A second option would be to heighten the activity of the single X chromosome in males. This is the route flies take. MSL-2, spliced into a functional form in males, serves to heighten transcriptional activation of the solitary X chromosome. The action of these male specific transcription factors is very similar to proteins of the trithorax complex . In the center ring here is the regulation by Sex lethal of sex specific RNA splicing in ovaries.
Biological systems operate at a level of complexity that continually astounds the student.
A tiny fly carries a whole circus at the core of its development, as far as sex determination is concerned. Sex-lethal expresses a set of three early transcripts and a set of seven late transcripts occurring from midembryogenesis through adulthood. The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Sex lethal has two transcripts, early and late, with different promoters and dramatically different splicing patterns. In contrast, the late SXL transcripts derive from an essentially constitutive promoter but are spliced sex specifically.