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Special Issue Review
INTRODUCTION
The genomes of four Strongyloides species (S. ratti, S. stercoralis, S. papillosus and S. venezuelensis), and two closely related species from the same evolutionary clade, the parasite Parastrongyloides trichosuri and the free-living species Rhabditophanes sp. have recently been sequenced. The genomes of each these six species have been formed into 42-60 Mb assemblies, with each containing 12 451-18 457 genes (Hunt et al. 2016). The genomes of S. ratti, S. stercoralis and P. trichosuri each comprises two autosomes and an X chromosome (Bolla and Roberts, 1968; Hammond and Robinson, 1994), while in S. papillosus and S. venezuelensis chromosomes I and X have fused (Albertson et al. 1979; Hino et al. 2014); Rhabditophanes sp. has five chromosomes (Hunt et al. 2016). All six genomes are AT rich, with GC values ranging from 21 to 25% in the four Strongyloides spp. to 31 and 32% in P. trichosuri and Rhabditophanes, respectively (Hunt et al. 2016). The genomes have been assembled to a high-quality - the reference genome S. ratti into single scaffolds for each of the two autosomes, and the X chromosome assembled into ten scaffolds. The high-quality assembly of these genomes makes the Strongyloides species an excellent model in which to understand nematode genomics. For Strongyloides spp., the parasitic and free-living adult female stages of their life cycle are genetically identical because the parasitic female reproduces by a genetically mitotic parthenogenesis (Viney, 1994). The differences between the parasitic and free-living adult stages are therefore due solely to differences in expression of genes of the Strongyloides genome. Because the parasitic and free-living adult stages are genetically identical, the genes and proteins upregulated in the parasitic stage, compared with the free-living stage can be directly attributed to a putative role in parasitism. Most parasitic nematode species do not have genetically identical parasitic and free-living adult stages. Strongyloides therefore offers a uniquely tractable system to study the genetic basis of nematode parasitism.
Specific gene and protein families are upregulated at different stages of the Strongyloides life cycle when compared with other life cycle stages (Fig. 1) (Hunt et al. 2016). Comparisons of the transcriptome and proteome of parasitic females with the...