The impact of individual inaccuracy of reciprocal herkogamy on legitimate pollen deposition and seed set in a distylous self‐incompatible herb

Abstract

Abstract Heterostyly is a floral polymorphism that has been observed in at least 28 different plant families across the globe. Heterostyly is characterized by reciprocal arrangement of the anthers and stigmas and the presence of a heteromorphic self‐incompatibility system, which together have been hypothesized to promote cross pollination and to avoid interference of self‐pollen and the deleterious effects of inbreeding. Recent analyses have shown that the reciprocal positioning of the sexual organs in heterostylous plant species is not always perfect. Inaccuracies in the positioning of styles and anthers within populations are expected to affect both male and female fitness of plants by disturbing pollen uptake and stigmatic deposition by insects. However, at present, little is known about how inaccuracies in the reciprocal positioning of the sexual organs affect stigmatic pollen deposition and subsequent seed set in heterostylous plant species. In this study, we quantified the level of reciprocity in a grassland and forest population of the distylous herb Primula veris and tested the hypothesis that inaccuracies in the positioning of the sexual organs were related to stigmatic deposition of pollen by insects and ultimately seed set in the field. Total inaccuracy in reciprocal positioning of the sexual organs was about four times larger in the forest population than in the grassland population (20.67 and 5.47 respectively). Individual inaccuracies in stigma positioning varied between 0.02 and 67.93 (L‐morph) and 0.03 and 45.34 (S‐morph) in the forest population, and between 0.00 and 13.19 (L‐morph) and 0.00 and 26.15 (S‐morph) in the grassland population. Legitimate pollen deposition decreased significantly with increasing inaccuracy, both in the grassland and forest population, but effects were most pronounced in the forest population. In both populations, short‐styled individuals suffered larger reductions in legitimate pollen deposition with increasing inaccuracies than long‐styled individuals. Seed set was significantly and positively affected by the number of legitimate pollen and decreased significantly with increasing inaccuracy, especially in the forest population where inaccuracies were most pronounced. Synthesis. Overall, these results indicate that deviations from perfect reciprocity significantly affect the reproductive success of heterostylous plants by diminishing legitimate stigmatic pollen deposition and concurrently reducing seed set. As a result of their lower fitness, maladaptive genotypes should be selected against in natural populations, leading to the maintenance of floral polymorphisms in general and the heterostylous syndrome in particular.