Self-compatibility and floral traits adapted for self-pollination allow homostylous Nymphoides geminata (Menyanthaceae) to persist in marginal habitats

Abstract

Many stylar polymorphic plants are equipped with reciprocal herkogamy and incompatibility systems so that each floral morph within a population can only mate with the compatible reciprocal morph. In the ecological or geographical margins of the range of stylar polymorphic taxa, homostyly may evolve in response to a lack of mate and/or poor pollinator visitation. This prediction is tested here by comparing floral traits, seed production without pollinators (i.e. autonomously), pollinator activity and ploidy level between the homostylous Nymphoides geminata that colonise geographically marginal habitats and its stylar polymorphic and self-incompatible closely related species N. montana that typically occur in natural water bodies in south-eastern Australia. Autonomous self-fertilisation is detected in glasshouse. Homostyly in N. geminata is also associated with floral adaptation to autonomous self-pollen deposition. There is a reduction in size and attractiveness of homostylous floral organs: over 80 % shorter stigma–anther separations and lower pollen:ovule ratio than the polymorphic flowers. The SEM images reveal that homostylous flowers possess similar pollen and stigma papillae morphology to those of short styles and long styles of the polymorphic flowers, respectively. Nymphoides geminata may be an example of recombinant homostyly: combining the pollen compatibility features of one morph with the stylar compatibility features of the reciprocal morph. Results of floral visitor observations and open pollinations indicate the ability of homostylous plants to reproduce successfully despite pollinator scarcity. Somatic chromosome counts indicate a possible lack of relationship between ploidy level and sexual system. Homostylous condition should provide an opportunity for reproductive assurance in environments with unreliable pollinators.