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Two heterotic groups and four heterotic patterns were identified for IRRI hybrid rice germplasm to develop hybrid rice in the tropics based on SSR molecular data and field trials. Information on heterotic groups and patterns is a fundamental prerequisite for hybrid crop breeding; however, no such clear information is available for tropical hybrid rice breeding after more than 30 years of hybrid rice commercialization. Based on a study of genetic diversity using molecular markers, 18 parents representing hybrid rice populations historically developed at the International Rice Research Institute (IRRI) were selected to form diallel crosses of hybrids and were evaluated in tropical environments. Yield, yield heterosis and combining ability were investigated with the main objectives of (1) evaluating the magnitude of yield heterosis among marker-based parental groups, (2) examining the consistency between marker-based group and heterotic performance of hybrids, and (3) identifying foundational hybrid parents in discrete germplasm pools to provide a reference for tropical indica hybrid rice breeding. Significant differences in yield, yield heterosis and combining ability were detected among parents and among hybrids. On average, the hybrids yielded 14.8 % higher than the parents. Results revealed that inter-group hybrids yielded higher, with higher yield heterosis than intra-group hybrids. Four heterotic patterns within two heterotic groups based on current IRRI B- and R-line germplasm were identified. Parents in two marker-based groups were identified with limited breeding value among current IRRI hybrid rice germplasm because of their lowest contribution to heterotic hybrids. Heterotic hybrids are significantly correlated with high-yielding parents. The efficiency of breeding heterotic hybrids could be enhanced using selected parents within identified marker-based heterotic groups. This information is useful for exploiting those widely distributed IRRI hybrid rice parents.

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Four heterotic groups were identified for tropical indica rice germplasm to develop hybrid rice in the tropics based on two studies. Heterotic groups are of fundamental importance in hybrid crop breeding. This study investigated hybrid yield, yield heterosis and combining ability within and among groups based on genetic distance derived from single-nucleotide polymorphism markers. The main objectives of the study were to (1) evaluate the magnitude of yield heterosis among marker-based groups, (2) identify possible heterotic groups for tropical indica hybrid rice, and (3) validate heterotic patterns concluded from a previous study. Seventeen rice parents selected from improved indica germplasm from the tropics with high genetic divergence and 136 derived hybrids were evaluated in five environments. The hybrids had more yield than their parents with an average of 24.1 % mid-parent heterosis. Genotype × environment interaction was the major factor affecting variations in yield and yield heterosis, which raised a necessity and a challenge to develop heterotic rice hybrid adapted to different regions and seasons in the tropics. Yield, yield heterosis and combining ability were significantly increased in inter-group than in intra-group hybrids. Four heterotic groups and three promising hybridization patterns, which could be used in tropical hybrid rice breeding, were identified based on marker-based grouping, yield and yield heterosis analyses in the two studies. The study reveals that molecular markers analysis can serve as a basis for assigning germplasm into heterotic groups and to provide guidelines for parental selection in hybrid rice breeding.