Abstract seedling-vigor is important for crop establishment. There have been reported quantitative trait locus (QTL) analyses on seedling-vigor related morphological traits. However, physiological understanding of these detected QTLs is rather limited. In this study, we employed a recombinant inbred population to detect QTLs for seedling-vigor traits and physiological traits related to seedling-vigor. Germination rate and seedling growth were measured to quantify seedling-vigor.

Total amylase activity, -amylase activity, reducing sugar content, root activity and seed weight were determined. Correlations were observed between the seedling-vigor and physiological traits. QTL analysis reveals that the intervals of RG393-C1087-RZ403 on chromosome 3, C246-RM26-C1447 and R830-R3166-RG360-C734b on chromosome 5, and the interval of Waxy on chromosome 6 are the four main chromosomal regions controlling seedling-vigor. Several QTLs for amylase activities, reducing sugar content and root activity were localized in the similar regions as the QTLs for seedling-vigor. The results suggest that these traits were under the control of pleiotropic and/or closely linked QTLs. The implications of the results in the understanding of the physiological basis of seedling-vigor were discussed.

Keywords Oryza sativa L. – Seedling vigor – Physiological traits – Molecular marker – Quantitative trait locus (QTL)

• Characterization of the main effects, epistatic effects, and their environmental interactions of QTLs on the genetic basis of yield traits in rice

    Y. Z. Xing, Y. F. Tan, J. P. Hua, X. L. Sun, C. G. Xu, and Q. Zhang. 2002.Theor Appl Genet 105: 248-257

Abstract Main effects, epistatic effects and their environmental interactions of QTLs are all important genetic components of quantitative traits. In this study, we analyzed the main effects, epistatic effects of the QTLs, and QTL by environment interactions (QEs) underlying four yield traits, using a population of 240 recombinant inbred lines from a cross between two rice varieties tested in replicated field trials. A genetic linkage map with 220 DNA marker loci was constructed. A mixed linear model approach was used to detect QTLs with main effects, QTLs involved in digenic interactions and QEs. In total, 29 QTLs of main effects, and 35 digenic interactions involving 58 loci were detected for the four traits. Thirteen QTLs with main effects showed QEs; no QE was detected for the QTLs involved in epistatic interactions. The amount of variations explained by the QTLs of main effect were larger than the QTLs involved in epistatic interactions, which in turn were larger than QEs for all four traits. This study illustrates the ability of the analysis to assess the genetic components underlying the quantitative traits, and demonstrates the relative importance of the various components as the genetic basis of yield traits in this population.

Key words Quantitative trait locus – Epistasis – Genotype by environment interaction – Oryza sativa L.

• Identification of quantitative trait loci and epistatic interactions for plant height and heading date in rice

    S. B. Yu, J. X. Li, C. G. Xu, Y. F. Tan, X. H. Li, Qifa Zhang. 2002.Theor Appl Genet 104: 619-625

Abstract Appropriate heading date and plant height are prerequisites for attaining the desired yield level in rice breeding programs. In this study, we analyzed the genetic bases of heading date and plant height at both single- locus and two-locus levels, using a population of 240 F2:3 families derived from a cross between two elite rice lines. Measurements for the traits were obtained over 2 years in replicated field trials. A linkage map was constructed with 151 polymorphic marker loci, based on which interval mapping was performed using Mapmaker/QTL.

The analyses detected six QTLs for plant height and six QTLs for heading date; collectively the QTLs for heading date accounted for a much greater amount of phenotypic variation than did the QTLs for plant height.

Two-way analyses of variance, with all possible two-locus combinations, detected large numbers (from 101 to 257) of significant digenic interactions in the 2 years for both traits involving markers distributed in the entire genome; 22 and 39 were simultaneously detected in both years for plant height and heading date, respectively. Each of the interactions individually accounted for only a very small portion of the phenotypic variation.

The majority of the significant interactions involved marker loci that did not detect significant effects by single-locus analyses, and many of the QTLs detected by single-locus analyses were involved in epistatic interactions. The results clearly demonstrated the importance of epistatic interactions in the genetic bases of heading date and plant height.

Key words Oryza sativa L. Molecular mapping Heading date Plant height Quantitative trait loci Digenic interaction

• Genetic analysis of the panicle traits related to yield sink size of rice

    Cui, K. H., S. B. Peng, Y. Z. Xing, S. B. Yu, and C. G. Xu. 2002. Acta Genetica Sinica 29(2):144-152.

Abstract In the study, ten panicle traits associated wit yield sink size were measured in a recombinant inbred population derived from Zhenshan 97¡ÁMinghui 63. Generally, spikelets per panicle were more closely correlated with number of secondary branch per panicle, spikelets on secondary branch per secondary branch, and spikelet density. A total of 53 QTL were detected for ten traits in two years. Approximately 43.4% QTLs were detected in both two years, suggesting environmental effects on traits. Five chromosomal regions (G359-RG532 and C567-C86-RG236 on chromosome 1, R712-RM29 on chromosome 2, P-RG424 on chromosome 6, C148-RM258 on chromosome 10)were detected to have effects on multiple panicle traits.

QTLs for traits, which were correlated, were generally localized in similar chromosomal regions, suggesting that pleiotropy and (or) linkage are the molecular basis of relationship between them. A large number of digenic interactions were detected, 18.2% of which were detected simultaneously in both two years. The proportion of common interactions was trait-depended, ranging 8.7% for spikelets on secondary branch per secondary branch to 32.6% for panicle length.

Approximately 26.7% of common two -locus combinations had pleiotropic effects by simultaneously influencing two or more traits. Overall, the results indicate that each panicle trait is controlled by several QTLs, genotype¡Áenvironment interaction, and a large number of epistatic interactions.

Key words rice, panicle traits, yield sink size, QTL mapping, epistatic interaction