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The USDA Soybean Isoline Collection has been an invaluable resource for the soybean genetics and breeding community. This collection, established in 1972, consists of 611 near-isogenic lines (NILs) carrying one or multiple genes conferring traits that had been determined to exhibit Mendelian inheritance. It has been used in multiple studies on the genetic basis, physiology, and agronomy of these qualitative traits. Here, we used publicly available genotype (SoySNP50K), phenotype, and pedigree data on this collection to characterize the isogenicity of the NILs and identify chromosomal positions of unmapped genes. A total of 368 NILs had at least 80% identity to their recurrent parent and, thus, were useful for what can be called introgression mapping. Both on-target and off-target introgressions were evaluated. The size of on-target introgressions into individual NILs ranged from 61 kb to 8.4 Mb, whereas off-target introgressions ranged from 2.6 kb to 54.8 Mb. The observed large off-target introgressions indicated that some NILs carry introgressions nearly the size of an entire chromosome. By applying introgression mapping to genes that had never been mapped, we identified the likely chromosomal positions of six such genes: ab, im, lo, Np, pc, and Rpm. The size of mapping intervals was large in some cases (10.28 Mb for im) but small in others (0.21 Mb for Np). The results reported herein will provide future researchers with a resource to help select informative NILs for future studies, and provide a starting point to further fine map, and ultimately clone and functionally characterize these six soybean genes., (© 2023 The Authors. The Plant Genome published by Wiley Periodicals LLC on behalf of Crop Science Society of America.)

CORE IDEAS: 'Fiskeby III' harbors a combination of abiotic stress traits, including iron deficiency chlorosis (IDC) tolerance. An IDC quantitative trait locus on chromosome Gm05 was identified in genome-wide association studies and biparental populations. Fine-mapping resolved a 137-kb interval containing strong candidate genes. Iron deficiency chlorosis (IDC) is an important nutrient stress for soybean [Glycine max (L.) Merr.] grown in high-pH soils. Despite numerous agronomic attempts to alleviate IDC, genetic tolerance remains the most effective preventative measure against symptoms. In this study, two association mapping populations and a biparental mapping population were used for genetic mapping of IDC tolerance. Quantitative trait loci (QTLs) were identified on chromosomes Gm03, Gm05, and Gm06. Heterogenous inbred families were developed to fine-map the Gm05 QTL, which was uniquely supported in all three mapping populations. Fine-mapping resulted in a QTL with an interval size of 137 kb on the end of the short arm of Gm05, which produced up to a 1.5-point reduction in IDC severity on a 1 to 9 scale in near isogenic lines., (© 2019 The Author(s).)