Evolution of Target Specificity in R1 Clade Non-LTR Retrotransposons

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Although most non-long terminal repeat (non-LTR) retrotransposons are inserted throughout the host genome, many non-LTR elements in the R1 clade are inserted into specific sites within the target sequence. Four R1 clade families have distinct target specificity: R1 and RT insert into specific sites of 28S rDNA, and TRAS and SART insert into different sites within the (TTAGG)n telomeric repeats. To study the evolutionary history of target specificity of R1-clade retrotransposons, we have screened extensively novel representatives of the clade from various insects by in silico and degenerate polymerase chain reaction (PCR) cloning. We found four novel sequence-specific elements; Waldo (WaldoAg1, 2, and WaldoFs1) inserts into ACAY repeats, Mino (MinoAg1) into AC repeats, R6 into another specific site of the 28S rDNA, and R7 into a specific site of the 18S rDNA. In contrast, several elements (HOPE, WISHBm1, HidaAg1, NotoAg1, KagaAg1, Ha1Fs1) lost target sequence specificity, although some of them have preferred target sequences. Phylogenetic trees based on the RT and EN domains of each element showed that (1) three rDNA-specific elements, RT, R6, and R7, diverged from Waldo; (2) the elements having similar target sequences are phylogenetically related; and (3) the target specificity in the R1 clade was obtained once and thereafter altered and lost several times independently. These data indicate that the target specificity in R1 clade retroelements has changed during evolution and is more divergent than has been speculated so far.

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