Transposable elements in sexual and ancient asexual taxa

Sexual reproduction allows deleterious transposable elements to proliferate in populations, whereas the loss of sex, by preventing their spread, has b een predicted eventually to result in a population free of such elements [H ickey, D. A. (1982) Genetics 101, 519-531]. We tested this expectation by s creening representatives of a majority of animal phyla for LINE-like and gy psy-like reverse transcriptases and mariner/Tc1-like transposases. All spec ies tested positive for reverse transcriptases except rotifers of the class Bdelloidea, the largest eukaryotic taxon in which males, hermaphrodites, a nd meiosis are unknown and for which ancient asexuality is supported by mol ecular genetic evidence. Mariner-like transposases are distributed sporadic ally among species and are present in bdelloid rotifers. The remarkable lac k of LINE-like and gypsy-like retrotransposons in bdelloids and their ubiqu itous presence in other taxa support the view that eukaryotic retrotranspos ons are sexually transmitted nuclear parasites and that bdelloid rotifers e volved asexually.