The evolution of P element repression in Drosophila melanogaster

P elements have invaded and spread through populations of Drosophila melanogaster in the last sixty years, despite the induction of a severe set of genetic abnormalities known as hybrid dysgenesis (HD). P element spread has occurred because of the co-evolution of several systems that repress P element transposition and HD. Such repression systems, singly or in combination, give rise to populations phenotypically defined as P, M' and Q, with Q further subdivided in Qmat (strong repression transmitted maternally only) and Qbip (strong repression transmitted biparentally). What are the evolutionary dynamics of these repression systems? Do they co-exist or will all populations subsequently gravitate towards one stable system? Qbip populations possess all of the fitness benefits of P, M' and Qmat strains, but have none of their apparent disadvantages. A type I deletion element, called SR, has been isolated from a Qbip strain and its presence has been demonstrated to correlate with strong repression. My thesis tests a working model that the Qbip phenotype, mediated by SR or 'SR-like' elements, is the evolutionary stable state for repression. A D. melanogaster strain devoid of P elements was transformed with SR. Gonadal dysgenesis assays failed to detect strong repression, but the more sensitive A4-4 eye-colour assay failed to detected intermediate strength repression that may be dependent on genomic position. Molecular analysis of naturally occurring Q stains revealed the existence of putative 'SR-like' elements which fulfil the structural requirements of strong repressors. Molecular and phenotypic analysis of flies from a cline on the east coast of Australia, which ten years ago had P populations in the north, Q at central sites and M' in the south, shows that Q and M' flies have increased their range at the expense of P lines. Competition cage experiments which comprise mixtures of repression systems indicate potential trends towards Qbip. The data support the proposition that Qbip is the evolutionary end state for repression in D. melanogaster.;