DNA transposons and the role of recombination in mutation accumulation in Daphnia pulex

We identified 55 families belonging to 10 of the known superfamilies of DNA transposons in the genome of D. pulex. DNA transposons constitute approximately 0.7% of the genome. We characterized each family and, in many cases, identified elements capable of activity in the genome. Based on assays of six putatively active element families in mutation-accumulation lines, we compared DNA transposon abundance in lines where sex was either promoted or prohibited. We find the major difference in abundance in sexuals relative to asexuals in lab-reared lines is explained by independent assortment of heterozygotes in lineages where sex has occurred.Our examination of the duality of sex as a mechanism for both the spread and elimination of DNA transposons in the genome reveals that independent assortment of chromosomes leads to significant copy loss in lineages undergoing sex. Although this advantage may offset the so-called 'two fold cost of sex' in the short-term, if insertions become homozygous at specific loci due to recombination, the advantage of sex may be decreased over long time periods. Given these results, we discuss the potential effects of sex on the dynamics of DNA transposons in natural populations of D. pulex.The role of recombination (hereafter used interchangeably with sex) in transposable element (TE) proliferation has been of great interest for nearly three decades [1]; however, the question of whether or not sex leads to a net increase or decrease in TE abundance over time persists. Generally, a switch to asexuality is thought to eliminate the possibility of reconstructing the least-loaded class via recombination, and thus to irreversibly larger mutation loads (that is, Muller's ratchet [2,3]). In the special case of TEs, however, sex can result in an increased rate of both gain and loss, thereby complicating the predictions of the net effects of reproductive strategy over long time periods. This is because, although there are several mechanisms of gain and