El gen Sex-lethal de la familia Sciaridae

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias. Departamento de Biología Molecular. Fecha de lectura: 07-07-2004In Drosophila melanogaster, the gene Sex-lethal (Srl) wntrols the processes of sex determination, sexual behaviour and dosage wmpensation (the products of the X-linked genes are present in equal amounts in males and females). Outside the genus Drosophila, Sxl has been characterised in Chvsomya rufiacies, Megaselia scalaris, Musca abmestim and Ceratitis capitala. In al1 these species - which belong to the suborder Brachycent - Sxl shows no sex-specific regulation and the same transcripts are found in males and females. In Sciara ocellaris (order Diptera, suborder Nematm), as in Drosophila melanogaster, gender depends on chromosome wnstitution: females are XX and males are XO. Dosage compensation in S. ocellaris also appears to be achieved by hyper-transcription of the single male X chromosome. The cloning and characterisation of the Sxl gene of S. ocellaris indicated that this gene appears not to play the key discriminative role in wntrolling sex determination and dosage wmpensation that it plays in Drosophila. To better understand the evolution of gene Sxl we undertook its cloning and characterisation in other insects of the suborder Nematocera - Sciara coprophila, Rhynchosciara americana and Trichosia pubescens - which represent three different genera of the Sciaridae. In these species, gender also depends on chromosome wnstitution, as in Drosophila melanogaster: females are XX and males are XO. Dosage wmpensation in Rhynchosciara and Trichosia also appears to be achieved by hyper-transcription of the single male X chromosome. The Sxl gene of the three species studied produces a single transcript encoding a single protein in both males and females. Comparison of the Sxl proteins of these Nematocera insects with those of the Brachycera showed their two RNA-binding domains (RBD) to be highly wnserved, whereas significant variation was observed in both the N- and C-terminal domains. The great majority of nucleotide changes in the RBDs were synonymous, indicating that purifying selection is acting on them. In both sexes of the threeNematocera insects, the Sxl protein co-localised with transcription-active regions dependent on RNA polymerase 11 but not on RNA polymerase 1. Together, these results indicate that Sxl does not appear to play a discriminatory role in the wntrol of sex detennination and dosage compensation in nematocerans. Thus, in the phylogenetic lineage that gave rise to the drosophilids, evolution co-opted for the Sxl gene, modified it, and wnverted it into the key gene wntrolling sex determination and dosage wmpensation. At the same time, however. certain properties of the recruited ancestral Sxl gene were beneficial, and these are maintained in the wolved Sxl gene, allowing it to exert its sex detennining and dose compensation functions in Drosophila