Rice Genetics IV - IRRI books - International Rice Research Institute

unavailable until recently with the advent of molecular marker technology and highdensitymolecular linkage maps. Genetic analyses based on high-density molecularmaps have been reported recently in maize and rice (Stuber et al 1992, Xiao et al1995, Yu et al 1997).In the past ten years, our group has conducted a series of genetic and molecularstudies with the long-term goal of elucidating the biological mechanism of heterosisusing rice as the model system. This series includes characterization of the relationshipbetween molecular marker heterozygosity and hybrid performance and heterosisin a wide range of rice germplasm (Zhang et al 1994, 1995, 1996, Saghai Maroof et al1997, Zhao et al 1999). The general finding of such analyses is that the correlationbetween molecular marker heterozygosity and heterosis varies with the germplasmanalyzed, which suggested the need for a detailed understanding of the genetic basisof heterosis. To characterize the genetic basis of heterosis, we have focused our effortson analyzing genetic materials derived from Shanyou 63, a cross betweenZhenshan 97 and Minghui 63, the most widely used hybrid in rice production in China.The hybrid of this cross is highly heterotic and was planted with an annual area of 6.7million hectares in its peak period, accounting for approximately 25% of the riceproduction of China. The most noticeable finding of the analysis is the existence of alarge number of epistatic interactions in the genome that play an important role in thegenetic basis of heterosis (Yu et al 1997). Another component of our work is theassessment of the relationship between gene expression and heterosis, with the objectiveof identifying genes involved in the manifestation of heterosis.In this chapter, we will report on our studies in characterizing the genetic basis ofheterosis and the analyses of the relationship between gene expression and heterosis.Analyzing the genetic basis of heterosis using an F 2:3 populationMost of this analysis has been published previously (Yu et al 1997). We will give abrief description here for the purpose of comparison.The experimental population and data collectionThe experimental population consisted of 250 F 3 families, each derived from selfed(bagged) seeds of a single F 2 plant, from a cross between Zhenshan 97 and Minghui63. The F 3 families, two parents, and F 1 were transplanted to a bird-net-equipped fieldon the experimental farm of Huazhong Agricultural University in the 1994 and 1995rice-growing seasons in Wuhan, China. Traits examined were yield per plant, tillersper plant, grain weight, and grains per panicle. The 250 families were assayed using150 molecular markers that detected polymorphisms between the parents.Single-locus QTLs for yield and yield component traitsA molecular marker linkage map was constructed using Mapmaker 3.0 (Lincoln et al1992a), which spanned a total of 1,842 cM in length and well integrated the markersfrom the two high-density restriction fragment length polymorphism (RFLP) linkagemaps (Causse et al 1994, Kurata et al 1994). The QTLs (quantitative trait loci) given174 Qifa Zhang et al