Molecular evolution of P transposable elements in the genus Drosophila. I. The saltans and willistoni species groups

A phylogenetic survey using the polymerase chain reaction (PCR) hasidentified four major P element subfamilies in the saltans and willistonispecies groups of Drosophila. One subfamily, containing about half of thesequences studied, consists of elements that are very similar to thecanonical (and active) P element from D. melanogaster. Within thissubfamily, nucleotide sequence differentiation among different copies fromthe same species and among elements from different species is relativelylow. This observation suggests that the canonical elements are relativelyrecent additions to the genome or, less likely, are evolving slowlyrelative to the other subfamilies. Elements belonging to the threenoncanonical lineages are distinct from the canonical elements and from oneanother. Furthermore, there is considerably more sequence variation, on theaverage, within the noncanonical subfamilies compared to the canonicalelements. Horizontal transfer and the coexistence of multiple,independently evolving element subfamilies in the same genome may explainthe distribution of P elements in the saltans and willistoni speciesgroups. Such explanations are not mutually exclusive, and each may beinvolved to varying degrees in the maintenance of P elements in naturalpopulations of Drosophila.