| A population of 105 DH lines derived from a cross between irrigated indica variety IR64 and upland japonica variety Azucena was used in the experiments. The genetic linkage map of this population, containing 175 markers distributed among 12 chromosomes. In the present research, the dynamic behavior of four seedling traits including seedling height, root length, leaf number, and seedling weight after 3-leaf stage cold treatment were analyzed for detecting QTLs associated with additive and epistatic effects as well as their QE interaction effects by combining the statistical procedures for analyzing conditional genetic effects (Zhu 1995) and the QTL mapping method based on mixed model approaches (Wang et al. 1999; Zhu 1999). The QTL mapping analysis revealed the following results:1. Through analyzing the data of four seedling traits at five measuring stages in two treatments, an important result could be obtained, that is, while a set of QTLs with relatively high magnitude of effects (those QTLs with significant genetic main effects) held the key to growth under different environments (treatments), few QTLs (those detected with significant QE effects) may, however, determine the differential response of the crop to environmental variation which could inversely guide the breeders to formulate an appropriate breeding strategy. In other words, selection on QTLs with additive main effects applied to different environments (treatments), while selection on QTLs with QE interaction could adapt to a special environment. And the alleles contributed to the seedling traits dispersed between two parents. In the study, the numbers ^ magnitude and direction of QTLs with conditional QE effects were different for different stages and traits determined the dissimilar growth resuming rates at different stages and for different seedling traits. Furthmore, the phenomenon of pleiotropic effects or close linkage of genes were the main causes for correlations among traits which resulted in selection for one will result in a correlated response with another.2. QTL mapping studies using age-specific measures clearly demonstrated distinct age-specific gene action. In our study, using unconditional and conditioanl mapping methods, there were 23 QTLs with additive and additive x treatment effects for seedling height, 22 QTLs for root length, 14 QTLs for leaf number, and 23 QTLs for seedling weight, respectively. Butonly 3, 1,0, and 2 QTLs with significant additive effects at all measuring stages for seedling height, root length, leaf number, and seedling weight, respectively. And other QTLs could be detected only at one or several stages. This suggested that genes controlling these seedling traits expressed selectively at different stages. Conditional mapping method could estimate and detect the net effects of gene expression that were independent of the causal genetic effects in earlier stages. And could reveal the gene expression pattern efficiently at different stage. This technique could serve as an important tool for studying the inheritance of response to stress.3. The present study showed the importance of epistasis as a genetic basis of the quantitative traits and revealed several important features of this phenomenon. In the study, with two mapping methods, there were 30 pairs of QTLs involved in epistasis for seedling height, 36 pairs for root length, 33 pairs for leaf number, and 25 pairs for seedling weight with proportion of 50%, 47%, 62%, and 42% for each seedling traits, respectively, without additive and/or additive x treatment interaction effects. Moreover, it was common for several loci to interact with more than one non-allelic locus at the same stage and/or across different stages, and, the interactions often showed differences in the direction of effects. It was indicated that the possibility of multilocus associations existed for trait development. Beside this, a lot of interactions could detected only by conditional mapping method indicated that many epistasis interactions... |
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