Evolution of the Drosophila Feminizing Switch Gene Sex-lethal

Abstract

In Drosophila melanogaster, the gene Sex-lethal (Sxl ) controls all aspects of female development. Since melanogaster males lacking Sxl appear wild type, Sxl would seem to be functionally female specific. Nevertheless, in insects as diverse as honeybees and houseflies, Sxl seems not to determine sex or to be functionally female specific. Here we describe three lines of work that address the questions of how, when, and even whether the ancestor of melanogaster Sxl ever shed its non-female-specific functions. First, to test the hypothesis that the birth of Sxl 's closest paralog allowed Sxl to lose essential ancestral non-femalespecific functions, we determined the CG3056 null phenotype. That phenotype failed to support this hypothesis. Second, to define when Sxl might have lost ancestral non-female-specific functions, we isolated and characterized Sxl mutations in D. virilis, a species distant from melanogaster and notable for the large amount of Sxl protein expression in males. We found no change in Sxl regulation or functioning in the 401 MY since these two species diverged. Finally, we discovered conserved non-sex-specific Sxl mRNAs containing a previously unknown, potentially translation-initiating exon, and we identified a conserved open reading frame starting in Sxl male-specific exon 3. We conclude that Drosophila Sxl may appear functionally female specific not because it lost non-female-specific functions, but because those functions are nonessential in the laboratory. The potential evolutionary relevance of these nonessential functions is discussed.

Molecular cloning and expression analysis of two sex-lethal homolog genes during development in the oriental river prawn, Macrobrachium nipponense

“…Apart from Drosophila, Sxl has also been transcribed in adult gonads and somatic tissues of both sexes in S. ocellaris and B. oleae by northern blot analyses (Maine et al, 1985;Ruiz et al, 2003;Lagos et al, 2005), and Sxl expression was found to be extraordinarily high in the ovaries (Sievert et al, 2000;Ruiz et al, 2003;Cline et al, 2010). The current study was the first to report Mnsxl gene expression in the abdominal ganglion, which is part of the central nervous system in shrimp.…”

Specific RNP capture with antisense LNA/DNA mixmers

“…2C,D ;Gebauer et al 2003), was identified as the specific binder to the wt mRNA. Ssx and Sxl are paralogs that arose from a gene duplication event (Traut et al 2006;Cline et al 2010). Ssx is highly homologous to Sxl, and was previously identified as an RBP without assigned binding targets (Sysoev et al 2016;Wessels et al 2016).…”

Sex Determination in Insects: Variations on a Common Theme

“…Once activated by a female signal, tra will perpetuate its female-promoting functions by engaging a positive feedback loop that locks tra into the ON state. Evidence for such an autoregulatory function of tra has been found in a number of different insect species [Pane et al, , 2005Lagos et al, 2007;Concha and Scott, 2009;Gempe et al, 2009;Hediger et al, 2010;Sarno et al, 2010;Verhulst et al, 2010a] with the notable exception of D. melanogaster and possibly other drosophilids [Bopp et al, 1996;Cline et al, 2010]. Here, it is the gene Sex lethal (Sxl) , a more recently acquired upstream regulator of tra , which performs the task of sustaining the female-promoting functions of tra (see below).…”

“…In the best-documented case, D. melanogaster , the embryo bases this decision on the numbers of X chromosomes present in the diploid zygote: the presence of 2 X chromosomes imposes female development, while male development follows when only 1 X is present [Cline and Meyer, 1996;Erickson and Quintero, 2007]. This rather peculiar practice of counting X chromosomes as a signal for determining the sex appears to be common to drosophilids but has found few followers outside the genus Drosophila [Cline et al, 2010]. The primary sex-determining signal in the honeybee, Apis mellifera , constitutes of a single locus and its multiple allelic variants [Beye et al, 2003;Gempe et al, 2009].…”