Originally published as Genetics Published Articles Ahead of Print on December 6, 2006.

Genetics, Vol. 175, 301-310, January 2007, Copyright © 2007
doi:10.1534/genetics.106.064378

The Evolutionary History of Drosophila buzzatii. XXXVI. Molecular Structural Analysis of Osvaldo Retrotransposon Insertions in Colonizing Populations Unveils Drift Effects in Founder Events

María Pilar García Guerreiro1 and Antonio Fontdevila

Grup de Biología Evolutiva, Departament de Genètica i Microbiologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain

1 Corresponding author: Grup de Biología Evolutiva, Departament de Genética i Microbiologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
E-mail: mariapilar.garcia.guerreiro{at}uab.es

Previous work on transposable element distribution in colonizing populations of Drosophila buzzatii revealed a high frequency of occupancy in several chromosomal sites. Two explanatory hypotheses were advanced: the founder hypothesis, by which founder genetic drift was responsible, and the unstable hypothesis that assigns this unusual distribution to bursts of transposition toward some chromosomal sites. Here, we study the molecular structure of three euchromatic Osvaldo clones isolated from sites occupied at high (A4 and B9) and low frequency (B4) in colonizing populations, to test these hypotheses. Large insertions, duplications, and indels in the Osvaldo coding region and LTR were detected in the A4 clone and a truncated Osvaldo with many substitutions was found in the B9 clone. These altered sequences indicate that the two copies of this retroelement are precolonization insertions. Interestingly, the LTR of the A4 clone and the reverse transcriptase region of B9 show identical sequences in all colonizing populations indicating, most probably, that they are identical by descent. Moreover, Osvaldo is inserted at the same nucleotide site in all colonizing populations. On the other hand an almost identical LTR sequence, except by 1 base deletion, was found in the B4 clone compared to the canonical active Osvaldo element. These results suggest that Osvaldo copies in highly occupied sites are, most probably, identical by descent and strongly favor the founder hypothesis. On the other hand, low-insertion-frequency sites could represent recent transposition events. This work emphasizes the importance of molecular population studies to disentangle the effects of genetic drift and transposition in colonization.