A gene that modifies the sex ratio in a bisexual strain of Sciara ocellaris

Abstract

SUMMARYThe influence of an X-linked recessive mutation, sepia, on the sex determination of a bisexual strain of Sciara ocellaris was studied. It induces an alteration in the sex ratio, especially in the progeny of heterozygous females, increasing the proportinn of males. These results cannot be explained by differential fecundity of the female parents of different genotypes nor by differential mortality between the sexes. The occurrence of gynandromorphs indicates that the mutant probably interferes with the processes of X-chromosnme elimination. Observations that heterozygous females which received the sepia allele from their mothers produced a higher frequency of gynandromorphs than females which received the mutated allele from their male parents, suggest that the mutation is interfering with the mechanism of chromosome elimination present in the egg cytoplasm.

Sex-determining mechanisms in insects

“…In this species the offspring are composed of male and female individuals, but the sex ratio varies widely (Metz, 1938;Davidheiser, 1943;Mori et al, 1979). It has been shown that this variation depends on temperature: at 18-20°C, the sex ratio distribution, although variable, shows a median at approximately 50%, but at 24-29°C the sex ratio moves towards the production of more females.…”

Temperature and the progeny sex-ratio in Sciara ocellaris (Diptera, Sciaridae)

“…We used an amphigenic strain of Sciara ocellaris that has been maintained in the laboratory since 1965 (Pavan and Perondini, 1967) and which has a progeny sex-ratio varying from a few males to a preponderance of males (Mori et al, 1979). In our investigation the number of flies produced by each female also varied widely, from progenies with only a few flies (progenies with less than 30 flies were not used) to progenies with 185 flies.…”

Sciara as an experimental model for studies on the evolutionary relationships between the zygotic, maternal and environmental primary signals for sexual development

“…However, whereas in S. coprophila females produce only either female-producer or male-producer oocytes, in S. ocellaris each female generates both classes of oocytes. Thus, in this species the offspring of each female is composed of males and females, although the sex ratio varies widely among the females (Metz 1938;Davidheiser 1943;Mori et al 1979). This variation depends on temperature: at 18-20…”

“…A strain of S. ocellaris has been described that carries sepia(s)-X chromosome (an X chromosome that carries the sepia mutation as a marker) that causes alterations in the sex ratio towards the production of more males, without differential mortality of female embryos (Mori et al 1979). Moreover, the offspring of this strain contain as well a significant number of gynandromorphs (individuals with some portions of the body typically male X0 and others typically female XX); i.e., individuals in which some somatic nuclei eliminate only one and other nuclei eliminate both two paternal-derived X chromosomes ( Mori et al 1979;Mori and Perondini 1980).…”