Genetic and molecular analysis of new female-specific lethal mutations at the gene Sxl of Drosophila melanogaster

We have isolated three female-specific lethal mutations at the gene Sex-lethal (Sxl): Sxl(fb), Sxl(fc) and Sxl(fd). We have carried out the complementation analysis between these mutations and other previously reported Sxl(f) mutations. It is possible to classify the alleles tested in this report into two complementation groups: the bc group defined by Sxl(fb), and Sxl(fc), and the LS group defined by Sxl(fLS) The other alleles tested affect both complementation groups albeit with different degrees. Contrary to what happens with mutations at the LS group, mutations at the bc group do not affect sex determination, nor late dosage compensation nor oogenesis. Both Sxl(fb) and Sxl(fc) present a DNA insertion of at least 5 kb between position -10 and -11 on the molecular map, within the fourth intron. On the contrary, Sxl(fd), a strong mutation affecting all Sxl functions, is not associated to any detectable DNA alteration in Southern blots, so that it seems to be a "point" mutation. In agreement with their phenotypes, both Sxl(fc)/Sxl(LS) and Sxl(fc) homozygous female larvae express only the late Sxl transcripts characteristic of females, while females homozygous for Sxl(fLS) express only the late Sxl transcripts characteristic of males. Moreover, Sxl(fc) presents a lethal synergistic interaction with mutations at either da or the X:A ratio, two signals that define the initial activity state of Sxl, while Sxl(fLS) do not. These data suggest that the two complementation groups are related to the two sets of early and late Sxl transcripts, which are responsible for the early and late Sxl functions, respectively: Sxl(fb) and Sxlfc would affect the early functions and Sxl(fLS) would affect the late Sxl functions.