Mating system evolution under strong clonality: towards self-compatibility or self-incompatibility?

Abstract

Clonal growth is a common feature among plant species with about 70% of all species displaying some form of clonal growth (Klimes et al. 1997). Because clonal growth strongly affects the spatial and temporal distribution of flowers, it can be expected that extensive clonal growth influences patterns of pollen dispersal and outcross mating opportunities (Handel 1985), and therefore may affect mating system evolution in plant species (Charpentier 2002; Barrett 2003). However, the direction in which clonal growth and architecture affect mating system evolution remains to a large extent puzzling. The multiplication and simultaneous blooming of reproductive shoots within a single genet can result in substantial geitonogamous pollination (i.e. pollen transfer from one flower to another within the same genet), which in turn leads to both female and male mating costs because of inbreeding depression and pollen discounting (i.e. the loss in outcross siring success as a result of self-pollination), respectively (Barrett 2003). Especially in species where ramets are connected by short internodes and genets are spatially clumped, pollen flow between distinct genotypes may be strongly limited, resulting in high frequencies of geitonogamy (Eckert 2000; Charpentier 2002). While pollen discounting affects both self-compatible and self-incompatible species, in self-compatible species geitonogamous pollen transfer between ramets within the same genet results in strong female mating costs due to selfing and associated inbreeding depression (Goodwillie et al. 2005). Although Eckert (2000), studying the self-compatible tristylous wetland plant Decodon verticillatus, has suggested that the advantages of clonal growth may be larger than the disadvantages imposed by increased geitonogamy and inbreeding depression, there may exist an upper limit to clone size above which geitonogamy becomes a too high fitness cost (Barrett 2003). Under these circumstances, it has been hypothesized that in the aquatic plant Sagittaria latifolia, monoecious populations have developed into dioecious