Figure 2.—

The phylogenic relationship among Diptera of Sxl and its closest paralog ssx (CG3056). With a phorid fly (M.sca) Sxl as the outgroup among the “higher” Diptera (Brachycera), we aligned Sxl and CG3056 (sister-of-Sex-lethal) with respect to the two RRMs and 11 residues immediately downstream, as well as the 27 (in melanogaster) C-terminal residues corresponding to the exon 8 isoform of D.mel Sxl. See materials and methods for details. A previous estimate of when the paralog-generating Sxl duplication occurred (traut et al. 2006) could say only that it was sometime after the point indicated by the open arrow. Our analysis placed that duplication later (solid arrow, bootstrap value 99%), closer to the time at which Sxl became a master sex-determining switch gene. The evolutionary divergence scale bar is in substitutions per nucleotide. D.mel, Drosophila melanogaster; D.vir, D. virilis; M.dom, Musca domestica (house fly); Ch.Ruf, Chrysomya rufifacies (blow fly); C.cap, Ceratitis capitata (medfly); M.Sca, Megaselia scalaris (scuttle fly).