Variation in floral morphology and plant reproductive success in four Ipomoea species (Convolvulaceae) with contrasting breeding systems

• This study tested the hypothesis that self‐compatibility would be associated with floral traits that facilitate autonomous self‐pollination to ensure reproduction under low pollinator visitation. In a comparison of two pairs of Ipomoea species with contrasting breeding systems, we predicted that self‐compatible (SC) species would have smaller, less variable flowers, reduced herkogamy, lower pollinator visitation and higher reproductive success than their self‐incompatible (SI) congeners.
• We studied sympatric species pairs, I. hederacea (SC)– I. mitchellae (SI) and I. purpurea (SC)–I. indica (SI), in Mexico, over two years. We quantified variation in floral traits and nectar production, documented pollinator visitation, and determined natural fruit and seed set. Hand‐pollination and bagging experiments were conducted to determine potential for autonomous self‐pollination and apomixis.
• Self‐compatible Ipomoea species had smaller flowers and lower nectar production than SI species; however, floral variation and integration did not vary according to breeding system. Bees were primary pollinators of all species, but visitation rates were seven times lower in SC than SI species. SC species had a high capacity for autonomous self‐pollination due to reduced herkogamy at the highest anther levels. Self‐compatible species had two to six times higher fruit set than SI species.
• Results generally support the hypothesis that self‐compatibility and autonomous self‐pollination ensure reproduction under low pollinator visitation. However, high variation in morphological traits of SC Ipomoea species suggests they maintain variation through outcrossing. Furthermore, reduced herkogamy was associated with high potential for autonomous self‐pollination, providing a reproductive advantage that possibly underlies transitions to self‐compatibility in Ipomoea.

     

TRANSMISSION ADVANTAGE FAVORS SELFING ALLELE IN EXPERIMENTAL POPULATIONS OF SELF‐INCOMPATIBLE WITHERINGIA SOLANACEA (SOLANACEAE)

The evolution of self‐fertilization is one of the most commonly traversed transitions in flowering plants, with profound implications for population genetic structure and evolutionary potential. We investigated factors influencing this transition using Witheringia solanacea, a predominantly self‐incompatible (SI) species within which self‐compatible (SC) genotypes have been identified. We showed that self‐compatibility in this species segregates with variation at the S‐locus as inherited by plants in F1 and F2 generations. To examine reproductive assurance and the transmission advantage of selfing, we placed SC and SI genotypes in genetically replicated gardens and monitored male and female reproductive success, as well as selfing rates of SC plants. Self‐compatibility did not lead to increased fruit or seed set, even under conditions of pollinator scarcity, and the realized selfing rate of SC plants was less than 10%. SC plants had higher fruit abortion rates, consistent with previous evidence showing strong inbreeding depression at the embryonic stage. Although the selfing allele did not provide reproductive assurance under observed conditions, it also did not cause pollen discounting, so the transmission advantage of selfing should promote its spread. Given observed numbers of S‐alleles and selfing rates, self‐compatibility should spread even under conditions of exceedingly high initial inbreeding depression.     

Breeding systems of Bromeliaceae species: evolution of selfing in the context of sympatric occurrence

Co-occurring congener plant species in the highly diverse Atlantic forests of southeastern Brazil may act as natural laboratories for evaluating evolution of reproductive shifts. We assessed the breeding systems in a sympatric assemblage of bromeliad species and compiled literature available for the family to compare our experiments with available information. We performed controlled experiments of autonomous selfing, self- and cross-pollination in 40 species of 11 genera of two subfamilies that, in general, overlap their blooming period and share the same pollination vectors. We also tested for differences between self-compatible (SC) and self-incompatible (SI) species with regard to ecological factors such as abundance, co-flowering and co-occurrence. Most species experimentally tested and surveyed in the literature (75%) were SC. Species from the subfamily Tillandsioideae were predominantly SC, while Bromelioideae showed greater variation in breeding systems. About 43% of the species studied set fruits spontaneously in the absence of pollinator vectors. We found that SC species were the more abundant, were more frequently associated with other relatives and overlapped their blooming period with other species more than SI species. Thus, our results suggest that self-compatibility is common for Bromeliaceae and do not support the traditional hypothesis of reproductive assurance used to explain the evolution of selfing. We discuss self-compatibility as a reproductive isolating mechanism in the presence of pollen flow among sympatric congener species.     

