| Summary: | <p>Abstract</p> <p>Background</p> <p>The evolutionary importance of horizontal gene transfer (HGT) from <it>Wolbachia </it>endosymbiotic bacteria to their eukaryotic hosts is a topic of considerable interest and debate. Recent transfers of genome fragments from <it>Wolbachia </it>into insect chromosomes have been reported, but it has been argued that these fragments may be on an evolutionary trajectory to degradation and loss.</p> <p>Results</p> <p>We have discovered a case of HGT, involving two adjacent genes, between the genomes of <it>Wolbachia </it>and the currently <it>Wolbachia</it>-uninfected mosquito <it>Aedes aegypti</it>, an important human disease vector. The lower level of sequence identity between <it>Wolbachia </it>and insect, the transcription of all the genes involved, and the fact that we have identified homologs of the two genes in another <it>Aedes </it>species (<it>Ae. mascarensis</it>), suggest that these genes are being expressed after an extended evolutionary period since horizontal transfer, and therefore that the transfer has functional significance. The association of these genes with <it>Wolbachia </it>prophage regions also provides a mechanism for the transfer.</p> <p>Conclusion</p> <p>The data support the argument that HGT between <it>Wolbachia </it>endosymbiotic bacteria and their hosts has produced evolutionary innovation.</p>
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