Abstract
Drosophila melanogaster retrotransposons of the gypsy group are considered to be potential errantiviruses. Their infectivity is caused by the functional activity of the third open reading frame (ORF3) encoding the Env protein, which was probably captured from baculoviruses. Mobile genetic elements (MGEs) of the gypsy group can be conventionally divided into three subgroups: with three ORFs, with a defective ORF3, and without the ORF3. To establish the patterns of evolution of gypsy retrotransposons in D. melanogaster, the members of the three subgroups were examined. Structural analysis of retrotransposons opus and rover, which carry a defective ORF3, as well as retrotransposons Burdock, McClintock, qbert, and HMS-Beagle, which lack the ORF3, suggests that the evolution of these MGEs followed the pattern of loosing the ORF3. At the same time, an MGE of the same subgroup, Transpac, may be an ancestral form, which had acquired the env gene and gave rise to the first errantiviruses. The capture of the ORF3 by retrotransposons provided their conversion to a fundamentally new state. However, the ORF3 in the genome is not subjected to strong selective pressure, because it is not essential for intragenomic transpositions. Because of this, the process of its gradual loss seems quite natural.
