Horizontal transmission, vertical inactivation, and stochastic loss of mariner-like transposable elements

Horizontal transmission has been well documented as a major mechanism forthe dissemination of mariner-like elements (MLEs) among species. Less wellunderstood are mechanisms that limit vertical transmission of MLEsresulting in the "spotty" or discontinuous distribution observed in closelyrelated species. In this article we present evidence that the genome of thecommon ancestor of the melanogaster species subgroup of Drosophilacontained an MLE related to the mellifera (honey bee) subfamily. Horizontaltransmission, approximately 3-10 MYA, is strongly suggested by theobservation that the sequence of the MLE in Drosophila erecta is 97%identical in nucleotide sequence with that of an MLE in the cat flea,Ctenocephalides felis. The D. erecta MLE has a spotty distribution amongspecies in the melanogaster subgroup. The element has a high copy number inD. erecta and D. orena, a moderate copy number in D. teissieri and D.yakuba, and was apparently lost ("stochastic loss") in the lineage leadingto D. melanogaster, D. simulans, D. mauritiana, and D. sechellia. In D.erecta, most copies are concentrated in the heterochromatin. Two copiesfrom D. erecta, denoted De12 and De19, were cloned and sequenced, and theyappear to be nonfunctional ("vertical inactivation"). It therefore appearsthat the predominant mode of MLE evolution is vertical inactivation andstochastic loss balanced against occasional reinvasion of lineages byhorizontal transmission.