Breeding System Evolution in Oxalis alpina: Asymmetrical Expression of Tristylous Incompatibility

Abstract

Premise of research. Tristylous populations of Oxalis alpina have three floral morphs (short-, mid-, and long-styled), and populations appear to be at different stages in the evolution of distyly via loss of the mid-styled morph. In tristylous populations transitioning to distyly, asymmetrical incompatibility reactions could affect style-morph frequencies and slow the evolution of distyly. We hypothesized that greater success of pollen from long-styled relative to short-styled morphs leads to an excess of mid-styled progeny from mid-styled morphs and favors retention of tristyly despite other forms of selection acting against the mid-styled morph. Methodology. We quantified morph ratios of progeny from naturally pollinated plants. We also compared pollen tube and seed production of mid-styled plants following single-source pollinations using pollen from long- versus short-styled pollen parents, and we investigated the proportion of short-styled progeny following pollinations of mid-styled plants with mixtures of pollen from short- and long-styled plants. Pivotal results. Three lines of evidence generally support the hypothesis that reduced performance of short-styled pollen donors may favor retention of mid-styled plants. (1) Naturally pollinated mid-styled plants exhibited reduced numbers of short-styled progeny for two years in one of two tristylous populations with modified tristylous incompatibility. (2) In populations transitioning toward distyly, mid-styled plants pollinated by short-styled plants had fewer pollen tubes and lower seed production than crosses using long-styled plants as pollen parents. (3) Hand-pollinations of mid-styled plants using mixtures of pollen from short- and long-styled plants resulted in reductions in the proportion of short-styled progeny, consistent with the differences in pollen tube numbers produced by the paternal morphs. Conclusions. Asymmetry in paternity among different pollen donors via pollen tube performance influences the evolution of the breeding system of O. alpina, by affecting the relative proportions of different floral morphs and potentially counteracting selection through inbreeding depression and differential fitness of alleles controlling floral morphs.