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Article Excerpt
SOYBEAN provides a major source of protein for human consumption and is one of the most important crops in the world. More than 20000 accessions of soybean have been collected in situ in China. As crop germplasm collections grow larger, genetic diversity studies become of increasing importance because they provide information on genetic relationships among germplasm accessions (Hoisington et al., 1999; Liu et al., 2003). Such knowledge can provide insight into crop origins and evolution, and may be useful for designing strategies to establish core collections (Matus and Hayes, 2002; Roussel et al., 2004). Collectively, this information is helpful for designing future breeding efforts to improve soybean yield, quality, production, and pest management.
Chinese soybean production is generally subdivided into Northern Spring, Huanghuai Valley Summer (Hsu), and Southern production regions. Within these three primary regions, soybean production can be further divided either into 10 sub-regions according to the mode of production and the geographic region of origin (Bu and Pan, 1982), or as seven ecotypes according to eco-geographic regions of origin and planting system. The seven primary Chinese ecotypes include Northeastern Spring soybean (NEsp); Northern Spring soybean (Nsp); Huanghuai Valley Summer soybean (Hsp); Hsu; Southern Spring soybean (Ssp); Southern Summer soybean (Ssu), and Southern Autumn soybean (Sau) (Fig. 1). Planting systems consist of single cropping in Northern regions, double cropping in the Huanghuai region, and multiple cropping in the Southern regions. Since most soybean breeding programs have focused on variation within ecotypes, this factor is of major importance in Chinese soybean classification systems. Comparative genetic studies have mainly concentrated on accessions within ecotypes, but some have used province of origin and/or latitude of cultivation.
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Evaluation of agronomic traits, pedigrees, geographic origins, isozymes, and DNA markers have been used for the assessments of soybean genetic diversity (Perry and McIntosh, 1991; Griffin and Palmer, 1995; Gizlice et al., 1996; Bernard et al., 1998; Dong et al., 2004). On the basis of isozyme data, Gorman (1984) reported that cultivated soybean accessions in Northeastern China were more diverse than those from Korea. However, wild soybean (G. soja Sieb. & Zucc.) accessions were more diverse in Korea than in China. Cui et al. (2000) analyzed Chinese soybean cultivars based on coefficients of parentage, and deduced a high level of genetic diversity among soybean breeding lines. On the basis of agronomic trait variation, Dong et al. (2001) defined three genetic diversity centers of Chinese wild soybean: the northeast, the Yellow River, and the coastal region of Southeastern China. Dong et al. (2004) also analyzed the phenotypic diversity of cultivated soybean resources based on agronomic traits, and pointed out that the phenotypic diversity center of cultivated soybean was the Yellow River Valley.
Because of (i) the limited data provided by isozymes, (ii) the influence of growing environment on agronomic trait evaluation, and (iii) possible errors or incomplete information in the documentation of pedigrees and origins of accession collections, these methods of assessing genetic diversity have largely been replaced by DNA marker analysis. DNA markers are stable and have proven to be genetically informative and useful for genotype discrimination (Keim et al., 1992; Skorupska et al., 1993; Nelson and Li, 1998). The SSRs (micro-satellites) are codominant polymorphic markers, with a high information content per locus (Powell et al., 1996; Diwan and Cregan, 1997; Abe et al., 2003), and are suited to high throughput fingerprintings of large numbers of accessions. A number of diversity studies have employed DNA markers, but the samples surveyed have been limited (Liu et al., 2000; Hai et al., 2002).
Because it is widely planted in different ecogeographic regions in China, soybean has evolved into various ecotypes following long-term natural and artificial selection. An overview of the genetic diversity of Chinese soybean using DNA molecular markers should therefore be useful for soybean breeding and genetic studies. In this paper, we report a diversity analysis based on allelic variation at 60 SSR loci among 129 Chinese soybean genotypes. These accessions were selected based on their ecotypes and phenotypic variations for morphological and agronomic traits from the >20 000 Chinese accessions held ex situ. We also sought to dissect the genetic structure of the Chinese soybean collection, and to apply this information to develop an optimal grouping system suitable for a stratified sampling strategy to generate a representative core collection of Chinese soybean.
MATERIALS AND METHODS
One hundred and twenty-nine accessions, consisting of 122 landraces and seven cultivars, were selected from the entire Chinese ex situ soybean germplasm collection to represent the range in phenotypic variability for 14 agronomic and morphological traits (Table 1). These materials included 17 NEsp, 25 Nsp, 20 Hsp, 18 Hsu, 16 Ssp, 20 Ssu, and 13 Sau ecotype accessions from 21 provinces that spanned a latitude from 21[degrees]35' N to 49[degrees]10' N. Of the seven cultivars, three and two cultivars were from NEsp and Hsu respectively, and one each...
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