NATURALLY-OCCURRING VARIATION IN COPIA EXPRESSION IS DUE TO BOTH ELEMENT (CIS) AND HOST (TRANS) REGULATORY VARIATION

Significant differences in levels of copia [Drosophila long terminal r epeat (LTR) retrotransposon] expression exist among six species repres enting the Drosophila melanogaster species complex (D. melanogaster, D rosophila mauritiana, Drosophila simulans, Drosophila sechellia, Droso phila yakuba, and Drosophila erecta) and a more distantly related spec ies Drosophila willistoni). These differences in expression are correl ated with major size variation mapping to putative regulatory regions of the copia 5' LTR and adjacent untranslated leader region (ULR). Seq uence analysis indicates that these size variants were derived from a series of regional duplication events. The ability of the copia LTR-UL R size variants to drive expression of a bacterial chloramphenicol ace tyltransferase reporter gene was tested in each of the seven species. The results indicate that both element-encoded (cis) and host-genome-e ncoded (trans) genetic differences are responsible for the variability in copia expression within and between Drosophila species. This findi ng indicates that models purporting to explain the dynamics and distri bution of retrotransposons in natural populations must consider the po tential impact of both element encoded and host-genome-encoded regulat ory variation to be valid. We propose that interelement selection amon g retrotransposons may]provide a molecular drive mechanism for the evo lution of eukaryotic enhancers which can be subsequently distributed t hroughout the genome by retrotransposition.