be interpreted as the result of multitrait adaptation for early selfing and high levels of autogamy, delayed selfing and higher levels of outcrossing, or intermediate levels of outcrossing. Continuous variation in these traits suggests the operation of continuous variation in selective optima or the combined effects of divergent selection and phylogenetic inertia.     

Evolutionary consequences of ecological factors: pollinator reliability predicts mating‐system traits of a perennial plant

The reproductive‐assurance hypothesis predicts that mating‐system traits will evolve towards increased autonomous self‐pollination in plant populations experiencing unreliable pollinator service. We tested this long‐standing hypothesis by assessing geographic covariation among pollinator reliability, outcrossing rates, heterozygosity and relevant floral traits across populations of Dalechampia scandens in Costa Rica. Mean outcrossing rates ranged from 0.16 to 0.49 across four populations, and covaried with the average rates of pollen arrival on stigmas, a measure of pollinator reliability. Across populations, genetically based differences in herkogamy (anther–stigma distance) were associated with variation in stigmatic pollen loads, outcrossing rates and heterozygosity. These observations are consistent with the hypothesis that, when pollinators are unreliable, floral traits promoting autonomous selfing evolve as a mechanism of reproductive assurance. Extensive covariation between floral traits and mating system among closely related populations further suggests that floral traits influencing mating systems track variation in adaptive optima generated by variation in pollinator reliability.     

Reproductive assurance and the evolutionary ecology of self-pollination in Clarkia xantiana (Onagraceae)

The reproductive assurance hypothesis posits that selection favors self-pollination in flowering plants where mates and/or pollinators are scarce. A corollary is that self-pollinating populations are expected to be superior colonizers of mate- and pollinator-scarce environments. The California annual Clarkia xantiana includes outcrossing populations (ssp. xantiana) and autogamously self-pollinating populations (ssp. parviflora). Outcrossing is ancestral, and the subspecies have parapatric distributions with a narrow contact zone. We tested aspects of the reproductive assurance hypothesis by examining geographic and subspecies variation in the densities of mates and pollinators (native bees) and the density dependence of pollinator visitation and pollen receipt. Plant and flower densities, pollinator density, and pollinator visitation rates were lowest in the region of exclusively self-pollinating populations. Pollinator assemblages there lacked Clarkia-associated pollinator taxa that were common elsewhere. Self-pollinating populations in the contact zone generally had densities and visitation rates intermediate between allopatric self-pollinating populations and outcrossing populations. Visitation rate and pollen receipt increased significantly with plant density. These findings suggest that selection for reproductive assurance influenced the origin of self-pollination and/or that reproductive assurance influenced the geographic distribution of self-pollination. Geographic variation in pollinator assemblages may have generated variation in the value of reproductive assurance.     

Frequency–dependent pollinator foraging in polymorphic Clarkia xantiana ssp. xantiana populations: implications for flower colour evolution and pollinator interactions

Under many circumstances pollinators are expected to practice positive frequency–dependent foraging in colour-polymorphic plant populations. Theory suggests, however, that competition for floral resources might favor negative frequency–dependent foraging by some pollinator species, possibly contributing to the maintenance of flower colour variation by negative frequency–dependent selection. We addressed this idea with pollination studies of the California annual plant Clarkia xantiana ssp. xantiana (Onagraceae), which is polymorphic for the presence of conspicuous petal spots and is pollinated by several specialist bee species. At the level of entire pollinator assemblages, we did not detect significant fixed flower colour preferences or frequency–dependent foraging. Three species of specialist bee pollinators, however, showed contrasting forms of frequency–dependent foraging. The most widespread species, Hesperapis regularis (Melittidae) exhibited positive frequency dependence. Two other common species, Lasiglossum pullilabre (Halictidae) and Ceratina sequoiae (Apidae), preferred to visit whichever morph (unspotted or spotted) was locally in the minority. All three species were found to be effective at transferring C. xantiana pollen; H. regularis appeared most effective. Our findings suggest that a mixture of positive and negative frequency–dependent selection on flower colour occurs in C. xantiana , with the form and intensity of selection varying in space and time with pollinator assemblages. Negative frequency–dependent selection via pollination dynamics may play a larger role in maintaining genetic variation in flower colour than was previously thought. Our results also suggest an unappreciated form of niche partitioning among specialist pollinators. Genetic polymorphism in flower colour may sometimes facilitate pollinator coexistence.     

