SEX RATIO OF RUMEX HASTATULUS: THE EFFECT OF ENVIRONMENTAL FACTORS AND CERTATION.

Abstract

Mechanisms of sex determination in dioecious seed plants are such that female and male progeny in a population are expected in approximately equal proportions (Lloyd, 1973). However, deviations from equality are known for many species (Lewis, 1942; Godley, 1964, 1976; Lloyd, 1973, 1974). Several prezygotic and postzygotic mechanisms may produce unequal sex ratios. Meiotic drive (Sandler and Novitski, 1957; Hamilton, 1967) and differential mortality or competitive ability of gametophytes carrying predominantly maleor female-determining genes are mechanisms whereby sex ratios are biased at the time of fertilization. Differential growth or mortality of sporophytes and sexual phenotype plasticity are mechanisms that can influence sex ratio after fertilization. Correns (1928) showed that the sex ratios of Silene alba and Rumex acetosa progenies could be manipulated by controlling the amount of pollen experienced by female partners. This phenomenon, termed certation, was explained by Correns as resulting from the differential growth of pollen tubes carrying femaleand male-determining gametes. Under low pollen densities, the sex ratio was near equality or slightly male-biased while at high pollen densities, the sex ratio was greatly female-biased. Evidently, pollen tubes carrying female-determining gametes reach the egg sooner than pollen