37. Construction of a molecular map of rice and gene mapping using a double haploid population of a cross between Indica and Japonica varieties

Li-Huang ZHU1, Ying CHEN1, Yuu-Bin XU2, Ji-Chen XU1, Hong-Wei CAI3 and Zhong-Zhuan LING4

1) Institute of Genetics, Academia Sinica, Beijing, 100101
2) Agronomy Dept., Zhejiang Agric. University, Hangzhou, 310029
3) Agronomy Dept., Beijing Agric. University, Beijing, 100094
4) Institute of Crop Cultivation and Breeding, Chinese Academy of Agri. Sciences, Beijing, 100081

Ever since McCouch et al. (1988) reported the first RFLP map of rice, a great effort has been devoted to develop permanent mapping populations including doubled haploids (DH) and recombinant inbred lines (RIL) for further molecular mapping (Guiderdoni et al. 1990; IRRI 1991; Tanksley et al. 1991; Wang 1992). Availability of such populations is important for mapping quantitative trait loci and for international cooperative research. Using a differential culture medium suitable for Indica/Japonica hybrids (Chen et al. 1978), we obtained a DH population by anther culture and subsequent chromosome doubling from a cross between Indica variety, Zhai-Ye-Qing 8 (ZYQ 8) and Japonica variety, Hing-Xi 17 (JX17). The DH lines developed from the cross show phenotypic stability through generations. For the most traits, phenotypic variance within each DH line was comparable to the mean phenotypic variance of parents. The degree of transgressive segregation for the most agronomic characters in the DH population was lower than that in the corresponding F2 Population. Therefore, the DH lines was considered to be a suitable population for RFLP analysis and Qene mapping.

To construct a RFLP framework map of the DH population, the two parents were surveyed by 181 RFLP markers (provided by S.D. Tanksley) with eight restriction enzymes (EcoRI, EcoRV, HindIII, ScaI, XbaI, BamHI, BglII and DraI), and 171 markers showed polymorphism. All of the 8 isozyme markers examined in this cross also showed polymorphism. These results indicate that the polymorphism level is very high and a saturated genetic map can be con- structed from the segregating population derived from these parents.

As the first step towards constructing a saturated map, 100 RFLP markers and 8 isozyme loci were scored on the DH population. Segregation data of genetic markers were analyzed, and a linkage map was constructed using MAP- MAKER software package developed by Lander et al. (1987). These 108 markers are distributed throughout the 12 chromosomes with mean map distance of 8.5 cM (Fig. 1). The number of markers on each chromosome ranges from three (chromosome 10) to 17 (chromosome 12). This map compares well with the latest RFLP map constructed in Tanksley's laboratory (personal communication) and lays down foundation for gene mapping with this DH population.

H.W. Cai and coworkers recently identified two new isozyme markers, Est-x and Mal-2 (unpublished data). Using our RFLP map, we placed these two markers on chromosome 1 and 7 respectively. Est-x is tightly linked to RG220 with a map distance of 2.7 cM, while Mal-2 is linked to RG511 with a distance of 1.8 cM (Fig. 1).

By inoculation with blast isolate Zhong 10-8-14, ZYQ8 and JXI7 were found to be resistant and susceptible, respectively. To identify marker(s) linked to this blast resistant gene, RAPD analysis was performed to examine the co-segregation between DNA markers and blast resistance. A RAPD marker (BP127) was identified to be linked to the blast gene and mapped on chromosome 8. The distance between Bp127 and the blast gene is 14.9 cM. This gene was named Pi-zh (Fig. 1). The linkage relationship was further confirmed by using the F2 population derived from the same cross as a mapping population (unpublished data).

This DH population is also segregating many other agronomic important traits including days to heading, plant height, spikelet number, etc. We are in the process of amplifying the seeds and mapping genes for these characters.


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