Differential chromatin amplification and chromosome complements in the germline of Strongyloididae (Nematoda)

Nematodes of the genus Strongyloides are intestinal parasites of vertebrates including man. Currently, Strongyloides and its sister genus Parastrongyloides are being developed as models for translational and basic biological research. Strongyloides spp. alternate between parthenogenetic parasitic and single free-living sexual generations, with the latter giving rise to all female parasitic progeny. Parastrongyloides trichosuri always reproduces sexually and may form many consecutive free-living generations. Although the free-living adults of both these species share a superficial similarity in overall appearance when compared to Caenorhabditis elegans, there are dramatic differences between them, in particular with respect to the organization of the germline. Here we address two such differences, which have puzzled investigators for several generations. First, we characterize a population of non-dividing giant nuclei in the distal gonad, the region that in C. elegans is populated by mitotically dividing germline stem cells and early meiotic cells. We show that in these nuclei, autosomes are present in higher copy numbers than X chromosomes. Consistently, autosomal genes are expressed at higher levels than X chromosomal ones, suggesting that these worms use differential chromatin amplification for controlling gene expression. Second, we address the lack of males in the progeny of free-living Strongyloides spp. We find that male-determining (nullo-X) sperm are present in P. trichosuri, a species known to produce male progeny, and absent in Strongyloides papillosus, which is consistent for a species that does not. Surprisingly, nullo-X sperm appears to be present in Strongyloides ratti, even though this species does not produce male progeny. This suggests that different species of Strongyloides employ various strategies to prevent the formation of males in the all-parasitic progeny of the free-living generation.