POTENTIAL FOR THE LOSS OF SELF-INCOMPATIBILITY IN POLLEN-LIMITED POPULATIONS OF MAYAPPLE (PODOPHYLLUM PELTATUM)

Self-compatibility (SC) appears to be a derived trait in many unrelated taxa. A major selective force that could lead to the evolution of SC from self-incompatibility (SI) is inadequate pollination. Here we show that seed set is strongly pollenlimited in populations of mayapple, Podophyllum peltatum (Berberidaceae). Hand-pollination led to a 26-fold increase in seed set as compared to natural levels in 1989, and to a fivefold increase in 1990. The species is SI, but in a survey of 49 colonies in Ohio we found three that had more than 50% fruit set from self pollen (as compared to about 90% fruit set from hand-outcrossed flowers). However, when hand-selfing yielded fruits, the number of seeds per fruit was only about 10% of the seed set from hand-outcrossing. We discuss the relative fecundity of putative SC and SI genotypes in terms of conditions needed for the spread of SC in mayapple populations (little or no morphological change would be needed to ensure within-flower selfing). Seed set from putative SC genotypes appeared to be low enough to prevent them from replacing SI genotypes, even in the face of infrequent pollinator visits.     

Evolution of autonomous selfing accompanies increased specialization in the pollination system of Schizanthus (Solanaceae)

The co-occurrence of elaborate flowers visited by specific groups of pollinators and capacity for autonomous selfing in the same plant species has puzzled evolutionary biologists since the time of Charles Darwin. To examine whether autonomous selfing and floral specialization evolved in association, we quantified the autofertility level (AFI) in nine Schizanthus species characterized by a wide range of pollination specialization, revealing AFI values of 0.02 to complete selfing. An independent contrasts analysis conducted on AFIs and number of functional pollinator groups showed that autonomous selfing evolved from an ancestral outcrossing system as plants became increasingly specialized (r = -0.82). To assess whether autonomous selfing together with specialization acts as a reproductive assurance mechanism, we estimated spatial and interannual variation in fruit set due to pollinator failure in two closely related high Andean Schizanthus species differing in their specialization levels. Variation in pollinator failure rate was more pronounced and autonomous selfing increased fruit production over biotically assisted pollination in the more specialized species. Our study suggests that specialized pollination deems species more vulnerable to pollinator fluctuation thus promoting the evolution of delayed autonomous selfing.     

Transitions between self-compatibility and self-incompatibility and the evolution of reproductive isolation in the large and diverse tropical genus Dendrobium (Orchidaceae)

Background and Aims The evolution of interspecific reproductive barriers is crucial to understanding species evolution. This study examines the contribution of transitions between self-compatibility (SC) and self-incompatibility (SI) and genetic divergence in the evolution of reproductive barriers in Dendrobium, one of the largest orchid genera. Specifically, it investigates the evolution of pre- and postzygotic isolation and the effects of transitions between compatibility states on interspecific reproductive isolation within the genus.Methods The role of SC and SI changes in reproductive compatibility among species was examined using fruit set and seed viability data available in the literature from 86 species and ∼2500 hand pollinations. The evolution of SC and SI in Dendrobium species was investigated within a phylogenetic framework using internal transcribed spacer sequences available in GenBank.Key Results Based on data from crossing experiments, estimations of genetic distance and the results of a literature survey, it was found that changes in SC and SI significantly influenced the compatibility between species in interspecific crosses. The number of fruits produced was significantly higher in crosses in which self-incompatible species acted as pollen donor for self-compatible species, following the SI × SC rule. Maximum likelihood and Bayesian tests did not reject transitions from SI to SC and from SC to SI across the Dendrobium phylogeny. In addition, postzygotic isolation (embryo mortality) was found to evolve gradually with genetic divergence, in agreement with previous results observed for other plant species, including orchids.Conclusions Transitions between SC and SI and the gradual accumulation of genetic incompatibilities affecting postzygotic isolation are important mechanisms preventing gene flow among Dendrobium species, and may constitute important evolutionary processes contributing to the high levels of species diversity in this tropical orchid group.     

