Mapping genetic loci for flowering time, maturity, and photoperiod insensitivity in soybean

Time of flowering and maturity in soybean (Glycine max (L.) Merr) are impor tant reproductive characters of agronomic interest. These traits are useful for developing soybean cultivars with a wider geographical adaptation. The objective of this research was to use molecular markers to identify chromo somal regions that control traits for flowering time, maturity and photoper iod insensitivity in soybean. Two single-cross populations, IX132 (PI 317.3 36 x `Corsoy') consisting of 101 progeny, and IX136 (PI 317.334B x `Corsoy' ) consisting of 100 progeny, were used. Days to R1 (the day when 50% of the plants in a plot have an open flower at one of the top nodes with a fully expanded leaf) was observed among F-6:7 RI lines in the field during 1991 a nd 1992 and in the growth chamber at 12 h and 20 h photoperiods using fluor escent and incandescent lamps. Days to R3 (the number of days after emergen ce when 50% of the plants in a plot had presented the first 5 mm pod at one of the top four nodes with a fully expanded leaf was observed in the field during 1991 and in the growth chamber with 12 h photoperiod. Days to R7 (t he number of days after emergence when 50% of pods in a plot had mature pod color) was observed in the field in 1991. A total of 139 markers (88 RFLPs and 51 SSRs) in the IX132 population and 125 markers (73 RFLPs and 52 SSRs ) in the IX136 population were used to map quantitative trait loci (QTL) af fecting these traits. Results show that a large-effect QTL for days to R1, R3, and R7, and photoperiod insensitivity was found at the same location on linkage group (LG) C2 in both populations. This result suggests that photo period insensitivity, flowering time, and maturity may be controlled by the same gene(s) or by tightly clustered genes in the same chromosomal region. In addition to the large effect QTL, minor QTL were also detected controll ing the four traits in both populations. Minor QTL account for as much as 1 7.8% and 12.1% of phenotypic variance in populations IX132 and IX136, respe ctively. Thus, time of flowering, maturity, and photoperiod insensitivity i n these soybean populations are proposed to be controlled by a major QTL wi th a large effect and modified by several minor QTL.