Molecular characterisation of two novel 17β-hydroxysteroid dehydrogenase genes from the soybean cyst nematode Heterodera glycines

Heterodera glycines Hg-hsd-1 and Hg-hsd-2 are two homologues of let-767, a short-chain dehydrogenase gene from the free-living nematode Caenorhabditis elegans. Both genes share similarity to 17β-hydroxysteroid dehydrogenases (17β-HSDs), enzymes known to be involved in the production of steroid hormones in mammals. Hg-hsd-1 and Hg-hsd-2 encode distinct protein sequences that share 51% amino acid identity with each other and 48% identity with LET-767. The Hg-HSD-1 protein sequence was most similar to those encoded by ESTs from the lesion nematode, Pratylenchus penetrans, and Hg-HSD-2 protein was most similar to sequences from another cyst nematode, Globodera pallida. Quantitative real-time PCR revealed the highest expression of both genes in developing females, with relatively low levels in second-stage juveniles, eggs, or males. Transcription levels of both genes declined as females aged, decreasing with increased cuticle pigmentation. Transcription was up-regulated in the cyst bodies and fluid that had been manually separated from the developing embryos, suggesting that Hg-hsd-1 and Hg-hsd-2 are most likely involved in gonad development or oogenesis in the female. It is possible that these 17β-hydroxysteroid dehydrogenases are involved in the conversion of sterols, which nematodes must ingest from dietary sources, into necessary steroid hormones.