The parasitic roundworms
Strongyloides stercoralis(in man) and Strongyloides ratti(in rats) employ environmentally controlled XX/XO sex determination with a pair of X chromosomes and two pairs of autosomes. Strongyloides papillosus(in sheep) has only two pairs of chromosomes, one of which combines the genetic material homologous to the S. rattichromosomes X and I. This species creates males through the elimination of one copy of the portion related to the X chromosome (chromatin diminution). It is not clear which one of these two sex-determining mechanisms is ancestral. We demonstrate that Strongyloides vituli(in cattle) has two pairs of chromosomes like its very close relative S. papillosuswhereas Parastrongyloides trichosuri, a closely related out-group to Strongyloidesspp. in Australian brushtail possums, has three chromosome pairs and employs XX/XO sex determination. The X chromosome of P. trichosuriis homologous to the X chromosome of S. ratti. Our data strongly suggest that the last common ancestor of Strongyloidesspp. and Parastrongyloidesspp. had two pairs of autosomes along with two or one X chromosome in females and males, respectively. The situation with two pairs of chromosomes is likely derived and occurred through the fusion of the X chromosome with an autosome.