Horizontal transfer and selection in the evolution of P elements

The roles of selection and horizontal transfer in the evolution of the cano nical subfamily of P elements were studied in the saltans and willistoni sp ecies groups of the genus Drosophila (subgenus Sophophora). We estimate tha t the common ancestor of the canonical P subfamily dates back 2-3 Myr at th e most, despite the much older age (more than 40 Myr) of the P family as a whole. The evolution of the canonical P subfamily is characterized by weak selection at nonsynonymous sites. These sites have evolved at three quarter s the rate of synonymous sites, in which no selective constraints were dete cted. Their recent horizontal transfer best explains the high degree of sim ilarity among canonical P elements from the saltans and willistoni species groups. These results are consistent with a model of P-element evolution in which selective constraints are imposed at the time of horizontal transfer . Furthermore, it is estimated that the spread and diversification of the c anonical subfamily involved a minimum of 11 horizontal transfer events amon g the 18 species surveyed within the past 3 Myr. The presence of multiple P subfamilies in the saltans and willistoni species groups is likely to be t he result of multiple invasions that have previously swept through these ta xa in a succession of horizontal transfer events. These results suggest tha t horizontal transfer among eukaryotes might be more common than anticipate d.