Molecular polymorphisms and heterotic potential of rice CMS lines

The Philippine government has launched a hybrid rice production program aimed at increasing national rice production and attaining rice-self sufficiency. A strong hybrid breeding component is crucial to this program's long-term sustainability. PhilRice has been diversifying its cytoplasmic-genetic male sterile (CMS) germplasm, the basis for the three-line hybrid system that is the major focus of hybrid rice breeding in the Philippines, in order to widen the genetic base of F1 hybrids, thus, enhancing heterosis or hybrid vigor while avoiding genetic vulnerability. Molecular tools have been used in combination with conventional strategies to facilitate hybrid breeding. In this study, the nuclear genome diversity of 22 CMS lines were assayed at loci amplified by 20 microsatellite and 25 RAPD primers, and 10 + 3/+3 AFLP primer combinations. Seventy-seven alleles were detected at microsatellite loci, and 73 RAPD and 72 AFLP markers showed polymorphism among the CMS lines. Microsatellites and AFLPs were the most effective in identifying specific CMS lines hence, hybrid variety protection. Cluster analysis based in 222 molecular markers separated to CMS lines into nine groups. The potential of eight CMS lines from seven groups to yield heterotic hybrids was assayed into 853 testcrosses developed using a wide array of testers. The percentage of F1 hybrids with yield advantage of at least 15 percent relative to the male parent and four check cultivars ranged from 6 percent for 28A to 25 percent for Lian A and averaged 16 percent across the eight CMS lines. Heterosis for grain yield was most associated with phenotypic acceptability, degree of panicle exsertion, total number of spikelets, and number of productive tillers. Other studies designed to exploit rice heterosis in the Philippines are underway combining molecular and conventional breeding strategies