Analysis of the sex ratio in Bradysia matogrossensis (Diptera, Sciaridae)

In sciarid flies, the control of sex determination and of the progeny sex r atio is exercised by the parental females, and is based on differential X-c hromosome elimination in the initial stages of embryogenesis. In some speci es, the females produce unisexual progenies (monogenic females) while in ot hers, the progenies consist of males and females (digenic females). The sex ratio of bisexual progenies is variable, and departs considerably from 1:1 . Bradysia matogrossensis shows both monogenic and digenic reproduction. In a recently established laboratory strain of this species, 15% of the femal es were digenic, 10% produced only females, 13% produced only males, and 62 % produced progenies with one predominant sex (33% predominantly of female and 29% predominantly male progenies). These progeny sex ratios were mainta ined in successive generations. Females from female-skewed progenies yielde d female- and male-producing daughters in a 1:1 ratio. In contrast, daughte rs of females from male-skewed progenies produce bisexual or male-skewed pr ogenies. The X-chromosome of B, matogrossensis shows no inversion or other gross aberration. These results suggest that the control of the progeny sex ratio (or differential X-chromosome elimination) involves more than one lo cus or, at least, more than one pair of alleles; The data also suggest that , in sciarid flies, monogeny and digeny may share a common control mechanis m.