V. Molecular Mapping and QTL Analysis

between wild and cultivated rice strains

H.W. Cai1 and H. Morishima2

National Institute of Genetics, Mishima, 411-8544) Japan

Nishinasuno, Tochigi, 329-2742 Japan;

2)15-2 Saiwaicho, Hiratuka, 254-0804 Japan

Flowering time is an important trait both for cultivated and wild plants to reproduce themselves in their habitats. Growth duration or days to heading (DTH) in rice can be divided into two phases; basic vegetative phase (BVP) and photoperiod sensitive phase (PSP) (Chang et al. 1969). Several major genes for DTH, BVP and PSP have been identified by many workers.

To identify quantitative trait loci (QTL) for heading date, we used 118 recombination inbred (RI) lines derived from a cross between wild rice 0. rufipogon (W1944) and cultivated rice 0. sativa (Pei-Khu). One hundred forty markers were mapped and employed for QTL analysis. Details of map construction are reported elsewhere (Cai and Morishima submitted). Heading dates were observed in 1995 (F6), 1996 (F7) and 1997 (F7) under natural day length. In addition, in 1996 the same set of materials were also tested in automatic short-day field (11.5 hours). Days to heading observed under short day condition and difference between DTH under short-day and long (natural) day conditions were tentatively regarded as BVP and PSP, respectively. Difference in DTH under natural day-length between 1995 and 1996 was regarded as temperature sensitive phase (TSP), as these two years markedly differed in summer temperature.

DTH, BVP, PSP and TSP were analyzed by simple interval mapping (SIM) and composite interval mapping (CIM) using MQTL software (Tinker and Mather 1995). SIM (0.05 significant level, F = 10.9) revealed three DTH loci and six BVP loci (data not shown). CIM (0.05 significant level, F = 10.9) revealed some more loci for DTH and BVP as shown in Table 1. In addition, four PSP loci were identified. Among DTH loci, qDTH-1 and qDTH-8 are accompanied by BVP and PSP loci closely or within the region flanked by the same markers. These two regions might affect both BVP and PSP genes or carry two linked genes. qDTH-3, qDTH-4-1, qDTH-5-1 and qDTH-l1-1 were accompanied by one BVP gene, respectively. QTL for TSP were not found. Composite interval mapping, by which data taken under multiple environments can be analyzed, enabled us to test interaction between QTL for DTH and year. qDTH-2-2 showed significant interaction with year effect. This locus might be responsible for temperature sensitivity.

Direction of additive gene effect mostly coincided with that predicted by phenotypes of parents. Wild alleles increase DTH, BVP and PSP. Only one BVP locus qBVP-4 showed opposite effect.In addition to the genes for heading behavior detected by conventional genic analysis, marker assisted studies revealed many QTLs (Li et a!. 1995, Xiao et a!. 1996). More recently, Yano eta!. (1997) detected two DTH QTLs with relatively large effect and three with smaller effect based on Nipponbare / Kasalath cross. It seems difficult to compare QTLs we found with the known loci reported by others. qPSP-6 flanked by R2l71 and RZ144 might be Sel, though it should be confirmed through fine linkage mapping.

Table 1. QTLs for heading behavior detected using composite interval mapping

 

References

Cai, H.W. and H. Monshima, 1998. QTL clusters reflect adaptive syndromes of domestication in rice. (submitted to TAG).

Chang, T.T., C.C. Li and B.S. Vergara, 1969. Component analysis of duration from seeding to heading in rice by the basic vegetative phase and the photoperiod-sensitive phase. Euphytica 18: 79-91.

Li, Z., S.R.M. Pinson, J.W. Stansel, W.D. Park, 1995. Identification of quantitative trait loci (QTLs) for head-ing date and plant height in cultivated rice (Oryza sativa L.). Theor. Appl. Genet. 91: 374-381.