Evolution of Autonomous Selfing in Marginal Habitats: Spatiotemporal Variation in the Floral Traits of the Distylous Primula wannanensis

Outcrossing plant species are more likely to exhibit autonomous selfing in marginal habitats to ensure reproduction under conditions of limited pollinator and/or mate availability. Distyly is a classical paradigm that promotes outcrossing; however, little is known about the variation in floral traits associated with distylous syndrome in marginal populations. In this study, we compared the variation in floral traits including stigma and anther height, corolla tube length, herkogamy, and corolla diameter between the central and peripheral populations of the distylous Primula wannanensis, and assessed the variation of floral traits at early and late florescence stages for each population. To evaluate the potential consequences of the variation in floral traits on the mating system, we investigated seed set in each population under both open-pollinated and pollinator-excluded conditions. The flower size of both short- and long-styled morphs was significantly reduced in late-opening flowers compared with early opening flowers in both central and peripheral populations. Sex-organ reciprocity was perfect in early opening flowers; however, it was largely weakened in the late-opening flowers of peripheral populations compared with central populations. Of these flowers, disproportionate change in stigma height (elongated in S-morph and shortened in L-morph) was the main cause of reduced herkogamy, and seed set was fairly high under pollinator-excluded condition. Our results provide empirical support for the hypothesis on the evolution of delayed autonomous selfing in marginal populations of distylous species. Unsatisfactory pollinator service is likely to have promoted reproductive assurance of distylous plants with largely reduced herkogamy mimicking “homostyles.”

Cryptic individual-level variation in propensity for selfing in a hermaphroditic snail

For mating-system evolution, individual-level variation is as important as variation among populations. In self-compatible hermaphrodites, individuals may vary in their lifetime propensity for selfing, which consists of a fundamental, likely genetic and an environmental component. According to the reproductive assurance hypothesis explaining partial selfing, a key environmental factor is mate availability, which fluctuates with population density. We quantified individual variation in selfing propensity in a hermaphroditic snail by manipulating mate availability in the laboratory, recording mating behaviour, estimating selfing rates from progeny arrays, and measuring female lifetime fitness. Our results revealed four classes of individuals with different selfing propensities: pure outcrossers, pure selfers, and two types of plastic individuals. These classes only became apparent in the laboratory; the field population is outcrossing. All classes were present both under low and increased mate availability; this large among-individual variation in selfing propensities meant that effects of the pairing treatment on the frequency and extent of selfing were non-significant despite large effect sizes and sufficient statistical power. We believe that selfing propensities may have a genetic component and when selected on cause mean selfing rates to evolve. We propose that heritable variation in selfing propensities offers a reconciliation between the reproductive assurance hypothesis and its weak empirical support: distributions of selfing propensities vary temporally and spatially, thus obscuring the relationship between population density and realised selfing rates.

What ecological factors favor parthenogenesis over sexual reproduction? A study on the facultatively parthenogenetic mayfly Alainites muticus in natural populations.

Different reproductive modes are characterized by costs and benefits which often depend on ecological contexts. Benefits of sex are expected to increase under complex biotic interactions, whereas parthenogenesis can be beneficial for reproductive assurance when females are mate limited. Here, we study how different ecological contexts influence the frequency of sex or parthenogenesis in the facultatively parthenogenetic mayfly Alainites muticus. We first verified that high parthenogenetic frequencies translate into female-biased population sex ratios. We then measured the population sex ratio (a proxy for parthenogenetic capacities), density of individuals (mate limitation) and community diversity (biotic interaction complexity) for 159 A. muticus populations, and used structural equation modeling to investigate their direct and indirect influences on sex ratios. We found no effect of community diversity or altitude on sex ratios. Furthermore, even when females can reproduce parthenogenetically, they generally reproduce sexually, indicating that the benefits of sex exceed its costs in most situations. Sex ratios become female-biased in low population densities, as expected if mate limitation selects for parthenogenesis. Mate limitation might be widespread in mayflies because of their very short adult lifespan and limited dispersal, which can generate strong selection for reproductive assurance and may provide a steppingstone towards obligate parthenogenesis.

