The origin and evolution of gamete dimorphism and the male-female phenomenon

Abstract

The classical theory for the origin of anisogamy is that the greatest number of successful fusions occurs when the gametic material available for the population is divided with a high degree of anisogamy. This assumes that a fixed amount of reserve material is necessary for development of the zygote and that only disassortative fusions occur (i.e. between small and large gametes). Assuming (as in previous literature) that a given gametic mass can be produced in unit time, then individual variations in gamete size may arise either from differences in the production time, or in the number of cell divisions at the time of production. Where zygote fitness is in some way related to zygote volume x , relative reproductive rates can be calculated for a range of variants with different gamete productivities (and therefore different gamete sizes). This model yields either', drive for small-producing (where the advantage of high productivity exceeds that of increased provisioning for the zygote) or drive for large-producing (in the reverse case). However in certain conditions (over part of the range of x where zygote fitness is proportional to volume x ) a marked disruptive effect can be generated in which the two extremes, (large and small gamete production) are favoured. Reasons are given why selection should always lead to the establishment of a stable dimorphism in multicellular organisms. When the model is modified to include inheritance of gamete size by simple mendelian dominance, it is shown that as the initial range of variants is increased, the range of x (where fitness is proportional to volumes) which generates stable dimorphism also increases. As high anisogamy is approached, the disadvantageous dominant homozygote is lost leaving two sexes (sperm producers and ovum producers) in a stable 1 : 1 ratio. Stages in the evolution of dissortative fusions are outlined. Though males with sperm which fused only with ova would be favoured throughout, females with assortatively-fusing ova may have been favoured initially. Because of a faster rate of adaptation in sperm than ova, or because of the instability of an isogametic population with assortativelyfusing ova, females face an evolutionary impasse in which the only stable solution is total committment to disassortative fusions. Males are dependent on females and propagate at their expense, rather as in a parasite-host relationship.