Expansion of a single transposable element family is associated with genome-size increase and radiation in the genus <i>Hydra</i>.

Abstract

Transposable elements are one of the major contributors to genome-size differences in metazoans. Despite this, relatively little is known about the evolutionary patterns of element expansions and the element families involved. Here we report a broad genomic sampling within the genus <i>Hydra</i>, a freshwater cnidarian at the focal point of diverse research in regeneration, symbiosis, biogeography, and aging. We find that the genome of <i>Hydra</i> is the result of an expansion event involving long interspersed nuclear elements and in particular a single family of the chicken repeat 1 (CR1) class. This expansion is unique to a subgroup of the genus <i>Hydra</i>, the brown hydras, and is absent in the green hydra, which has a repeat landscape similar to that of other cnidarians. These features of the genome make <i>Hydra</i> attractive for studies of transposon-driven genome expansions and speciation.