Seed dormancy in rice is an important agronomic trait as it relates to preharvest sprouting. Rice varieties differ significantly in their degree of seed dormancy. N22, an indica-type traditional cultivar, displayed the strongest seed dormancy among the rice cultivars (Gu et al. 2003). Based on conventional genetic analysis, a mono - and digenic model for seed dormancy in N22 were proposed respectively (Seshu and Sorrells 1986, Gu et al. 2004). But the chromosome locations of those genes have not been determined. In this study, we compared QTL mapping of seed dormancy by use of two BC₁-type populations: NanJing 35 (a japonica breeding line with nondormant) /N22//NanJing 35 (Population I, PI) and USSR5 (nondormant japonica cultivar)/N22//USSR5 (Population II, PII), and a F₂ population of a hybrid USSR5/N22 (Population III, PIII).

All parents and populations (PI, PII and PIII contained 168, 82 and 148 individuals, respectively) were grown in the experimental farm at Weigang Campus of Nanjing Agricultural University in 2002. Seeds were collected from each individual on the 35th day after heading and immediately placed under conditions of 30C and 100% relative humidity for 7 days. And the germination percentage of each individual was measured as the degree of seed dormancy. Using data from PI, PII and PIII, linkage maps consisting of 137, 123 and 121 SSR markers were constructed, respectively. QTL analysis was performed using the threshold of LOD>2.0 for Windows QTL Cartographer 1.13a based on composite interval mapping (Wang et al. 1999).

In population I, four QTL for seed dormancy, qSdn-1, qSdNj-3, qSdn-5 and qSdn-7, were identified on chromosome 1, 3, 5 and 7, respectively (Table 1, Fig. 1). The N22 alleles increased seed dormancy, except for qSdNj-3 on chromosome 3. The QTL qSdn-5, which had major effect with a LOD value of 7.19, and explained 16.0% of the observed phenotypic variation, was significantly different from other loci. One of them, qSdn-1 was identified with LOD score of 4.59, and explained 8.6% of the observed phenotypic variation. And the other 2 QTLs, qSdNj-3 and qSdn-7, have minor effects, which were detected with LOD value less than 3.00, and only explained 6.4% and 3.8% of the observed phenotypic variation respectively.

In population II, two QTL for seed dormancy, qSdn-1 and qSdn-11, were identified on chromosome 1 and 11, respectively (Table 1, Fig. 1) . These 2 QTL were also identified in population III with negative additive effect (Table 1, Fig. 1). Furthermore, the QTL qSdn-1, which had the highest significant effect with LOD values of 4.05 and 3.53, and explained 18.7% and 12.0% of the observed phenotypic variation in population II and III, respectively, was de-

tected in all the three populations. Such highly repeatable effect in the three populations may indicate the stability of this QTL and its potential usefulness for improving rice pre-harvest sprouting tolerance using marker-assisted selection (MAS). In addition, qSdn-5 was detected in population III with the lowest effect and was not detected in population II, though this locus was detected with highest significant level in population I. On the basis of the results from a F₂ population, dominance effect could be estimated. And the three QTL detected in population III displayed partial dominance (Table 1). All the data on seed dormancy in three populations showed that genetic mechanisms underlying seed dormancy of cultivated rice: N22 was more complicated than what was revealed by traditional genetics methods.

Reference

Gu, X.Y., Z.X. Chen and M.E. Foley, 2003. Inheritance of seed dormancy in weedy rice. Crop Sci. 43: 835-843.

Seshu, D.V. and M.E. Sorrells, 1986. Genetic studies on seed dormancy in rice. In: Rice genetics 1. International Rice Research Institute, Manila, Philippines. pp. 369-382.

Gu, X.Y., S.F. Kianian and M.E. Foley, 2004. Multiple loci and epistases control genetics variation for seed dormancy in weedy rice (Oryza sativa). Genetics. 166: 1503-1516.

Wang, S.C., B.Z. Zeng and C.J. Basten, 1999. QTL Cartographer Windows, Version 1.13. Department of Statistics, North Carolina State University, Raleigh, NC. (http://statgen.ncsu.edu/qtlcart/WQTLCart.htm).