Poor male function favours the coexistence of sexual and asexual relatives

Classical models of the evolution of sex typically assume that an asexual l ineage, once derived, is reproductively separate from the sexual lineage fr om which it was derived. However, many asexuals, including hermaphrodite pl ants, produce male gametes capable of fertilising the eggs of co-existing s exuals, giving rise to sexual and asexual progeny. This male function of as exuals may be poor, and it has been proposed that this could favour sexuali ty and adversely affect the successful establishment of asexual lineages. W e show that things are more complicated than this; the effect is frequency dependent and poor male function may sometimes favour asexuality. In a spat ially distributed population of flowering plants, it can prevent the succes sful invasion of either reproductive mode by the other via long-range dispe rsal. Consequently invasions must be driven by short-range dispersal, anti are therefore extremely slow. Thus poor male function favours long-term co- existence of sexuals and asexuals. When coupled with the superior ability o f asexuals to colonise virgin territory after an Ice Age, it may explain cu rrent ecological distribution patterns.