Fig. 1. Distribution of IR64 alleles by RFLP markers (A) and doubled haploid lines (B).
International Rice Research Institute, P. 0. Box 933, Manila, Philippines
A doubled haploid population was recently developed by Guiderdoni et al. (1992). The population, derived from a cross between indica rice variety IR64 and japonica variety Azucena, consists of 135 doubled haploid lines. Since many characters, such as resistance to blast, bacterial blight and aroma are segregating in this population, it can be used to map genes for these characters. As a first step in gene mapping, a restriction fragment length polymorphism (RFLP) map was developed covering all 12 rice chromosomes.
Six restriction enzymes (DraI, EcoRV, HindIII, ScaI, XbaI, EcoRI) were used for the parental polymorphism survey. A total of 135 polymorphic RFLP markers were used to survey 135 doubled haploid lines. Because japonica rices are generally easier to regenerate in tissue culture than indicas, we were interested to assess the relative frequency of IR64 and Azucena alleles in the population used in this study. The frequency of IR64 alleles for each marker and each line was calculated and is shown in Figures 1A and 1B. The distribution was roughly symmetrical around 0.5, suggesting that there was no overall bias toward either parent. Taking the population as a whole, we determined that it carried 49.5% IR64 loci and 50.5% Azucena loci, indicating that an equal amount of genetic material from each parent has been transmitted to the progenies through anther culture.
As shown in Figure 1, some markers and lines had a higher frequency of alleles from one parent. Of the 135 marker loci analyzed, the allele frequency at 52 marker loci deviated significantly (p<0.05) from the expected 1:1 ratio. The allelic frequency at 22 loci was biased toward IR64 and it was biased towards Azucena alleles at 30 loci. These skewed marker loci were clustered on 9 chromosomal segments shown by letter A (skewed for Azucena) and I (skewed for IR64) in Fig. 2. Similar results were obtained based on an analysis of the allele frequency in each doubled haploid line. A chi-square test indicated that 25
Fig. 1. Distribution of IR64 alleles by RFLP markers (A) and doubled haploid
lines (B).
lines (18.5%) contained significantly more IR64 loci and 29 lines (21%) contained more Azucena loci (expected ratio, 1:1; p<0.05).
The 135 polymorphic markers identified in the parental survey were used to construct a linkage map (Fig. 2) using the computer program MAPMAKER (Lander et al. 1987) and map distances estimated by the Kosambi function. The RFLP linkage map covered 1,811.3 cM. This genome coverage was about the same as the RFLP maps published recently (Causse et al. 1994; Saito et al. 1991; Nagamura et al. 1993). The order of the markers on the map was comparable to that published by Tanksley et al. (1992). The average distance between markers was 13.4 cM, with seven gaps larger than 35 cM. This map lays down the foundation for mapping genes for characters segregating in this population and it can serve as an additional mapping population to place DNA markers onto the RFLP map.
References
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