Post-pollination mechanisms and the maintenance of outcrossing in selfcompatible, tristylous, decodon verticillatus (Lythraceae)

Abstract

The mating systems of self-compatible plants may be vulnerable to ecological factors that influence pollen transfer. Post-pollination mechanisms may, however, buffer the mating system against ecological variation. We combined an examination of mating-system variation with experimental studies of pollen competition in a clonal, self-compatible, tristylous plant, Decodon verticillatus (Lythraceae), to assess proximate mechanisms influencing the mating system in natural populations. Outcrossing rates (t) were estimated for 10 populations in a variety of habitats and exhibiting a wide range of size, density and patchiness. We also investigated whether self-compatibility results in departures from disassortative mating, by estimating morph-specific rates of outcrossing and disassortative mating (d). All population-level estimates of t indicated significant self-fertilization (average t= 0.72) and weak disassortative mating (average d≈0.30). There were small but significant differences in outcrossing rates among style morphs, with the mid-styled morph outcrossing at a lower rate (0.62) than the short-styled morph (0.81) and the long-styled morph intermediate (0.71). The expectation of wide variation in t among populations was not supported. Population- level estimates were distributed over a narrow range (0.63-0.86) with few significant differences. A possible mechanism underlying this pattern was revealed by mixed-donor pollinations. Outcross pollen experienced a substantial siring advantage over self pollen. Outcrossing in D. verticillatus may be maintained by both floral polymorphism and post-pollination mechanisms. © 1994 The Genetical Society of Great Britain.