QTL Analysis for Rice Grain Length and Fine Mapping of an Identified QTL
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Theor Appl Genet (2006) 112: 1258–1270 DOI 10.1007/s00122-006-0227-0 ORIGINAL PAPER X. Y. Wan Æ J. M. Wan Æ L. Jiang Æ J. K. Wang H. Q. Zhai Æ J. F. Weng Æ H. L. Wang C. L. Lei Æ J. L. Wang Æ X. Zhang Æ Z. J. Cheng X. P. Guo QTL analysis for rice grain length and fine mapping of an identified QTL with stable and major effects Received: 8 August 2005 / Accepted: 15 January 2006 / Published online: 14 February 2006 Ó Springer-Verlag 2006 Abstract Grain length in rice plays an important role in map-based cloning of this gene and for marker-aided determining rice appearance, milling, cooking and eating QTL pyramiding in rice quality breeding. quality. In this study, the genetic basis of grain length was dissected into six main-effect quantitative trait loci (QTLs) and twelve pairs of epistatic QTLs. The stability of these QTLs was evaluated in four environments using Introduction an F7 recombinant inbred line (RIL) population derived from the cross between a Japonica variety, Asominori, Grain length and shape determine appearance in rice, and an Indica variety, IR24. Moreover, chromosome and affect milling, cooking and eating quality, and are segment substitution lines (CSSLs) harboring each of the therefore important agronomic traits in rice breeding six main-effect QTLs were used to evaluate gene action (Luo et al. 2004). Preferences for grain shape vary across of QTLs across eight environments. A major QTL de- different consumers. Long and slender grain varieties are noted as qGL-3a, was found to express stably not only in preferred in most Asian countries including China, the isogenic background of Asominori but also in the India, Pakistan and Thailand, and also in the USA, recombinant background of Asominori and IR24 under while short grain cultivars are preferred in Japan and Sri multiple environments. The IR24 allele at qGL-3a has a Lanka. In addition, grain dimension is an important positive effect on grain length. Based on the test of ad- indicator of the evolution of cereal crops because vanced backcross progenies, qGL-3a was dissected as a humans tended to select large seeds during the early single Mendelian factor, i.e., long rice grain was con- domestication, as evidenced by the fact that most culti- trolled by a recessive gene gl-3. High-resolution genetic vated species have larger seeds than their wild relatives and physical maps were further constructed for fine (Harlan 1992). However, small seed is usually favored by mapping gl-3 by using 11 simple sequence repeat (SSR) natural selection because it is frequently associated with markers designed using sequence information from more seeds per plant, early maturity, and wider geo- seven BAC/PAC clones and a BC4F2 population con- graphic distribution. Therefore, from the standpoints of sisting of 2,068 individuals. Consequently, the gl-3 gene both biological development and breeding, it is neces- was narrowed down to a candidate genomic region of sary to understand the genetic basis and formation 87.5 kb long defined by SSR markers RMw357 and mechanism of rice grain shape. RMw353 on chromosome 3, which provides a basis for Since rice grain length is quantitatively inherited (McKenzie and Rutger 1983), it is difficult for breeders to efficiently improve grain appearance using conven- Communicated by T. Sasaki tional selection methods. Thus, it should be particularly X. Y. Wan Æ J. M. Wan (&) Æ L. Jiang Æ J. F. Weng Æ H. L. Wang helpful for enhancing breeding efficiency to use markers National Key Laboratory for Crop Genetics and Germplasm closely linked to genes or major quantitative trait loci Enhancement, Jiangsu Plant Gene Engineering Research Center, (QTLs) for grain length in order to screen target geno- Nanjing Agricultural University, 210095, Nanjing, China types directly in early generations. Independent studies E-mail: [email protected] Tel.: +86-10-62186628 have identified many QTLs associated with rice grain Fax: +86-10-62186628 length using primary mapping populations (Huang et al. 1997; Redon˜ a and Mackill 1998; Tan et al. 2000; Li et al. J. K. Wang Æ H. Q. Zhai Æ C. L. Lei Æ J. L. Wang Æ X. Zhang 2004b; Aluko et al. 2004). However, in all these studies Z. J. Cheng Æ X. P. Guo Æ X. Y. Wan Æ J. M. Wan Institute of Crop Science, Chinese Academy of Agricultural the grain length was evaluated in a single environment, Sciences, 100081, Beijing, China so the stability of the resultant QTLs could not be 1259 determined. The expression stability of a QTL is critical gene, by using secondary F2 and F3 populations derived in determining its usefulness in marker-assisted selection from a cross between the target CSSL and the recurrent (MAS) breeding. Of the 21 QTLs for grain length de- parent; and (3) to locate the gl-3 gene to a narrow tected in these studies cited above, one has been con- genomic region based on SSR markers produced from sistently identified near the centromeric region of rice sequence information of the japonica cultivar, cv. chromosome 3, with the average percentage of pheno- Nipponbare, and then compare the location relationship typic variation explained (PVE) of 33.5%. One basic between the gw3.1 (Li et al. 2004a) and gl-3 genes from question regarding this QTL still remains, i.e., whether different rice germplasm, respectively. the QTL corresponds to only one locus or to a chro- mosomal region consisting of a cluster of genes, each with relatively small genetic effects. This question has to Materials and methods be answered before we further develop MAS strategies or clone this QTL. Therefore, it is necessary to establish Plant materials whether the QTL is a single Mendelian factor and to narrow down its location on the genetic linkage map. Four populations, RILs, CSSLs, BC F and BC F , Primary populations such as F , RIL, BC and DH 4 2 4 3 2:3 were used in this study. are not appropriate for fine mapping of a single QTL, Seventy-one F RILs were derived from the cross because they segregate whole parental chromosomal 7 between Asominori and IR24 by single-seed descent segments simultaneously (Yamamoto et al. 2000). On (Tsunematsu et al. 1996). To produce a series of CSSLs the contrary, chromosome segment substitution lines in a largely Asominori background, 19 selected RILs (CSSLs) or near-isogenic lines (NILs) have distinct were crossed and then backcrossed with Asominori, advantages for QTL identification. Most importantly, without selection, until the BC F generation. Then 66 genetic interactions between donor alleles are limited to 3 1 individuals were selected at BC F on the basis of a those between genes on homozygous substituted tracts 3 1 whole genome survey (116 RFLP loci) and they were since each CSSL/NIL carries one or a few donor seg- denoted as CSSL1-CSSL66, which have representation ments in the near-isogenic background of the recurrent of the whole IR24 genome, except for the 9.8 cM region parent, thus reducing the effects of interferences from defined by the interval C1468-G1015 on chromosome 3 genetic background (Howell et al. 1996). In addition, (Kubo et al. 1999; Wan et al. 2004a). secondary F and F populations can be derived from a 2 3 Of four long-grain lines (CSSL16, 17, 18, and 46) further backcross between a selected CSSL/NIL and the harboring the gl-3 allele, CSSL18 was used to build the recurrent parent, and then be used in the fine mapping secondary F (BC F )andF (BC F ) populations by and positional cloning of a QTL (Yamamoto et al. 1998; 2 4 2 3 4 3 backcrossing to Asominori with subsequent self- Frary et al. 2000; Yano et al. 2000; Yan et al. 2004). pollination. Recently, a grain-weight QTL, gw3.1, was narrowed down to a 93.8-kb interval in the centromeric region of rice chromosome 3, by using a set of NILs derived from the cross between an Oryza sativa, cv, Jefferson and O. Field experimental design rufipogon (IRGC105491) based on five generations of backcrossing and seven generations of selfing. The This study was carried out in ten environments, E1-E10, dominant O. rufipogon allele at gw3.1 conferred small including four locations and four cropping seasons seed size and might associate with domestication in rice (Table 1). Asominori, IR24 and their 71 RILs were (Li et al. 2004a). Otherwise, stability of QTLs for rice grown in four environments (E1–E4), and the parental grain dimension was studied across eight environments varieties and their 66 CSSLs in eight environments (E1– using a population of 66 CSSLs derived from three E8). Each experimental plot consisted of two replicates, backcrosses of IR24 to Asominori (Asominori/IR24// consisting of ten rows each of ten plants, grown in a 3*Asominori) (Wan et al. 2005). A grain-length QTL, randomized block design. At maturity, each plot was qGL-3, with an average PVE of 32.8%, was mapped to harvested in bulk. After drying, grains were stored at an 18.1-cM interval in the centromeric region of rice room temperature for 3 months, and then milled using chromosome 3 and was consistently detected in all the the method described by Yamamoto et al. (1995). The eight environments, indicating that the IR24 allele at milled rice thus obtained was used for determining grain qGL-3 had a significant and stable effect on increasing length. grain length in the homozygous genetic background of Using a spacing pattern of 25 cm (between rows) · Asominori. 13.3 cm (within rows), 214 BC4F2 plants were grown in The objectives of this study were: (1) to determine environment E9, and 214 BC4F3 families (16 plants per whether or not the epistatic effect of QTLs in the re- family) in E10.