Variation in the timing of autonomous selfing among populations that differ in flower size, time to reproductive maturity, and climate

? Premise of the study: Early reproductive maturity is common in dry and ephemeral habitats and often associated with smaller flowers with increased potential for within-flower (autonomous) self-pollination. We investigated whether populations from locations that differ in moisture availability, known to vary for whole-plant development rate, also varied in the timing of autonomous selfing. This timing is of interest because the modes of selfing (prior, competing, and delayed) have different fitness consequences. ? Methods: We measured timing of anther dehiscence, stigma receptivity, and herkogamy under pollinator-free conditions for plants from three populations of Collinsia parviflora that differed in annual precipitation, flower size, and time to sexual maturity. Using a manipulative experiment, we determined potential seed production via prior, competing, and delayed autonomous selfing for each population. ? Key results: Stigma receptivity, anther dehiscence, and selfing ability covaried with whole-plant development and climate. Plants from the driest site, which reached sexual maturity earliest, had receptive stigmas and dehiscent anthers in bud. Most seeds were produced via prior selfing. The population from the wettest site with slowest development was not receptive until after flowers opened. Although competing selfing was possible, all selfing was delayed. The intermediate population was between these extremes, with significant contributions from both competing and delayed selfing. ? Conclusions: Our results demonstrate that within-species variation in the timing of selfing occurs and is related to both environmental conditions and whole-plant development rates. We suggest that, if these results can be generalized to other species, mating systems may evolve in response to ongoing climatic change.     

Evolution of mating system and the genetic covariance between male and female investment in Clarkia (onagraceae): selfing opposes the evolution of trade-offs

Sex allocation theory has assumed that hermaphroditic species exhibit strong genetically based trade-offs between investment in male and female function. The potential effects of mating system on the evolution of this genetic covariance, however, have not been explored. We have challenged the assumption of a ubiquitous trade-off between male and female investment by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. In flowering plants, the result of such selection would be similar pollen:ovule (P:O) ratios across selfing genotypes, precluding a negative genetic correlation (r(g)) between pollen and ovule production per flower. Moreover, if selfing genotypes with similar P:O ratios differ in total gametic investment per flower, a positive r(g) between pollen and ovule production would be observed. In outcrossers, by contrast, male- and female-biased flowers and genotypes may have equal fitness and coexist at evolutionary equilibrium. In the absence of strong stabilizing selection on the P:O ratio, selection on this trait will be relaxed, resulting in independence or resource-based trade-offs between male and female investment. To test this prediction, we conducted artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems. The predominantly self-fertilizing species (Clarkia exilis) consistently exhibited a significant positive r(g) between pollen and ovule production while the outcrossing species (C. unguiculata) exhibited either a trade-off or independence between these traits. Clarkia exilis also exhibited much more highly canalized gender expression than C. unguiculata. Selection on pollen and ovule production resulted in little correlated change in the P:O ratio in the selfing exilis, while dramatic changes in the P:O ratio were observed in unguiculata. To test the common prediction that floral attractiveness should be positively genetically correlated with investment in male function, we examined the response of petal area to selection on pollen and ovule production and found that petal area was not consistently genetically correlated with gender expression in either species. Our results suggest that the joint evolutionary trajectory of primary sexual traits in hermaphroditic species will be affected by their mating systems; this should be taken into account in future theoretical and comparative empirical investigations.     

The joint evolution of mating system and pollen performance: Predictions regarding male gametophytic evolution in selfers vs. outcrossers

