Sex chromosome evolution in the clam shrimp Eulimnadia texana

Sex chromosome degeneration is documented in various animal taxa and is predicted to be due, in part, to a buildup of transposable elements (TE) on the non-recombining sex chromosome (Y in mammals and W in many crustaceans). Recombination in XX (or ZZ) individuals is predicted to reduce the buildup of TEs on the X (Z), but because there are no YY (WW) individuals, such TE buildup goes unchecked on the Y or W. Herein, we report an analysis of TEs in the genome of a crustacean that has both ZZ and WW individuals. The purported W chromosome did show linkage with several known sex-linked loci. However, the prediction of a buildup of TEs throughout the purported W was not found: neither the predicted accumulation of the TE types LINE and LTR nor the expected higher TE numbers were observed on the purported W. We could not exclude the possibility of TE buildup in a smaller non-recombining region of the W chromosome, which is predicted in species in the earliest stages of sex chromosome development. These results allow a glimpse into the earliest stages of sex chromosome evolution in these branchiopod crustaceans.