The history of the uhu transposable element in the Hawaiian Drosophila

Abstract

The uhu transposable element belongs to the class of elements that have short inverted repeats. It was originally isolated from Drosophila heteroneura, a Hawaiian picture-winged Drosophila endemic to the Island of Hawaii. Biogeographic and DNA sequence divergence data suggest an ancient origin for the uhu element in the Hawaiian Drusophila. Biogeographic data suggests that uhu arose more than 7 million years ago. Sequence divergence data and phylogenetic analysis suggests that uhu was present in a common ancestor of the species. The maximum distance between two isolates suggests that uhu has been in the Hawaiian Drosophila for 20 million years. Using in situ hybridization to polytene chromosomes, the copy number of uhu in the planitibia subgroup and the adiastola subgroups of the Hawaiian Drosophila is found to be higher in the species endemic to the younger islands than in the species endemic to the older islands. This trend is also seen for the loa transposable element in the planitibia subgroup. No complete loa elements are found in D. picticornis from the island of Kauai, while there are 10 to 20 potentially complete copies of loa in the other species. For the uhu element, the percentage of sites that are variable for the presence or absence of uhu is high in the species on the younger islands, while nearly all the sites in D. picticornis are fixed. This would indicate that uhu has more recently been active in the species on the younger islands. Since all of the species are single island endemics, and believed to have evolved on the island, the increase in copy number and evidence for transpositional activity is consistent with the idea that there has been increase in the activity of transposable element associated with a speciation event.