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.”

Allee Effects Determine Pollinator Services at Local and Geographic Scales

Climatic factors have attracted much attention in the study of species’ distributions, while little is known about the role of biotic interactions. Here, we tested for variation in pollinator service across the distribution of a plant species, and evaluated the driving mechanisms. We monitored insect pollinators using time-lapse cameras in populations of North American Arabidopsis lyrata from the southern to the northern range limit. We spotted 67 pollinating insect taxa, indicating that this plant-pollinator network is a generalist system. Pollinator service increased with latitude. Higher pollinator visitation was correlated with the richness of other flowering plants and with plant census size, which was largest in northern populations. Furthermore, pollinator service reached a maximum at intermediate local flower density. Synthesis: This study indicates that pollination service underlies Allee effects on a local and species’ range scale, and that plant populations at range limits receive only marginal pollination service if they are small.

High fruit sets in a rewardless orchid: a case study of obligate agamospermy in Habenaria

Low fruit set and pollination limitation are common characteristics of non-autogamous orchids, especially in rewardless species. The flowers of many Habenaria species are often characterised by long spurs and are mostly pollinated by long-tongued hawkmoths or butterflies. Unlike the flowers of other Habenaria species, the flowers of Habenaria malintana (Blanco) Merr. have very short spurs with no nectar or scent; however, this species is able to maintain high fecundity in south-west China. Breeding system experiments suggested that H. malintana is an obligate agamospermous orchid. Seed set did not need to be triggered by pollen grain deposition on stigmas, and ~100% fruit set was found in different populations and years. In pollen germination experiments, hand-deposited pollen failed to germinate on stigmas. The flowers of H. malintana failed to attract any pollinators, as we did not observe any floral visitors, and no pollinia removal or deposition occurred in both 2013 and 2014 at two study sites. These results strongly suggested that H. malintana has completely abandoned sexual reproduction and has adopted obligate agamospermy to achieve high reproductive output. We suggest that this strategy may have evolved to provide reproductive assurance and reduce the cost of flowers in response to unreliable pollinator service.

The evolution of plant reproductive systems: how often are transitions irreversible?

Flowering plants are characterized by striking variation in reproductive systems, and the evolutionary lability of their sexual traits is often considered a major driver of lineage diversification. But, evolutionary transitions in reproductive form and function are never entirely unconstrained and many changes exhibit strong directionality. Here, I consider why this occurs by examining transitions in pollination, mating and sexual systems, some of which have been considered irreversible. Among pollination systems, shifts from bee to hummingbird pollination are rarely reversible, whereas transitions from animal to wind pollination are occasionally reversed. Specialized pollination systems can become destabilized through a loss of pollinator service resulting in a return to generalized pollination, or more commonly a reliance on self-pollination. Homomorphic and heteromorphic self-incompatibility systems have multiple origins but breakdown to self-compatibility occurs much more frequently with little evidence for subsequent gains, at least over short time-spans. Similarly, numerous examples of the shift from outcrossing to predominant self-fertilization are known, but cases of reversal are very limited supporting the view that autogamy usually represents an evolutionary dead-end. The evolution of dioecy from hermaphroditism has also been considered irreversible, although recent evidence indicates that the occurrence of sex inconstancy and hybridization can lead to the origin of derived sexual systems from dioecy. The directionality of many transitions clearly refutes the notion of unconstrained reproductive flexibility, but novel adaptive solutions generally do not retrace earlier patterns of trait evolution.