Shifts in mating systems under contrasting environments

The remarkable diversity of mating systems in flowering plants has fascinated biologists for more than a century [see, for example, Darwin’s monographs on plant sexual systems]. In particular, the evolutionary shift from outcrossing towards selfing has received much theoretical and empirical attention because of its profound biological importance for plant populations. Severe pollen limitation can be particularly important during the early phases of colonization and population establishment and is expected to cause selection for floral traits that facilitate autonomous self-fertilization. This may be particularly true in novel or disturbed environments that lack specialized pollinator communities.

A new report in Annals of Botany investigates floral morphology and mating system functioning in the annual, hoverfly-pollinated Blackstonia perfoliata occurring in two contrasting pollination environments. Two large populations were studied in a pollinator-rich coastal dune area where the species originally occurred and which is typically rich in insect pollinators. These populations were compared with two recently established B. perfoliata populations that were similar in size and density, and occurred in a newly created industrial area that is poor in pollinators and co-flowering plant species. Self-compatible, hoverfly-pollinated B. perfoliata experienced stronger failure of pollinator-mediated seed production in recently established populations. However, total seed production in these populations was not restricted by pollen limitation, and indices of reproductive assurance indicated that autonomous selfing significantly enhanced total seed set in these populations.

Differences in dichogamy and herkogamy contribute to higher selfing in contrasting environments in the annual Blackstonia perfoliata (Gentianaceae). Ann Bot (2013) 111 (4): 651-661. doi: 10.1093/aob/mct031
The establishment of plant populations in novel environments may generate pronounced shifts in floral traits and plant mating systems, particularly when pollinators are scarce. In this study, floral morphology and mating system functioning are compared between recently established and older populations of the annual plant Blackstonia perfoliata that occur in different pollinator environments.
Hand-pollination and emasculation experiments were conducted to assess the extent of pollinator-mediated pollen deposition and pollen limitation, and the contribution of autonomous selfing to total seed production. Detailed measurements of key floral traits were performed to compare the flower morphology and mating system functioning between plants from both pollination environments.
Pollinator-mediated pollen deposition was about twice as low in the recently colonized and pollinator-poor environment compared with the old and pollinator-rich sites, but total seed set was little affected by any type of pollen limitation. The contribution of autonomous selfing to total seed production was higher in the pollinator-poor sites than in the pollinator-rich sites (index of reproductive assurance = 0·56 and 0·17, respectively), and seed production was only poorly affected by selfing, whereas in the pollinator-rich populations selfing reduced total reproductive output by about 40 % compared with outcross pollination. Plants originating from pollinator-poor environments produced smaller flowers that showed significantly lower levels of dichogamy (i.e. protogyny) and herkogamy. These reductions resulted in a 2-fold higher capacity for autonomous selfing under pollinator-free conditions (index of autonomous selfing = 0·81 and 0·41 in plants originating from the pollinator-poor and pollinator-rich environment, respectively).
The results illustrate that plant populations colonizing novel environments can differ markedly in floral morphology and mating system functioning. Due to a temporal shift in the male phase, the breeding system of B. perfoliata shifted from delayed selfing under pollinator-rich conditions towards competing selfing in recently established populations, providing greater reproductive assurance when pollinators and/or reproductive partners are limited.