Inbreeding depression is high in a self‐incompatible perennial herb population but absent in a self‐compatible population showing mixed mating

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self‐incompatibility (SI) and outcrossing toward self‐compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self‐fertilization. In contrast, populations that show intermediate selfing rates (a mixed‐mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self‐fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed‐mating populations that are similar to those expected for outcrossing populations. Using hand‐pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between‐population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.     

The loss of self‐incompatibility in a range expansion

It is commonly observed that plant species' range margins are enriched for increased selfing rates and, in otherwise self‐incompatible species, for self‐compatibility (SC). This has often been attributed to a response to selection under mate and/or pollinator limitation. However, range expansion can also cause reduced inbreeding depression, and this could facilitate the evolution of selfing in the absence of mate or pollinator limitation. Here, we explore this idea using spatially explicit individual‐based simulations of a range expansion, in which inbreeding depression, variation in self‐incompatibility (SI), and mate availability evolve. Under a wide range of conditions, the simulated range expansion brought about the evolution of selfing after the loss of SI in range‐marginal populations. Under conditions of high recombination between the self‐incompatibility locus (S‐locus) and viability loci, SC remained marginal in the expanded metapopulation and could not invade the range core, which remained self‐incompatible. In contrast, under low recombination and migration rates, SC was frequently able to displace SI in the range core by maintaining its association with a genomic background with purged genetic load. We conclude that the evolution of inbreeding depression during a range expansion promotes the evolution of SC at range margins, especially under high rates of recombination.?     

Density-dependent mating and reproductive assurance in the temperate forest herb Paris quadrifolia (Trilliaceae)

In animal-pollinated plants, autonomous selfing provides reproductive assurance under conditions of infrequent pollinator visits or a lack of mates, but few data are available for wind-pollinated species or species with combined insect and wind-pollination, for which it is often assumed that pollen availability does not limit reproduction. In this study, the capacity of autonomous selfing was investigated in the temperate forest herb Paris quadrifolia, and an emasculation experiment was performed under natural field conditions to investigate the contribution of autonomous selfing to total seed set across a continuous gradient of densities of flowering conspecifics. In the absence of wind or pollinators, autonomous selfing was observed through anthers approaching stigmas at the end of flowering and the capacity for autonomous pollination was about 0.34. Under natural conditions, considerable outcross pollination was observed, but the proportion of ovules successfully fertilized significantly decreased with decreasing density of conspecifics when flowers were emasculated, but not when flowers were left intact. These results indicate that autonomous selfing resulted in reproductive assurance (RA = 0.16) and thus support the hypothesis that autonomous selfing can also provide reproductive assurance in wind-pollinated species.     

Rapid loss of self‐incompatibility in experimental populations of the perennial outcrossing plant Linaria cavanillesii

Transitions from self‐incompatibility to self‐compatibility in angiosperms may be frequently driven by selection for reproductive assurance when mates or pollinators are rare, and are often succeeded by loss of inbreeding depression by purging. Here, we use experimental evolution to investigate the spread of self‐compatibility from one such population of the perennial plant Linaria cavanillesii into self‐incompatible (SI) populations that still have high inbreeding depression. We introduced self‐compatible (SC) individuals at different frequencies into replicate experimental populations of L. cavanillesii that varied in access to pollinators. Our experiment revealed a rapid shift to self‐compatibility in all replicates, driven by both greater seed set and greater outcross siring success of SC individuals. We discuss our results in the light of computer simulations that confirm the tendency of self‐compatibility to spread into SI populations under the observed conditions. Our study illustrates the ease with which self‐compatibility can spread among populations, a requisite for species‐wide transitions from self‐incompatibility to self‐compatibility.     

Mating in the pseudogamic apomictic Anemopaegma acutifolium DC: another case of pseudo‐self‐compatibility in Bignoniaceae?

