Genetic analysis of net-like cracking in soybean seed coats

Cracking of seed coats in soybean (Glycine max (L.) Merr.) deteriorates the external appearance of seeds and reduces their commercial value. Two types of cracking have been reported that occur in some cultivars: Type I with irregular cracks and Type II with net-like cracks. This study was conducted to determine the genetic basis of net-like cracking. Genetic analysis was performed using F-1 plants produced by crossing Uzuramame, a Japanese landrace with black seed coats having net-like cracking and a Clark mutant with black seed coats, their F-2 population and F-3 lines. Degree of cracking in individual plants was calculated by averaging cracking index (no cracking: 0 to severe cracking: 4) of total or 100-seed samples (average cracking index, ACI). Uzuramame exhibited intense cracking, whereas the Clark mutant showed slight cracking. Intermediate degree of cracking in F-1 plants suggested incomplete dominance. ACI of F-2 plants was continuously distributed. Gene number involved was estimated to be 1.4 by Wright's method. All F-3 lines derived from F-2 plants with ACI more than 2.8 displayed severe cracking phenotypes. In contrast, F-3 lines derived from F-2 plants with ACI less than 2.8 segregated from low to high ACI (0.5 to 3.2). When F-2 plants were classified as slight (ACI<2.8) or severe (ACI>2.8) cracking, the frequency distribution of the F-2 plants fitted to a 3:1 ratio. Genotypes of SSR marker Satt264 that is closely linked to SoyPRP1 locus for proline-rich cell wall protein had a minor effect on ACI. Further, seed weight was positively associated with ACI (r = 0.46**). Our results suggest that net-like cracking is controlled primarily by a major gene, and SoyPRP1 and gene(s) contributing to seed weight may have minor effects on the intensity of cracking.