The Association Between Breeding System and Transposable Element Dynamics in Daphnia Pulex

Transposable elements (TEs) are major sources of genetic variation, and mating systems are believed to play a significant role in their dynamics. For example, insertion number is expected to be higher in sexual than in asexual organisms due to the inability of TEs to colonize new genomes in the absence of sex. The goal of this study was to determine the impact of the loss of sexual reproduction on TE load. Daphnia pulex has two reproductive modes, obligate and cyclical parthenogenesis, which differ with respect to the production of diapausing eggs. Cyclical parthenogens produce them meiotically, while obligate parthenogens produce them clonally. Pokey is a TTAA-specific DNA transposon, and is a stable component of Daphnia genomes. We used a PCR-based approach, TE-Display, to estimate the number of Pokey insertions in 22 cyclic and 22 obligate isolates of D. pulex. As expected, the copy number of Pokey insertions is significantly higher in cyclic than in obligate isolates. However, the distribution of elements among isolates within each breeding system is similar, which is congruent with the recent establishment of obligate lineages from a cyclic ancestor. We also assayed 46 isolates from eight cyclic populations and found that very few Pokey insertions were observed in more than one isolate, suggesting that Pokey has been active recently. Sequencing of PCR products from the TE-Display analysis shows that Pokey inserts into both coding and noncoding regions of the genome. However, there is no obvious similarity among sequences downstream of the TTAA Pokey insertion site.