Self‐compatibility in apomictic pseudogamic species is considered fundamental to assure reproduction by seeds in extreme situations, making apomictic species more advantageous than sexual ones in these scenarios. Anemopaegma acutifolium is a polyploidy, apomictic sporophytic species with no endosperm development in ovules of unpollinated pistils, which indicates obligate pseudogamy. Thus, the aim of the present work is to study the breeding system and post‐pollination events to test if there is similar pseudogamous development irrespective of pollination treatment. We analysed fruit and seed set obtained in controlled experimental pollinations, as well as embryo number per seed, and the progress of ovule penetration, fertilisation and early endosperm development between self‐ and cross‐pollinated pistils. We found that the species is self‐fertile and that spontaneous selfing fruit set is also possible, although emasculated flowers never form fruits. Selfed pistils were as efficient as crossed ones for all parameters analysed, except for a delay in endosperm development observed in the former that may be an effect of the late‐acting self‐incompatibility. Therefore, the avoidance of selfed pistil abortion seems to be promoted by the presence of adventitious embryos and a normal endosperm. We conclude that A. acutifolium shows apomixis‐related pseudo‐self‐compatibility, as in other self‐fertile apomictic species of Bignoniaceae, which confer reproductive assurance and increases fruit‐set and persistence ability in fast‐changing tropical habitats.     

Costs of selfing prevent the spread of a self‐compatibility mutation that causes reproductive assurance

In flowering plants, shifts from outcrossing to partial or complete self‐fertilization have occurred independently thousands of times, yet the underlying adaptive processes are difficult to discern. Selfing's ability to provide reproductive assurance when pollination is uncertain is an oft‐cited ecological explanation for its evolution, but this benefit may be outweighed by costs diminishing its selective advantage over outcrossing. We directly studied the fitness effects of a self‐compatibility mutation that was backcrossed into a self‐incompatible (SI) population of Leavenworthia alabamica, illuminating the direction and magnitude of selection on the mating‐system modifier. In array experiments conducted in two years, self‐compatible (SC) plants produced 17–26% more seed, but this advantage was counteracted by extensive seed discounting—the replacement of high‐quality outcrossed seeds by selfed seeds. Using a simple model and simulations, we demonstrate that SC mutations with these attributes rarely spread to high frequency in natural populations, unless inbreeding depression falls below a threshold value (0.57 ≤ δ_threshold ≤ 0.70) in SI populations. A combination of heavy seed discounting and inbreeding depression likely explains why outcrossing adaptations such as self‐incompatibility are maintained generally, despite persistent input of selfing mutations, and frequent limits on outcross seed production in nature.     

The divergence history of the perennial plant Linaria cavanillesii confirms a recent loss of self‐incompatibility

Many angiosperms prevent inbreeding through a self‐incompatibility (SI) system, but the loss of SI has been frequent in their evolutionary history. The loss of SI may often lead to an increase in the selfing rate, with the purging of inbreeding depression and the ultimate evolution of a selfing syndrome, where plants have smaller flowers with reduced pollen and nectar production. In this study, we used approximate Bayesian computation (ABC) to estimate the timing of divergence between populations of the plant Linaria cavanillesii that differ in SI status and in which SI is associated with low inbreeding depression but not with a transition to full selfing or a selfing syndrome. Our analysis suggests that the mixed‐mating self‐compatible (SC) population may have begun to diverge from the SI populations around 2810 generation ago, a period perhaps too short for the evolution of a selfing syndrome. We conjecture that the SC population of L. cavanillesii is at an intermediate stage of transition between outcrossing and selfing.     

Breeding systems in Balsaminaceae in relation to pollen/ovule ratio,pollination syndromes,life history and climate zone

