Mapping versatile QTL for soybean downy mildew resistance.

KEY MESSAGE: Three versatile QTL for soybean downy mildew resistance in Japan were detected using five RIL populations and confirmed using recombinant fixed pairs or a backcrossed line. Downy mildew reduces soybean seed quality and size. It is a problem in Japan, where 90% of soybean grown is used as food. In the USA, 33 downy mildew races have been reported, but race differentiation in Japan is unclear. To identify quantitative trait loci (QTL) for downy mildew resistance effective in the Kanto and Tohoku regions, we performed QTL analysis using five populations of recombinant inbred lines (RILs) originated from 'Natto-shoryu' × 'Tachinagaha' (NT), 'Natto-shoryu' × 'Suzumaru', 'Satonohohoemi' × 'Fukuibuki' (SF), 'Kinusayaka' × 'COL/Akita/2009/TARC/1,' and 'YR-82' × 'Harosoy' over a 4-year period (2014-2017). We evaluated spontaneously developed symptoms of the RILs and applied 112-233 polymorphic markers to each population. Out of 31 QTL detected, we found five on chromosome 3 in three populations and another five on chromosome 7 in three populations. Other QTL were detected in one population, nine of them in different years. In the NT population, two QTL were detected in a 3.0-Mb region on chromosome 7 and in an 8.1-Mb region on chromosome 18 by evaluating nine recombinant fixed pairs in both Kanto and Tohoku regions. In the SF population, a QTL on chromosome 8 was detected in both regions. This QTL was introduced into the 'Satonohohoemi' background by backcrossing, and its effect was confirmed in both regions. In summary, two QTL on chromosomes 7 and 18 from the NT population and one QTL on chromosome 8 from the SF population were confirmed to be effective in both Tohoku and Kanto regions.

Identification and dissection of single seed weight QTLs by analysis of seed yield components in soybean.

Single seed weight (SSW), or seed size, is a seed yield components (SYC) in soybean, and it is suggested that the genetic factors regulating SSW are involved in the control of other SYCs. The quantitative trait loci (QTLs) for SSW and their effects on the other SYCs were investigated using a recombinant inbred line population derived from typical small- and large-seeded cultivars that were cultivated in two different environments. QTL analysis detected four environmentally stable QTLs for SSW, two of which coincided with the defined loci, qSw17-1 and Ln. The effects of the other loci, qSw12-1 and qSw13-1, were confirmed by analyzing residual heterozygous line progenies derived from the recombinant population. These four QTL regions were also involved in the control of an additional SYC, namely the large-seeded allele at each locus that reduced either the number of pods per plant or the number of ovules per pod. These results suggest the presence of at least two different regulatory mechanisms for SSW. Isolation of genes responsible for these QTLs provides an important tool in the understanding and utilization of SSW diversity for soybean breeding.

Effects on flowering and seed yield of dominant alleles at maturity loci E2 and E3 in a Japanese cultivar, Enrei.

'Enrei' is the second leading variety of soybean (Glycine max (L.) Merr.) in Japan. Its cultivation area is mainly restricted to the Hokuriku region. In order to expand the adaptability of 'Enrei', we developed two near-isogenic lines (NILs) of 'Enrei' for the dominant alleles controlling late flowering at the maturity loci, E2 and E3, by backcrossing with marker-assisted selection. The resultant NILs and the original variety were evaluated for flowering, maturity, seed productivity and other agronomic traits in five different locations. Expectedly, NILs with E2 or E3 alleles flowered later than the original variety in most locations. These NILs produced comparatively larger plants in all locations. Seed yields were improved by E2 and E3 in the southern location or in late-sowing conditions, whereas the NIL for E2 exhibited almost the same or lower productivity in the northern locations due to higher degrees of lodging. Seed quality-related traits, such as 100-seed weight and protein content, were not significantly different between the original variety and its NILs. These results suggest that the modification of genotypes at maturity loci provides new varieties that are adaptive to environments of different latitudes while retaining almost the same seed quality as that of the original.