Distribution and conservation of mobile elements in the genus Drosophila

Essentially nothing is known of the origin, mode of transmission, andevolution of mobile elements within the genus Drosophila. To betterunderstand the evolutionary history of these mobile elements, we examinedthe distribution and conservation of homologues to the P, I, gypsy, copia,and F elements in 34 Drosophila species from three subgenera. Probesspecific for each element were prepared from D. melanogaster and hybridizedto genomic DNA. Filters were washed under conditions of increasingstringency to estimate the similarity between D. melanogaster sequences andtheir homologues in other species. The I element homologues show the mostlimited distribution of all elements tested, being restricted to themelanogaster species group. The P elements are found in many members of thesubgenus Sophophora but, with the notable exception of D. nasuta, are notfound in the other two subgenera. Copia-, gypsy-, and F-element homologuesare widespread in the genus, but their similarity to the D. melanogasterprobe differs markedly between species. The distribution of copia and Pelements and the conservation of the gypsy and P elements is inconsistentwith a model that postulates a single ancient origin for each type ofelement followed by mating-dependent transmission. The data can beexplained by horizontal transmission of mobile elements betweenreproductively isolated species.