NATURAL REPRESSORS OF P-INDUCED HYBRID DYSGENESIS IN DROSOPHILA-MELANOGASTER - A MODEL FOR REPRESSOR EVOLUTION

Type I repressors control P element transposition and comprise full le ngth elements and elements with small 3' deletions in the final exon. Using a sensitive assay for measuring the strength of repression of P element transposition in somatic and germline tissues, we have isolate d and characterized a naturally occurring type I repressor element fro m a Q population of Drosophila melanogaster. We demonstrate that the a lmost complete repression of transposition in this population is a mix ture of KP elements with intermediate levels of repression, and the st rong contribution of a single 2.6 kb P element deletion derivative, wh ich we call SR (Strong Repressor). A deletion in the final intron of S R allows for the constitutive production of a putative 75 kDa represso r protein in germline tissues in addition to the production of the 66 kDa repressor in the soma, which would result in a biparental mode of inheritance of repression. Based on the four observed classes of natur al Q populations, we propose a model in which populations containing S R-like elements, capable of producing strong type I repressor constitu tively, have a selective advantage over populations which rely either on maternally transmitted P cytotype or on KP-induced weak levels of r epression, Such populations may subsequently spread and constitute an evolutionary stable strategy for the repression of hybrid dysgenesis i n Drosophila melanogaster.