Heterosis and heterotic grouping effects on grain yield, height, tiller density, and days to heading in hybrid rice (oryza sativa l.)

Heterotic groups are necessary for high vigor in hybrid rice. However, hybrids produced from crosses between parents from different rice subspecies (i.e., Indica x Japonica) have extensive incompatibility issues exhibited by low seed sets. The study objectives were to evaluate the heterosis in grain yield and yield-related traits between hybrids produced from low and high parental genetic distances (PGDs) and demonstrate the heterotic group approach in rice. From PGDs, eight and three hybrids were assigned to the low and high PGD hybrid groups, respectively. Neighbor-joining clustering and model-based population structure analyses classified the hybrid parents into four heterotic groups, with the low and high PGD hybrid groups found consisting of intra-subpopulation and inter-subpopulation crosses, respectively. Replicated yield trials conducted at Beaumont, Texas, transpired in 2019 and 2020. The hybrids exhibited normal seed sets, with at least one of each hybrid's parents determined to possess the wide compatibility S5n allele necessary for normal seed sets in wide crosses. Trait and standard heterosis values estimates included the number of days to heading, plant height, tiller density, and grain yield. Higher trait values and heterosis for tiller density and grain yield occurred in the high than the low PGD hybrid group, especially in the inter-subpopulation crosses with indica rice. PGD had consistent positive correlations with heterosis for grain yield (r = 0.41 to 0.60) and tiller density (0.28 to 0.36) in both years. PGDs aid in determining highly heterotic cross combinations for tiller density and grain yield and in forming heterotic groups. Heterotic grouping is advisable through cluster and structure analyses of genome-wide markers instead of identifying genetically-distant crosses based on pedigree information.