Unisexual Progenies and Sex Determination in Sciara

Abstract

Two types of reproduction are found in Sciara. (1) "Digenic," in which progenies are essentially bisexual; (2) "Monogenic," in which progenies are unisexual. Thirteen species have been studied, 6 extensively, 7 slightly. Of the former, 3 show digenic and 3 monogenic reproduction. In some species both types of reproduction are found. Sex ratios in digenic type are variable, not consistently 11. Genetic difference between the two not yet analysed. Sex determination involves two aspects: (1) sex of progeny as a whole and (2) sex of individual fly. Former is determined by the sex chromosomes of mother, [male]-producing [female][female] being XX and [female]-producing [female][female] being X[image]X. Genetic tests indicate that X and X[image] chromosomes are similar, but not identical in constitution. Not yet certain how sex of individual is determined. Sex-linked inheritance of Drosophila type is found. Male germ-line has 2 sex chromosomes, indicating XY constitution and a process of selective fertilization. But in both sexes certain chromosomes present in the germ-line are absent from somatic cells. Male soma lacks one more chromosome than female. Latter chromosome is presumably a sex chromosome (always the paternal one), which suggests that sex may be determined by constitution of soma and that c?[male] germ-line may be XX, like the [female]. On this view, sex of individual would be determined at time of chromosome elimination from somatic nuclei during cleavage. Certain large chromosomes are apparently limited entirely to germ-line in both sexes; presumably their function is similarly limited.