Structural, genomic, and phylogenetic analysis of Lian, a novel family of non-LTR retrotransposons in the yellow fever mosquito, Aedes aegypti.

Abstract

A retrotransposon named Lian-Aa1 was discovered in an intron of an AaHR3-1 gene of the yellow fever mosquito, Aedes aegypti. This retrotransposon contained a long open reading frame with 1,219 amino acids that included endonuclease, reverse transcriptase, and RNase H domains. It was shown that in the Rock strain of Ae. aegypti, there were up to 1,380 copies of Lian elements, equivalent to 0.8% of the entire genome. Five additional copies of Lian elements were isolated, mapped by restriction digestion, and partially sequenced. The 5' and 3' ends of the Lian family were determined by comparing the terminal sequences of the six copies and were subsequently confirmed by the identification of putative target duplications flanking Lian-Aa1 and Lian-Aa2. The Lian family is likely a novel family of non-long-terminal-repeat (non-LTR) retrotransposons that terminate in a repeat of (CTGA-TAC)2. On average, the six copies of Lian elements showed only 0.6% sequence divergence at the nucleotide level in both a 735-bp region at the 5' end and a 1,124-bp coding region. Genomic Southern blots also revealed a very high degree of similarity among hundreds of Lian elements, suggesting very recent activity of Lian. Furthermore, all six analyzed Lian elements were closely associated with one or more different families of repetitive elements. It is possible that these associations could reflect the complex relationship between Lian elements and the rest of the Ae. aegypti genome. Phylogenetic analyses based on the reverse transcriptase, domains of 36 non-LTR retrotransposons including Lian-Aa1 identified five major subgroups that were supported by bootstrap replications. In contrast to the majority of non-LTR retrotransposons, Lian-Aa1 has an RNase H domain that is similar to a few other non-LTR retrotransposons and some retroviruses, which is consistent with the previously proposed independent assortment of different domains during the evolution of retroelements.