Male contributions to female-male mutualism overcome the twofold cost of sex.

The increasing rate of anisogamy (fusion between eggs and sperm) against asexual thelytoky is necessarily halved under a 1:1 sex ratio because males lay no eggs. The present dominance of anisogamy despite this twofold cost of male production is the greatest enigma in evolutionary biology. Various benefits of sex are insufficient to overcome this cost. How can anisogamy be maintained? Here, we propose hypotheses that anisogamy evolved as a female-male mutualism in which females produce large eggs to ensure zygote development and males produce many sperm to ensure fertilisation. In the real world with finite resources, thelytoky cannot express its twofold increasing potential, and various male functions reduce the thelytokys performance. Thus, the twofold cost is an artefact of the infinite resource assumption. Simple mathematical models and simulations showed that when males and thelytokous females are equally competitive and acquire the same amount of resources but males can develop with fewer resources than females, anisogamy outcompetes thelytoky. When males need more growth costs than females, as in sexually selected species, the excess mating activity of "harmful" males reduces the fitness of thelytokous females. The significance of sexes, a question since Aristotles time, is explained by male contribution to sustaining mutualism.