MAS in Hybrid Rice Breeding

Attributes such as fertility restoration, PGMS, TGMS, and reverse TGMS are either difficult to evaluate or can be evaluated only in the progeny of test crosses. In hybrid rice-breeding programs, these genes are often transferred to different genetic backgrounds to develop inbred lines by successive backcrossing. Development of molecular tags for these traits would allow selection at the seedling stage, resulting in considerable savings in both time and effort.

Komori et al. (2003) fine mapped the Rf-1 gene that restores the pollen fertility in BT-type male sterile cytoplasm by using nine PCR-based markers developed from tightly linked RFLP markers on chromosome 10. Due to the tight linkage of the Rf-1 to the flanking markers S12564 Tsp509I and C1361 MwoI, it will now be possible to transfer the Rf-1 gene more efficiently and precisely. For the same restorer gene, a number of PCR-based markers have been developed and utilized by many other investigators (Akagi et al. 1996; Ichikawa et al. 1997; Mishra et al. 2003). The discovery of the tight linkage of the marker R2349 with the wide-compatibility gene S5 (Liu et al. 1997) provides an opportunity to transfer the S5n alleles to different varieties in intersubspecific hybrid breeding.

Introgression of the TGMS or reverse TGMS gene through conventional breeding is cumbersome because it involves identification of TGMS plants in the segregating generation followed by induction of fertility by rationing at appropriate temperatures. Lang et al. (1999) developed both dominant and codomi-nant STS markers from the RAPD markers linked to the tms3 gene and reported an accuracy of 85% in MAS at the vegetative stage. For tms4(t) on chromosome 2, Dong et al. (2000) converted an AFLP marker E5/M12-600 mapped at a distance of 3.3 cM into an STS for marker-assisted transfer of this gene to different genetic backgrounds. Wang et al. (2003b) developed one STS marker, C-365-1, and another CAPS marker, G227-2, that flanked the tms5 gene at a distance of 1.04 and 2.08 cM, respectively. Lopez et al. (2003) reported a successful transfer of tms2 from Norin PL12 to an aromatic Thai cultivar KDML 105 using linked

SSR markers RM2 and RM11 on chromosome 7. The accuracy of selecting sterile plants during segregating generation was more than 90%. Jia et al. (2001) se-quenced and converted a closely linked AFLP marker, rev1, 4.2 cM from the rtms1 gene into a SCAR marker that could facilitate MAS of the rtms1 gene.

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