| PubMed Abstract |
Transposable elements of the mariner family are widespread among insects and other invertebrates, and initial analyses of
their relationships indicated frequent occurrence of horizontal transfers between hosts. A specific PCR assay was used to
screen for additional members of the irritans subfamily of mariners in more than 400 arthropod species. Phylogenetic analysis
of cloned PCR fragments indicated that relatively recent horizontal transfers had occurred into the lineages of a fruit fly
Drosophila ananassae, the horn fly Haematobia irritans, the African malaria vector mosquito Anopheles gambiae, and a green
lacewing Chrysoperla plorabunda. Genomic dot-blot analysis revealed that the copy number in these species varies widely, from
about 17,000 copies in the horn fly to three copies in D. ananassae. Multiple copies were sequenced from genomic clones from
each of these species and four others with related elements. These sequences confirmed the PCR results, revealing extremely
similar elements in each of these four species (greater than 88% DNA and 95% amino acid identity). In particular, the consensus
sequence of the transposase gene of the horn fly elements differs by just two base pairs out of 1,044 from that of the lacewing
elements. The mosquito lineage has diverged from the other Diptera for over 200 Myr, and the neuropteran last shared a common
ancestor with them more than 265 Myr ago, so this high similarity implies that these transposons recently transferred horizontally
into each lineage. Their presence in only the closest relatives in at least the lacewing lineage supports this hypothesis.
Such horizontal transfers provide an explanation for the evolutionary persistence and widespread distribution of mariner transposons.
We propose that the ability to transfer horizontally to new hosts before extinction by mutation in the current host constitutes
the primary selective constraint maintaining the sequence conservation of mariners and perhaps other DNA-mediated elements.
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