• Pollen/ovule (P/O) ratios are often used as proxy for breeding systems. Here, we investigate the relations between breeding systems and P/O ratios, pollination syndromes, life history and climate zone in Balsaminaceae.
• We conducted controlled breeding system experiments (autonomous and active self‐pollination and outcrossing tests) for 65 Balsaminaceae species, analysed pollen grain and ovule numbers and evaluated the results in combination with data on pollination syndrome, life history and climate zone on a phylogenetic basis.
• Based on fruit set, we assigned three breeding systems: autogamy, self‐compatibility and self‐incompatibility. Self‐pollination led to lower fruit set than outcrossing. We neither found significant P/O differences between breeding systems nor between pollination syndromes. However, the numbers of pollen grains and ovules per flower were significantly lower in autogamous species, but pollen grain and ovule numbers did not differ between most pollination syndromes. Finally, we found no relation between breeding system and climate zone, but a relation between climate zone and life history.
• In Balsaminaceae reproductive traits can change under resource or pollinator limitation, leading to the evolution of autogamy, but are evolutionary rather constant and not under strong selection pressure by pollinator guild and geographic range changes. Colonisation of temperate regions, however, is correlated with transitions towards annual life history. Pollen/ovule‐ratios, commonly accepted as good indicators of breeding system, have a low predictive value in Balsaminaceae. In the absence of experimental data on breeding system, additional floral traits (overall pollen grain and ovule number, traits of floral morphology) may be used as proxies.

     

Rapid floral senescence following male function and breeding systems of some tropical orchids

No comparative study of floral senescence following male function among a range of tropical orchid genera has previously been undertaken. The timing and pattern of floral senescence and occurrence of fruit formation were studied following self-, geitonogamous and cross-pollination in 14 epiphytic and two terrestrial orchid species to determine their breeding system and assess the occurrence of floral abscission following pollinaria removal. Both pollination and pollinaria removal caused rapid floral senescence, and the pattern and timing of the floral changes were the same in all treatments. Six Dendrobium species and Pelatantheria insectifera were self-incompatible (SI) and eight other species, including one terrestrial species, were self-compatible (SC). Capsules produced from outcrossing in four SC species, Phalaenopsis cornu-cervi, Eria pubescens, Cleisostoma appendiculatum and Arundina graminifolia, were larger and heavier than those produced after selfing. Reductions in flower life span following pollinaria removal were positively correlated with flower size and longevity of unpollinated flowers but not with position in the inflorescence or nature of the breeding system. Rapid flower senescence following pollinaria removal reported here suggests that it may be widespread in tropical species. The significant association of the response with size of flowers and inflorescences among the species studied suggests that the cost of flower maintenance outweighs the benefit of remaining open for female function after pollinaria have been removed. Both SC and SI species were found among tropical orchids, but variation in capsule size following self- and cross-pollination indicates that there may be a reduction in seed production following selfing, even in SC species, and that fruit formation alone should not be taken as reliable evidence of full self-compatibility.     

Does selfing provide reproductive assurance in the perennial herb Bulbine vagans (Asphodelaceae)?

Selection for selfing to provide reproductive assurance depends on the balance between increased reproductive output when pollinators or potential mates are scarce and the extent that inbreeding depression erodes such fertility gains. We use glasshouse and field experiments to examine the benefits of autonomous and facilitated selfing in Bulbine vagans . Autonomous selfing was delayed until after opportunities for outcrossing and reproductive output was 0.67 relative to manual selfing and open pollination. Values less than one probably reflected insufficient autonomous deposition of self pollen. In the field, reproductive output of emasculated flowers was 0.50 relative to intact flowers that could both outcross and self, indicating that outcross pollen was limited and that selfing boosted reproductive output. Because all pollen was removed from anthers before intact flowers closed, facilitated selfing rather than autonomous selfing occurred. In the glasshouse, inbreeding depression was 0.45, but under natural conditions would probably exceed 0.5. Values greater than 0.5 negate the automatic gene transmission advantage afforded by selfing and increasingly erode the benefits of reproductive assurance. We conclude that in B. vagans delayed and facilitated selfing can confer reproductive assurance, providing the latter does not usurp ovules that could be outcrossed.     

