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Transgressive Segregation of Tiller Angle in Rice Caused by Complementary Gene Action

Dep. of Agronomy, Zhejiang Agric. Univ., Hangzhou 310029, China
Dep. of Plant Breeding and Biometry, Cornell Univ., Ithaca, NY 14853

^* Corresponding author (e-mail: srm4{at}cornell.edu).

Transgressive segregation is often observed for quantitative traits in the offspring of both intraspecific and interspecific hybrids. Understanding its genetic base is important for the improvement of these characters. In this research, we began with four indica cultivars, 5002, Zhu-Fei 10, HA79317-7, and Zhen-Long 13, all having similar tiller angles. F₂ populations were derived from the crosses 5002/Zhu-Fei 10 and HA79317-7/Zhen-Long13, and divergent selection for tiller angle was practiced in each F₂ population. As a result of selection, two types of true-breeding phenotypic extremes were obtained, one with larger tiller angle and the other with smaller tiller angle. for loci contributing to variation in tiller angle, the alleles of similar effect were proved to be dispersed in the original parents and associated in the extreme selections. Crossing two extreme strains derived from different original crosses revealed new transgressive segregation in the F₂, demonstrating that different loci controlled tiller angle in the two cross combinations examined in this study. When the extreme phenotypes were crossed with each other, strains with the largest tiller angle represented recombinants containing all increasing alleles from the four original parents. Four quantitative trait loci (QTLs) were inferred to be responsible for the transgressive segregation in the two original crosses. The intervarietal transgression could be explained by complementary action of additive genes that had been dispersed among the original parents. To test for allelism of the QTLs for large tiller angle and a previously identified major-gene mutant, lazy (la), one of the extreme strains from the second cycle of selection having the largest tiller angle was crossed with the lazy growth mutant, H-79. Results indicated that la was not allelic to any of the QTLs. Breeding experiments demonstrated that multiple crossing followed by one cycle of selection could be used to accumulate into one strain the genes that were dispersed in the parents.

Received for publication November 8, 1996.

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