Gamete Motion, Search, and the Evolution of Anisogamy, Oogamy, and Chemotaxis

Abstract

A new model is proposed for the evolution of anisogamy from isogamy based on considerations of gamete motion and zygote fitness, without regard to pre-existing mating types. The conditions were examined under which a large population of gametangia producing equal-sized gametes can be invaded successfully by a single gametangium producing gametes of a different size. Assuming an inverse relationship between size and velocity for gametes of a single species within a population, an expression for gametic encounter probabilities is developed. Analytical solutions for this problem are provided in 2 and 3 dimensions. When coupled with considerations of zygote fitness, analysis indicates that the invasion will be successful if the invading gametangium produces gametes of a size different from the population being invaded; this result is confirmed by numerical simulations. However, regions of local stability for isogamy at small gamete sizes are indicated by both analytical results and numerical simulations. A complex adaptive topography is postulated for the evolution of anisogamy with a fitness saddle separating the low adaptive peak of isogamy and the higher adaptive peak for anisogamy. The model also allows to make rough estimates of the length of fertilization window for gametes given empirical data concerning their sizes and swimming speeds. The gametes of isogamous taxa will generally have a longer period of fertilization competency than gametes of anisogamous taxa. Using considerations of gamete motion, decreasing benefit of flagella to female gametes as the female gamete size increases is predicted, thus providing impetus for the evolution of oogamy in taxa with large female gametes. Effective target size for macrogametes can be greatly increased through the release of chemotactic substances. Using data from the fungal genus Allomyces, a chemotactic search pattern was analyzed, based on only 2 microgametic swimming behaviors: a swimming glide and a jerk. A sophisticated search pattern emerges from these 2 behaviors. A numerical simulation of this search pattern was developed and its effectiveness was demonstrated.