Sex chromosome evolution in the clam shrimp, Eulimnadia texana

Chromosomes that determine sex are predicted to evolve differently thanautosomes: a lack of recombination on one of the two sex chromosomes ispredicted to allow an accumulation of deleterious alleles that eventually leadsto reduced functionality and potential physical degradation of the nonrecombining chromosome. Because these changes should occur at an elevatedevolutionary rate, it is difficult to find appropriate species in which to testthese evolutionary predictions. The unique genetic sex-determining mechanism of the crustacean Eulimnadia texana prevents major chromosomedegeneration because of expression of both ‘proto-sex’ (i.e. early stage ofdevelopment) chromosomes in homozygous form (ZZ and WW). Herein, weexploit this unique genetic system to examine the predicted accumulation ofdeleterious alleles by comparing both homogametic sexual types to theirheterogametic counterpart. We report differences in crossing over in a sexlinked region in the ZW hermaphrodites (3%) relative to the ZZ males(21%), indicative of cross-over suppression in the ZW hermaphrodites.Additionally, we report that both ZZ and WW genotypes have reduced fitnessrelative to ZW hermaphrodites, which is consistent with the prediction ofharboured recessive mutations embedded on both the Z and the W chromosomes. These results suggest that the proto-sex chromosomes in E. texanaaccumulate recessive deleterious alleles. We hypothesize that recessivedeleterious alleles of large effect cannot accumulate because of expression inboth ZZ and WW individuals, keeping both chromosomes from losingsignificant function