The advanced generations of soybean chromosome segment substitution lines (CSSLs) carrying a unique recurrent parent background represent a permanent population, and provide ideal resources for crop improvement and quantitative trait loci (QTL) mapping, especially for minor QTL mapping. The great difference in root weight between two parental lines for CSSLs make it suitable for mapping cysts per gram root (CGR).

A research team led by Prof. WANG Congli from the Northeast Institute of Geography and Agroecology of the Chinese Academy of Sciences, collaborating with Prof. CHEN Qingshan's team from the Northeast Agricultural University, for the first time investigated SCN transgressive resistance mechanism in soybean using CSSLs. This study was published in Plant Genome.

The researchers demonstrated a wide range of distribution and transgressive lines in CSSLs for FI and CGR through phenotypic screening.  The association between FI and CGR was very low, indicating that different genes control FI and CGR. Whole-genome resequencing of CSSLs displayed 3766 hyplotype blocks containing 580,524 SNP (single nucleotide polymorphism) markers.  

Following the single marker analysis nonparametric and multiple-QTL model (MQM) mapping tests revealed 38 QTLs contributing 5.6-36.2% phenotypic variance to FI and CGR on 20 chromosomes. Gene interactions from both susceptible parents inhibit nematode reproduction which produces transgressive resistance, showing multiple minor effect QTLs. 

Therefore, both FI and CGR should be considered for breeding purposes in the absence of strong resistance genes and the identified CSSLs with low FI and CGR might be potential candidates for replacement of susceptible varieties when resistant varieties are not available. 

The findings directly confirm the presence of multiple minor QTLs in the soybean accessions either through genome‐wide association study (GWAS) or biparental population analysis, even in the susceptible genotype, and indirectly explain the wide range of phenotypic variation among soybean accessions even carrying the same major resistance genes rhg1 and Rhg4.  

The study represents SCN evaluation with CGR for the first time, and uses CSSLs for the first time to evaluate SCN FI and CGR. It provides a new way to explore transgressive resistance sources from the positive allele combination from susceptible parents.