MOLECULAR EVOLUTION OF P-TRANSPOSABLE ELEMENTS IN THE GENUS DROSOPHILA .1. THE SALTANS AND WILLISTONI SPECIES GROUPS

A phylogenetic survey using the polymerase chain reaction (PCR) has id entified four major P element subfamilies in the saltans and williston i species groups of Drosophila. One subfamily, containing about half o f the sequences studied, consists of elements that are very similar to the canonical (and active) P element from D. melanogaster. Within thi s subfamily, nucleotide sequence differentiation among different copie s from the same species and among elements from different species is r elatively low, This observation suggests that the canonical elements a re relatively recent additions to the genome or, less likely, are evol ving slowly relative to the other subfamilies. Elements belonging to t he three noncanonical lineages are distinct from the canonical element s and from one another. Furthermore, there is considerably more sequen ce variation, on the average, within the noncanonical subfamilies comp ared to the canonical elements. Horizontal transfer and the coexistenc e of multiple, independently evolving element subfamilies in the same genome may explain the distribution of P elements in the saltans and w illistoni species groups. Such explanations are not mutually exclusive , and each may be involved to varying degrees in the maintenance of P elements in natural populations of Drosophila.