Breeding strategies to enhance heterosis in rice

Indica germplasm has served as a rich reservoir for the identification of heterotic combinations in rice. However, the yield potential of indica parental lines is about 9-9.5 t ha¯¹ under tropical conditions. Further improvement in the yield potential of parental lines and broadcasting of their genetic base should lead to the identification of hybrid combinations with even higher heterosis. To develop elite lines with higher yield potential, breeding work on the development of the new plant type (NPT), using tropical japonica (TJ) germplasm, began in 1989 at IRRI. Since then, major progress has been made in developing high-yielding NPT lines. Two high-yielding NPT lines have been released as varieties in China from this program. To further improve the new plant type for wider acceptability and its ability to withstand harsh tropical environments, elite indica lines were crossed with NPT lines. NPT lines derived from IJ (indica/japonica) x indica elite lines possess higher yield, the desired grain quality, and resistance to several diseases. They also have strong stems and hence don't lodge even after heavy rains and strong winds. We are continuing to improve the NPT lines for grain quality and for resistance to various diseases and insects. Improved NPT lines with high yield potential, developed from the crosses between indicas and tropical japonicas, offer new opprotunities in heterosis breeding in rice. We are now investigating whether the present level of heterosis (15% to 20%) observed in indica x indica crosses can be further increased by exploiting the newly developed germplasm. In fact, enhanced heterosis (greater than 40%) observed at IRRI in indica/tropical japonica crosses encouraged us to develop CMS lines in the genetic background of elite TJ lines possessing wide compatibility (WC) genes. A very low frequency of restorer lines among elite TJ lines did not allow us to use them as the male parent. In addition, TJ CMS lines showed two major weaknesses: degeneration of panicles and low outcrossing rate. Consequently, we are emphasizing developing TGMS lnes in the genetic background of elite TJ and indica/TJ derivative lines. The latter appear to be better adapted, and higher yielding in the tropics. They also possess acceptable indica grain type. Heterosis between indica (TGMS)/(indica/TJ) derivative lines is being studied to identify heterotic combinations. We have also identified six putative heterotic groups in rice that are likely to help us in using heterotic combinations.