Population Density as Related to Sex and to Evolution in Cladocera

Abstract

Sex ratios in Cladocera are controlled largely by population density. Under ordinary favorable conditions the population is entirely [female] reproduction is by diploid parthenogenesis. [male][male] (also diploid) occur ordinarily only under conditions of crowding[long dash]dense population[long dash]as a result of accumulated by-products of metabolism which presumably lower the rate of metabolism of the ovarian (partheno-genetic) egg during the critical period for sex determination, rate of metabolism being the mechanism which controls sex determination. Sexual egg production is caused by food scarcity, ordinarily a result of overpopulation. [male] production in nature briefly antedates sexual egg production so that mature [male][male] and sexual eggs (haploid and requiring fertilization) occur simultaneously. Cladocerans referred to live mostly in temporary surface water ponds. Fertilized "winter" eggs are essential to carry the race over periods of drought and complete freezing of habitats. Spontaneous aggregations sometimes occur. These produce local crowding in an otherwise sparse Daphnia population with a like result[long dash][male] and sexual egg production. In the evolution of Cladocera, races have appeared which are less and less dependent upon the [male] sex. The climax of this evolutionary tendency is a type which is entirely maleless yet when food becomes sparse produces the winter egg (dauerei) which remains diploid and develops partheno-genetically. It is surmised that in view of (1) the obvious reproductive advantages of an all-[female] partheno-genetic race and (2) the lack of a handicap from the genetic standpoint, inasmuch as mutations occur in parthenogenesis, further evolution of thelytokous races may readily occur.