Studies of sexual selection in plants historically have focused on pollinator attraction, pollen transfer, gametophytic competition, and post-fertilization discrimination by maternal plants. Pollen performance (the speeds of germination and pollen tube growth) in particular is thought to be strongly subject to intrasexual selection, but the effect of mating system on this process has not been rigorously evaluated. Here we propose four predictions derived from the logic that pollen performance should evolve with mating system as an adaptive response to: (1) the competitive environment among pollen genotypes and (2) variation among female genotypes regularly encountered by a given pollen genotype. First, as previously predicted, due to the higher potential for intense selection among diverse pollen genotypes in outcrossing relative to selfing taxa, pollen should evolve to germinate and/or to grow more rapidly in outcrossers than in selfers. Second, due to stronger selection on pollen performance in outcrossing than in selfing taxa, heritable variation in pollen tube growth rate is more likely to be purged in outcrossers. In selfers, by contrast, genetic variation in pollen tube growth rates may readily accumulate because selfing reduces the number of genetically distinct male gametophytes likely to be deposited on any given stigma, thereby relaxing selection on male gametophytic traits. A summary of published studies presented here provides preliminary support for this prediction. Third, due to the high probability that the pollen of outcrossing individuals will be exposed to multiple pistil genotypes, we predict that the pollen of habitually outcrossing taxa will evolve to perform more consistently across female genotypes than the pollen of selfing taxa. Fourth, we predict that epistatic interactions between pollen and pistil genotypes are more likely to evolve in selfers than in outcrossers. We suggest several empirical approaches that may be used to test these predictions.     

Minor quantitative trait loci underlie floral traits associated with mating system divergence in Mimulus

The genetic basis of species differences provides insight into the mode and tempo of phenotypic divergence. We investigate the genetic basis of floral differences between two closely related plant taxa with highly divergent mating systems, Mimulus guttatus (large-flowered outcrosser) and M. nasutus (small-flowered selfer). We had previously constructed a framework genetic linkage map of the hybrid genome containing 174 markers spanning approximately 1800 cM on 14 linkage groups. In this study, we analyze the genetics of 16 floral, reproductive, and vegetative characters measured in a large segregating M. nasutus x M. guttatus F2 population (N = 526) and in replicates of the parental lines and F1 hybrids. Phenotypic analyses reveal strong genetic correlations among floral traits and epistatic breakdown of male and female fertility traits in the F2 hybrids. We use multitrait composite interval mapping to jointly locate and characterize quantitative trait loci (QTLs) underlying interspecific differences in seven floral traits. We identified 24 floral QTLs, most of which affected multiple traits. The large number of QTLs affecting each trait (mean = 13, range = 11-15) indicates a strikingly polygenic basis for floral divergence in this system. In general, QTL effects are small relative to both interspecific differences and environmental variation within genotypes, ruling out QTLs of major effect as contributors to floral divergence between M. guttatus and M. nasutus. QTLs show no pattern of directional dominance. Floral characters associated with pollinator attraction (corolla width) and self-pollen deposition (stigma-anther distance) share several pleiotropic or linked QTLs, but unshared QTLs may have allowed selfing to evolve independently from flower size. We discuss the polygenic nature of divergence between M. nasutus and M. guttatus in light of theoretical work on the evolution of selfing, genetics of adaptation, and maintenance of variation within populations.     

Population-level variation in the expression of heterostyly in three species of Rubiaceae: does reciprocal placement of anthers and stigmas characterize heterostyly?

Heterostyly (i.e., reciprocal placement of anthers and stigmas between two or three floral morphs) is hypothesized to enhance outcrossing and reduce selfing. However, few studies have documented reciprocity among individual plants; instead, mean anther and stigma heights for floral morphs are usually reported, masking interindividual variation. We measured eight floral dimensions for individuals in five populations of three heterostylous Rubiaceae. The three methods used to quantify reciprocity yielded different conclusions regarding the degree to which populations conformed to expectations for heterostylous plants. Only Psychotria poeppigiana had stigma and, to a lesser degree, anther heights in discrete classes. Variation among plants of Bouvardia ternifolia and Psychotria chiapensis yielded a continuum of anther and stigma heights across populations. Comparison of distances between stigma and anthers indicated that only flowers of B. ternifolia had, as expected, a constant value for this distance. Finally, regression relationships between anther and stigma heights and corolla length showed that only in one population each of B. ternifolia and P. poeppigiana, and in P. chiapensis, was distance between anthers and stigmas the same across the range of corolla sizes for both floral morphs. Variation among these species in expression of heterostyly was not clearly linked to phylogenetic relationship or pollinator syndromes. Two approach herkogamous (AH) species were studied for comparison. Flowers of Psychotria brachiata were consistently AH, but flowers of P. pittieri were highly variable. Determining fitness consequences of population-level variation in sexual systems requires studies linking floral morphology to pollinator behavior and pollen transfer.     