Post‐pollination process in a partially self‐compatible distylous plant,Primula sieboldii (Primulaceae)

Pollen germination and pollen‐tube growth under natural conditions were observed in a population of a distylous species, Primula sieboldii, in which partial self‐compatibility has been demonstrated in some long‐styled genets. We observed post‐pollination processes microscopically in styles collected after self‐morph and inter‐morph hand pollination (with standardized pollen load on the stigmas) in four genets each from the following three ‘genet types’: self‐incompatible long‐styled (SI), partially self‐compatible long‐styled (SC) and self‐incompatible short‐styled morph genets. Irrespective of the genet type, pollen germination began within 24 h after pollination and tubes of pollen reached to the style base with 48–96 h after inter‐morph pollination. Although pollen tubes germinated after self‐pollination in the SC genets, the number of germinated pollen tubes was significantly lower than in the case of inter‐morph pollination. Few pollen tubes germinated after self‐pollination of the SI or short‐styled genets. In SC genets, the rate of pollen‐tube growth did not differ between self‐morph and inter‐morph pollination (～1.9 mm/day). Therefore, differences in self‐compatibility between SC and SI genets in P. sieboldii are likely to be attributable to differential pollen germination rates rather than to differential pollen‐tube growth rates.     

Autonomous selfing provides reproductive assurance in an alpine ginger Roscoea schneideriana (Zingiberaceae)

Background and Aims

Reproductive assurance, the ability to produce seeds when pollinators or mates are scarce, is thought to be the major advantage of selfing in flowering plants. However, few studies have performed a direct cost–benefit analysis of the selective advantage of selfing, particularly given a long-term perspective among populations or across several flowering seasons within population. This study examined the fertility consequences of autonomous selfing in Roscoea schneideriana (Zingiberaceae), a small perennial Himalayan ginger typically found in habitats at around 3000 m a.s.l.

Methods

The floral biology of R. schneideriana was studied in natural populations; the capacity for autonomous selfing was estimated using pollinator exclusion experiments; the timing of selfing was quantified by anther removal at different times during flowering; whether autonomous selfing increases seed production was tested by emasculating flowers; and the magnitude of inbreeding depression was estimated by comparing relative performance of progeny from self- and cross-pollinations. Pollinator observations were also conducted in the natural populations.

Key Results

The hooked stigmas of most flowers curl towards the anther and can contact pollen grains at an early stage of anthesis. Flowers with potential pollinators excluded set of as many seeds per fruit as hand-selfed and opened flowers. Autonomous selfing mostly occurs within 2 d of anthesis and can increase seed production by an average of 84 % in four populations during the flowering seasons of 2005–2007. Visits by effective pollinators were extremely rare. The cumulative inbreeding depression of R. schneideriana was 0·226.

Conclusions

Autonomous selfing in R. schneideriana is achieved by stigmas curling towards the anthers early in flowering. It is suggested that under the poor pollination conditions, autonomous selfing has been selected for in this alpine ginger because it provides substantial reproductive assurance with very low costs.Key words: Zingiberaceae, Roscoea, autonomous self-pollination, reproductive assurance, inbreeding depression, pollinator failure, Himalayan species     

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.     

The evolution of self-compatible and self-incompatible populations in a hermaphroditic perennial, <Emphasis Type="Italic">Trillium camschatcense</Emphasis> (Melanthiaceae)

The evolution of selfing from outcrossing ancestors is known to have occurred repeatedly in angiosperms. Theoretical studies have argued that the transition from outcrossing to selfing is accomplished more easily than the reverse case, and phylogenetic analyses involving self-compatible (SC) and self-incompatible (SI) species has basically supported this assumption. The evolutionary direction of self-compatibility and self-incompatibility was examined in Trillium camschatcense, which contains geographically widespread SC populations, and restricted SI populations. Ecological surveys have revealed that the SC populations were suitable for outcrossing, and selfing in these populations did not confer any fitness advantage. Since reproductive fitness indicates the possibility of an evolutionary shift from self-compatibility to self-incompatibility, the phylogenetic relationships of SI and SC populations of T. camschatcense were investigated based on cpDNA variations and nuclear DNA microsatellite polymorphisms. Although phylogenetic analyses did not provide credible evidence to determine evolutionary direction, the SI populations turned out to be monophyletic with extremely low genetic differentiation. Based on these results, we proposed two possible scenarios for the evolutionary backgrounds of SI and SC populations in T. camschatcense. The plausibility of each scenario was evaluated based on the reproductive and geographical features of the mating systems. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.