Submitted to: Nematology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/23/2006
Publication Date: 10/1/2006
Citation: Skantar, A.M., Guimond, N.A., Chitwood, D.J. 2006. Molecular characterization of two novel 17 beta-hydroxysteroid dehydrogenase genes from the soybean cyst nematode, heterodera glycines. Nematology. 8(3):321-333. Interpretive Summary: Soybean cyst nematode (SCN) is the major pathogen of soybean in the United States, responsible for nearly $1 billion in annual losses. The use of nematode-resistant soybeans is the major method of control used today, but new genetic sources of resistance are needed in order to combat constantly changing nematode populations that overcome resistance. One approach being taken by ARS researchers is to develop genetically based nematode controls to exploit differences between the hormonal systems of nematodes and those of their host plants. Many hormones are chemicals known as steroids, compounds that are produced in organisms from the cholesterol they contain. Because nematodes cannot make their own cholesterol, they must ingest it or related compounds from the plant roots on which they feed and then convert these compounds into steroid hormones, which play important roles in maturation and reproduction. The enzymes responsible for this conversion in SCN are unknown. In this paper, researchers at Beltsville, MD, identified two genes with similarity to genes that encode enzymes involved in steroid hormone production in mammals. Both genes are very highly expressed in developing female stages and may be involved in the development of the reproductive system or eggs. This first discovery of genes possibly responsible for hormone production in SCN is important because SCN females are responsible for the damage caused to soybean roots, and identifying these genes provides information that can be used to design new ways to prevent SCN damage to soybean. This research will be used by scientists developing novel controls for SCN and other nematodes.
Technical Abstract: Heterodera glycines Hg-hsd-1 and Hg-hsd-2 are two homologs of let-767, a short-chain dehydrogenase gene from the free-living nematode Caenorhabditis elegans. Both genes share similarity to 17 beta-hydroxysteroid dehydrogenases, enzymes known to be involved in the production of steroid hormones in mammals. Hg-hsd-1 and Hg-hsd-2 encode distinct proteins 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 proteins encoded by ESTs 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 female cysts aged, corresponding 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 17 beta-hydroxysteroid dehydrogenases are involved in the conversion of sterols, which nematodes must ingest from dietary sources, into necessary steroid hormones.