Consistent accumulation of transposable elements in species of the Hawaiian Tetragnatha spiny-leg adaptive radiation across the archipelago chronosequence

Abstract

The ecological and phenotypic diversity observed in oceanic island radiations presents an evolutionary paradox: a high level of genetic variation is typically required for diversification, but species colonizing a new island commonly suffer from founder effects. This reduction in population size leads to lower genetic diversity, which ultimately results in a reduction in the efficiency of natural selection. Then, what is the source of genetic variation which acts as the raw material for ecological and phenotypic diversification in oceanic archipelagos? Transposable elements (TEs) are mobile genetic elements that have been linked to the generation of genetic diversity, and evidence suggests that TE activity and accumulation along the genome can result from reductions in population size. Here, we use the Hawaiian spiny-leg spider radiation (Tetragnatha) to test whether TE accumulation increases due to demographic processes associated with island colonization. We sequenced and quantified TEs in 23 individuals representing 16 species from the spiny-leg radiation and 4 individuals from its sister radiation, the Hawaiian web-building Tetragnatha. Our results show that founder effects resulting from colonization of new islands have not resulted in TE accumulation over evolutionary time. Specifically, we found no evidence for increase in abundance of specific TE superfamilies, nor an accumulation of ‘young TEs’ in lineages which have recently colonized a new island or are present in islands with active volcanoes. We also found that the DNA/hAT transposon superfamily is by far the most abundant TE superfamily in the Tetragnatha radiation. This work shows that there is no clear trend of increasing TE abundance for the spiny-leg radiation across the archipelago chronosequence, and TE accumulation is not affected by population oscillations associated with island colonization events. Therefore, despite their known role in the generation of genetic diversity, TE activity does not appear to be the mechanism explaining the evolutionary paradox of insular diversification in the Tetragnatha spiny-leg radiation.