Evol Ecol Res 1: 769-784 (1999) Full PDF if your library subscribes.
Why are equally sized gametes so rare? The instability of isogamy and the cost of anisogamy
and Peter A. Abrams2
1Ocean Research Institute, University of Tokyo, Minamidai 1-15-1, Nakano-ku, Tokyo 164-8639, Japan and 2Department of Zoology, University of Maryland, College Park, MD 20742, USA
Author to whom all correspondence should be addressed.
The aim of this study was to determine the circumstances in which equally sized gametes (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; (2) whether gamete size does or does not affect gamete success; 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 stable under a wide range of conditions, particularly when mutations of large effect are deleterious. However, when size is linked to mating type, isogamy can only be stable when there are significant direct effects of size on gamete survival and mating success; even then, it may only be locally stable. When isogamy is stable, it results in a lower rate of increase than in asexual forms and, in some cases, can be associated with a lower rate of increase than anisogamous forms of the same species. Thus, the cost of anisogamy is generally less than two-fold. Different explanations for the rarity of isogamy are compared.
Keywords: anisogamy, convergence stability, cost of sex, dimorphism, evolutionary stability, isogamy, mathematical model.
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