International Botanical Congress 20 July Viena (2005-A-4877-IBC)

Why are equally-sized gametes so rare? The instability of isogamy and the cost of anisogamy

H. Matsuda¹, P. A. Abrams²;
¹Yokohama National University, Yokohama, Japan, ²University of Toronto, Toronto, ON, Canada.

The aim of this study was to determine the circumstances in which isogamy can be maintained in a population that has already evolved mating types. We analysed the evolutionary dynamics of gamete sizes when there are two mating types. The models and conclusions differ depending on: (1) whether size-determining loci are linked to loci-determining mating types or not; (2) whether gamete size affects gamete success or not; and (3) whether viable mutations with large effects on size are possible or not. In all cases, the reproductive success of a zygote depends on the sum of the sizes of the two uniting gametes, and the number of gametes produced is inversely proportional to gamete size. When size is not closely linked to mating type, it is possible for isogamy to be likely stable, particularly when mutations of large effect are deleterious. However, when size is linked to mating type, isogamy can only be stable when there are signiffcant direct effects of size on gamete survival and mating success. The cost of anisogamy is generally less than two-fold.

Matsuda H, Abrams PA . (1999) Why are equally-sized gametes so rare? The instability of isogamy and the cost of anisogamy. Evolutionary Ecology Research1:769-784.