Pollinator-Mediated Selection on Floral Traits in the Arctic Plant Parrya nudicaulis (Brassicaceae)

The evolution of floral traits is largely attributed to pollinator-mediated selection; however, the importance of pollinators as selective agents in pollen-limited environments is poorly resolved. In pollen-limited arctic and subarctic regions, selection is expected to either favor floral traits that increase pollinator attraction or promote reproductive assurance through selfing. We quantified phenotypic selection on floral traits in two arctic and two subarctic populations of Parrya nudicaulis. Additionally, we measured selection in plants in both open-pollination and pollen-augmentation treatments to estimate selection imposed by pollinators in one population. Seed production was found to be limited by pollen availability and strong directional selection on flower number was observed. We did not detect consistently greater magnitudes of selection on floral traits in the arctic relative to the subarctic populations. Directional selection for more pigmented flowers in one arctic population was observed however. In some populations, selection on flower color was found to interact with other traits. We did not detect consistently stronger selection gradients across all traits for plants exposed to pollinator selection relative to those in the pollen-augmentation treatment; however directional selection tended to be higher for some floral traits in open-pollinated plants.

Cypripedium wardii (Orchidaceae) employs pseudopollen with both reward and deception to attract both flis and bees as pollinators

Flowering plants always attract animals providing rewards or deceptive signals to gain reproductive success. However, there is no well-documented reporting about a pollination mechanism with both rewards and deceptive signals by a same object. We found Cypripedium wardii flowers seem to attract visitors by the white pseudopollen-like trichomes on labella in our preliminary field observation. To explore the pollination mechanism of Cypripedium wardii, especially, the ecological function of the pseudopollen-like trichomes, we conducted field observations, analyses of the traits of visitors and flowers, and breeding system experiments. The white trichomes composed by multicellular moniliform hairs on the floral labella played a crucial role to attract pollinators, causing a high natural fruit set ratio in C. wardii. We established the direct connection of the white trichomes and real pollen. We propose that flowers of C. wardii provide pseudopollen to attract suitable bees and hoverflies as pollinators. And our evidence indicate that the pseudopollen owns both deceptive and rewarding ecological functions. Our study provide a clear pollination mechanism with both rewards and deceptive signals by a same object in angiosperm for the first time. However, an inbreeding depression seem to be caused by this strategy. And we speculated that the pollen mimicry strategy with both rewarding and deceptive functions in C. wardii may be an adaptation to the habitat fragmentation of this species to gain a reproductive assurance. Keywords: bee pollinators, Cypripedium, deception, hoverfly pollinators, inbreeding depression, orchid, pseudopollen, reward

Selection for two different mating systems in Aquilegia canadensis

Because plants are sessile, they depend on biotic and/or abiotic vectors to transfer pollen from the male pollen-producing anthers to the female pollen-receiving stigmas. As a result, plant mating systems evolve through selection on the floral traits that influence how much pollen is transferred from anthers to stigmas within flowers (self-pollination) vs. between flowers on different individuals (outcrossing). Thus, mating systems are influenced by the traits that dictate the relative abundance of self-versus outcrossed pollen on stigmas. Spatial separation between anthers and stigmas within flowers (herkogamy) is expected to regulate self-pollination yet there are few estimates of how natural selectin acts on this trait. Aquilegia canadensis (columbine, Ranunculaceae) is a short-lived herbaceous plant of rocky outcrops throughout eastern North America that makes seed through both self-fertilization which is influenced by herkogamy, and outcrossing, which is likely influenced by the plant’s floral display size (flower number and size). Selfing provides reproductive assurance in natural populations of columbine, whereas outcrossing appears to produce much fitter offspring, and there is a trade-off between thes two components of the mating system. We, therefore, predicted correlational selection between herkogamy and display size: selection would favour reduced herkogamy among individuals with small floral displays (to enhance reproductive assurance) and increased herkogamy among individuals with large floral displays (to reduce selfing when outcrossing is likely). We tested this prediction by using multivariate linear regression to estimate phenotypic selection through seeds/fruit and seeds/plant on floral traits and plant size for 1015 plants from nine populations of A. canadensis at the Queen’s University Biological Station. Although we detected positive direction selection on display size mostly through flower number, we did not detect selection on herkogamy or correlational selection between herkogamy and display size. As expected, large size is universally favoured yet selection of floral morphology is weak.