No evidence of transposable element bursts in the Galápagos Scalesia adaptive radiation despite hybridization, diversification and ecological niche shifts

Transposable elements (TEs) have been hypothesized to play a pivotal role in driving diversification by facilitating the emergence of novel phenotypes and the accumulation of divergence between species. Hybridization and adaptation to novel niches have been proposed to destabilize mechanisms constraining TE proliferation, potentially inducing a ‘TE burst’ that promotes TE accumulation on the genome. The rapid speciation and ecological diversification characteristic of adaptive radiations offer a unique opportunity to examine the link between TE accumulation and speciation, diversification, hybridization and adaptation. Here, focusing on all 15 species of the genus Scalesia (Asteraceae), a radiation endemic to the Galápagos Islands, we test whether diversification, hybridization, or shifts in ecological niche are associated with changes in TE accumulation in genomes. Our analyses reveal little to no variation in TE accumulation among Scalesia species nor its hybrid populations. Shifts in ecological niches, linked to climatic variation, did not result in discernible changes in TE accumulation, a surprising finding given the anticipated selective pressure imposed by aridity, a factor often linked to genome size reduction. We found no distinct patterns in the temporal accumulation of TEs, and no effects at the class or superfamily level. Our findings suggest that while TEs may play a key role in evolution at the locus level, their macroevolutionary association with diversification or speciation appears weak. Rather than actively driving evolutionary diversification, TEs may simply be'along for the ride.