Evolution of gamete motility differences II. Interaction with the evolution of anisogamy

Abstract

In this paper the population genetic analysis of the evolution of a gamete motility dimorphism is carried further. In contrast to the preceding paper ( Hoekstra, Janz & Schilstra, 1984 ) the existence of two mating types (based on pheromonal gamete attraction) is assumed. This broadens the conditions for a stable gamete motility dimorphism, provided there is initially some linkage between the loci for gamete motility and pheromone production/response. The effect of anisogamy on gamete motility is added to the assumptions of Parker, Baker & Smith (1972) for the evolution of anisogamy; this appeared to broaden the conditions for stable anisogamy only when the loci for gamete size and for mating type are closely linked. However, anisogamy can also evolve solely as a consequence of its effect on gamete motility, provided the size difference between the gamete types is sufficiently large. This alternative explanation of the evolution of anisogamy is corroborated by the fact that pheromonal gamete attraction mechanisms are generally absent in isogamous species but present in anisogamous species.