Analysis on fertility and inheritance of hybrid male sterility of indica rice 9814HS-1

Abstract

Hybrid rice has generally a yield-increasing heterosis of 10%−15% compared with inbred parents. Food and Agriculture Organization (FAO) has recommended hybrid rice as one of the best choices for solving the food shortage in the developing countries. Male sterility provides a basic genetic tool for hybrid rice breeding. Hybrid male sterility refers to the phenomenon that F1 hybrid from two parents is abnormal or nonfunctional in the development of male organs. A new germplasm source of hybrid male sterility is valuable for establishing a novel system for using heterosis by hybrid breeding of three-way crossing, which helps to develop elite hybrid rice with better comprehensive traits. To date, hybrid male sterility occurs commonly from the crosses between species (e.g. Oryza rufipogon × Oryza sativa ) or subspecies (e.g. indica rice × japonica rice). Nevertheless, it is not clear if the hybrid male sterility could be induced from a cross within a subspecies. Here, we reported firstly the hybrid male sterility resulted from a cross between two indica rice parents, i.e., two WA-type maintainer lines of indica rice T98B and M114B. The InDel marker analysis showed that T98B and M114B had an indica -genotype frequency at 0.94 and 1.00, respectively, indicating both of them belong to indica rice. Field observation showed that 9814HS-1, the F1 hybrid of T98B (as female parent) and M114B (as male parent), was eco-typically male sterile. The pollens of 9814HS-1 were completely aborted, resulting in a seed setting rate at 0.00% when heading after early October at Changsha or before mid-March at Sanya, while both parents T98B and M114B were normally fertile under the same conditions. Interestingly, when T98B and M114B were crossed with eight other indica rice varieties, all of their F1 hybrids were normally fertile with a seed setting rate ranged from 80.35% to 91.87%, implying that the hybrid male sterility of 9814HS-1 is induced from the specific parental incompatibility between T98B and M114B. The restorability test showed that 9814HS-1 could be restored for fertility by all of six representative indica rice lines, with seed setting rates ranged from 85.24% to 93.21% in their F1 hybrids, indicating that the broad spectrum of restorability of 9814HS-1. This male sterility provides a good basis for hybrid breeding of three-way cross. Genetic analysis showed that both of the F1 hybrid and its reciprocal cross from T98B and M114B were male sterile, suggesting the hybrid male sterility was controlled by nuclear genes. The Chi-square test supported that the separation ratio of sterile: Fertile plants in both F2 and BC1F1 populations was confirmed to be 1:3, which we infer that the hybrid male sterility in 9814HS-1 is controlled by two pairs of non-allelic nuclear genes and occurs only when they are in a heterozygous state. This study provides a new perspective for re-recognizing rice reproductive barriers and opens up a new way for the multiple-line hybrid rice breeding.