Maintainer and restorer frequencies and heterotic potential in Philippine hybrid rice germplasm

Heterosis or hybrid vigor is the foundation of hybrid rice breeding. Genetic diversity is acknowledge to be the basis for heterosis. Hence, PhilRice has diversified its hybrid rice genepool through collaborations with Chinese R & D institutions that have led to the introduction of new CMS [cytoplasmic-genetic male sterile and restorer lines into PhilRice's breeding program. Genetic diversity analysis of CMS lines, for example, has revealed nine groups within the PhilRice's CMS genepool. Knowledge on the magnitude of heterosis achievable using diverse CMS materials and a wide array of testers would provide baseline information that would be useful in charting the future of Philippine hybrid rice breeding. Using 850 F1 derived from eight CMS lines, the maintaining and restoring ability of 450 testers/inbreds were examined. On average, 19 percent of the testers were effective restorers while 28 percent were partial or effective maintainers of eight CMS lines. Crosses made with CMS Line A were most heterotic (34 percent), and crosses to CMS 28A were mostly sterile (65 percent). The heterosis for ten vegetative and reproductive traits were also examined. Trait wise, more than 20 percent of the F1 hybrids showed at least 20 percent superiority over the male parent (MP) for number of productive tillers, spikelet number, grain weight, and grain yield. Distribution of MP heterosis was normal for maturity, plant height, panicle length, grain length, and grain width, and was bimodal or skewed for number of productive tillers, spikelet number, spikelet fertility, grain weight, and grain yield. These results suggest the greater likelihood for selecting heterotic combinations for the latter group of characters. For grain yield, 21 percent of the hybrids showed at least 20 percent heterosis. These results indicate that breeding progress is attainable using the current germplasm at PhilRice