Evolution of the T1 retroposon family in the Anopheles gambiae complex

The T1 family of retrotransposable elements is interspersed and moderately repeated in five member species of the Anopheles gambiae sibling-species complex and has diverged little since the radiation of the complex. T1 includes two closely related but independent subfamilies, defined by the presence or absence of linked sets of restriction sites, in all but one species, although the relative abundance of the subfamilies differs within each. Sequence analysis of a 349-bp region from 21 clones isolated from A. gambiae confirmed the bipartite organization by revealing 19 coordinated nucleotide differences between the two subfamilies--T1 alpha and T1 beta. Sequence divergence is not only greater between than within subfamilies, but divergence within T1 beta is less than that within T1 alpha. Between-species comparisons of genomic consensus restriction maps revealed that T1 alpha is fixed for species-diagnostic differences in all species. With one exception, these subfamilies account for approximately 70% of detectable T1 copies in the genome. The results support retroposition as the dominant mechanism underlying the evolution of the T1 family.