DNA sequence polymorphism of the Rhg4 candidate gene conferring resistance to soybean cyst nematode in Chinese domesticated and wild soybeans

Rhg4 is one of the major resistant genes conferring resistance to soybean cyst nematode races 1, 3 and 4. In order to better understand its sequence diversity among different Chinese soybean populations and the impact of human activities on it, we designed 5 primer sets based on its sequence deposited in Genbank (Genbank accession No. AF506518) to obtain the Rhg4 sequence from 104 Chinese cultivated and wild soybean genotypes, and then analyzed the DNA sequence polymorphism in different Chinese soybean populations. The alignment of Rhg4 sequence included 5,216 nucleotide base pairs. A total of 67 single nucleotide polymorphisms (SNPs) including 59 single base changes and 8 DNA insertion-deletions (InDels) were identified with a SNP frequency of 1/78. Except for a 14-base InDel, there were 29 SNPs in coding regions, and among them, 13 were non-synonymous (9 in functional domains with 1 in a leucine-rich repeats region, 2 in a transmembrane region and 6 in a Ser/Thr kinase domain). The probability of substitution at each site was not the same, there were two hot spots, one was in the 5′-untranslated region between positions 124 and 804, and the other was in the region between positions 2520 and 3733. Sequence diversity analysis among 104 soybean genotypes showed π = 0. 00102 and θ = 0. 00218 for Rhg4. A domestication bottleneck was found because of lower sequence diversity and 58% unique SNPs loss in landraces compared with Glycine soja. Intensive selection increased the sequence diversity of cultivars, which had higher diversity and more unique SNPs than landraces.