On the evolution of sex determination and the sex ratio in haplodiploid populations

Abstract

Sex determination by ploidy, either of the entire genome (in haplodiploid populations) or of special chromosomes (XY or X0 systems) is a common occurrence. It is commonly held that observed sex ratios are an evolutionary consequence of the sex-determination system; for example, that the XY system helps produce and/or maintain sex ratios to 1: 1 (Edwards, 196 1; Williams, 1975). It should be noted, however, that factors such as segregation distortion in the sex chromosomes (see e.g. Curtsinger and Feldman, 1980, and references therein), sex-dependent fitness components (see e.g. Thomson and Feldman 1975, Charlesworth, 1977) as well as hermaphroditism, parthogenesis, and sex conversion (see e.g. Williams, 1975, Chapter 10) are all expected to affect the sex ratio in those XY systems for which they are relevant. On the other hand, sex ratios in species with other sex-determining systems may also be close to 1: 1 (Scudo, 1964; Spieth, 1974; Charlesworth, 1977). Thus the cytogenetic pattern can be regarded as neither a necessary nor a sufficient factor for the explanation of observed sex ratios. Nevertheless, the connection between sex determination and the sex ratio remains compelling to evolutionary theorists. The evolution of a sex ratio close to 1: 1 (or rather the evolution of sex determination which should determine such a ratio) was first explained by Fisher (1930) as a consequence of maximization of the number of grandoffspring through optimal allocation of parental investment in male and female offspring (see also Bodmer and Edwards, 1960). A crucial evolutionary assumption in Fisher’s argument is that each sex must supply