Assembly, comparative analysis, and utilization of a single haplotype reference genome for soybean

Abstract

Cultivar 'Williams 82' has served as the reference genome for the soybean research community since 2008, but is known to have areas of genomic heterogeneity among different sub-lines. This work provides an updated assembly (version Wm82.a6) derived from a specific sub-line known as 'Wm82-ISU-01' (seeds available under USDA accession PI 704477). The genome was assembled using Pacific BioSciences HiFi reads and integrated into chromosomes using HiC. The 20 soybean chromosomes assembled into a genome of 1.01Gb, consisting of 36 contigs. The genome annotation identified 48,387 gene models, named in accordance with previous assembly versions Wm82.a2 and Wm82.a4. Comparisons of Wm82.a6 with other near-gapless assemblies of 'Williams 82' reveal large regions of genomic heterogeneity, including regions of differential introgression from the genotype 'Kingwa' within approximately 30 Mb and 25 Mb segments on chromosomes 03 and 07, respectively. Additionally, our analysis revealed a previously unknown large (~20 Mb) heterogeneous region in the pericentromeric region of chromosome 12, where Wm82.a6 matches the 'Williams' haplotype while the other two near-gapless assemblies do not match the haplotype of either parent of 'Williams 82'. In addition to the Wm82.a6 assembly, we also assembled the genome of soybean line 'Fiskeby III', a rich resource for abiotic stress resistance genes. A genome comparison of Wm82.a6 with 'Fiskeby III' revealed the nucleotide and structural polymorphisms between the two genomes within a QTL region for iron deficiency chlorosis resistance. The Wm82.a6 and 'Fiskeby III' genomes described here will enhance comparative and functional genomics capacities and applications in the soybean community.