Journal of Molecular Evolution

, Volume 78, Issue 1, pp 50–65 | Cite as

Evolution of the Sex-lethal Gene in Insects and Origin of the Sex-Determination System in Drosophila

Original Article


Sex-lethal (Sxl) functions as the switch gene for sex-determination in Drosophila melanogaster by engaging a regulatory cascade. Thus far the origin and evolution of both the regulatory system and SXL protein’s sex-determination function have remained largely unknown. In this study, we explore systematically the Sxl homologs in a wide range of insects, including the 12 sequenced Drosophila species, medfly, blowflies, housefly, Megaselia scalaris, mosquitoes, butterfly, beetle, honeybee, ant, and aphid. We find that both the male-specific and embryo-specific exons exist in all Drosophila species. The homologous male-specific exon is also present in Scaptodrosophila lebanonensis, but it does not have in-frame stop codons, suggesting the exon’s functional divergence between Drosophila and Scaptodrosophila after acquiring it in their common ancestor. Two motifs closely related to the exons’ functions, the SXL binding site poly(U) and the transcription-activating motif TAGteam, surprisingly exhibit broader phylogenetic distributions than the exons. Some previously unknown motifs that are restricted to or more abundant in Drosophila and S. lebanonensis than in other insects are also identified. Finally, phylogenetic analysis suggests that the SXL’s novel sex-determination function in Drosophila is more likely attributed to the changes in the N- and C-termini rather than in the RNA-binding region. Thus, our results provide a clearer picture of the phylogeny of the Sxl’s cis-regulatory elements and protein sequence changes, and so lead to a better understanding of the origin of sex-determination in Drosophila and also raise some new questions regarding the evolution of Sxl.


Sex-lethal Sex-determination Drosophila Regulatory elements 



Coding sequence


Hidden Markov model


Amino acid


RNA recognition motif


RNA-binding domain

Supplementary material

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Supplementary material 1 (PDF 647 kb)
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Supplementary material 2 (PDF 458 kb)
239_2013_9599_MOESM3_ESM.pdf (100 kb)
Supplementary material 3 (PDF 100 kb)


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Copyright information

© Springer Science+Business Media New York 2013

Authors and Affiliations

  1. 1.Institute of Molecular Evolutionary Genetics and Department of BiologyPennsylvania State UniversityState CollegeUSA

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