高山植物扁蕾的延迟自交机制

扁蕾(Gentianopsis barbata)具有鲜艳的花和显著的腺体,并且花开放的前5 d柱头和花药始终处于不同的位置(雌雄异位),这些花综合征表明该植物应为异花传粉。为检验这一假设,我们对青藏高原植物扁蕾的海北站种群进行了3年的传粉生物学研究实验。与花综合征所表明的繁育系统相反,两年的野外观察发现昆虫的访花频率十分低,不去雄并隔离昆虫处理也能产生大量种子,说明这一种群的繁殖主要是依赖于自花传粉。尽管利用种子结实评价的柱头可授性从花开放4 d后开始下降,但随着花的发育进程,雄蕊的伸长能使得花药与柱头完全接触。实验也证明,柱头可授性和花粉活力都超过5 d,说明花药和柱头的接触能够发生自花授粉。扁蕾的这种自花传粉机制应属于典型的延迟自交类型。自花授粉发生在单花花期快要结束前,自交之前仍然保持异交传粉机制,这种延迟自交避免了自交与异交竞争造成的花粉或者种子折损,并为扁蕾在青藏高原极端环境下由于访花昆虫缺乏造成的异交失败提供了繁殖保障。     

Delayed stigma receptivity in Collinsia heterophylla (Plantaginaceae): genetic variation and adaptive significance in relation to pollen competition, delayed self-pollination, and mating-system evolution

To increase our knowledge about mating-system evolution, we need to understand the relationship between specific floral traits and mating system. Species of Collinsia (Plantaginaceae) vary extensively in mating system; this variation is associated with variation in floral morphology and development and with the timing of self-pollination. Counterintuitively, large-flowered, more outcrossing species tend to have delayed stigma receptivity, reducing the amount of time that the stigma is receptive to cross-pollination before autonomous self-pollination. To understand how the timing of stigma receptivity is related to mating-system evolution, we studied in detail the timing of both stigma receptivity and self-pollination (anther-stigma contact) in two greenhouse-grown populations of large-flowered Collinsia heterophylla. Crosses on emasculated flowers at different stages of floral development always produced seeds, suggesting that cross-fertilization can be effected by pollen arriving prior to physiological receptivity. Phenotypic and genetic variation within populations in the timing of stigma receptivity and anther-stigma contact was substantial, although slightly less for the contact. Despite strong interspecific and interpopulation correlations, we did not find an among-genet phenotypic correlation between the traits. This indicates that each trait may respond independently to selection, and the trait association may be the result of correlational selection.     

The genetics of floral divergence and postzygotic barriers between outcrossing and selfing populations of Arenaria uniflora (Caryophyllaceae)

The genetic architecture of floral traits involved in the evolution of self-pollination provides a window into past processes of mating system divergence. In this study, we use two generations of crosses between highly selfing and predominantly outcrossing populations of Arenaria uniflora (Caryophyllaceae) to determine the minimum number, average dominance relationships, and pleiotropic effects of genetic factors involved in floral divergence. Comparison of the F1 and F2 phenotypic means with the expectations of a completely additive model of gene action revealed a primarily additive genetic basis for floral characters associated with mating system variation. The exception was flower life span, which showed partial dominance of the outcrosser phenology. In contrast to similarly divergent species, the substantial differences in flower size between these A. uniflora populations appear to involve relatively few genes of large effect (minimum number of effective factors = 2.2 +/- 2.8 SE). In addition, correlations among traits in the F2 generation indicate that pleiotropy may be an important feature of the genetic architecture of floral evolution in A. uniflora. The evolution of selfing via major modifiers of floral morphology is consistent with other evidence for ecological selection for preemptive self-pollination in A. uniflora. Analyses of the genetic basis of autonomous selfing were complicated by hybrid breakdown in both F1 and F2 generations. Only F1 hybrids showed reductions in female fertility, but about 30% of F1 and F2 hybrids exhibited partial or complete male sterility. Male sterile flowers were characterized by short stamens, reduced petals, and a lack of protandry, as well as indehiscent anthers. This morphological breakdown mimics environmental disruptions of floral development and may result from novel genic interactions in hybrids.