Influence of Gender Bias on Distribution of Hybrid Sterility in Rice.

Hybrid sterility genes define species identities, setting reproductive barriers between distantly related relatives. They induce allelic-specific selective gametic abnormalities by killing pollens, embryo sacs, or both, and thus resulting in the male specific transmission ratio distortion (TRD), female specific transmission ratio distortion ( TRD), and/or sex-independent transmission ratio distortion (TRD) in hybrids. Although more than 50 hybrid sterility genes have been reported, comprehensive analysis on the distributional pattern of TRD systems in species is limited. In this review, we surveyed the TRD systems and the underlying possible mechanisms in these species. In rice, pollen killers which cause TRD are often observed in higher frequency than egg killers and gamete eliminators, which are factors affecting TRD and TRD, respectively. Due to the rather massive population of pollen grains, their reduction in the number caused by hybrid sterility possesses a smaller selective disadvantage to the hybrid individuals, in contrast to female gamete abortion. The pattern of TRD distribution displays less abundancy in TRD. It suggests that fixation of TRD might require a certain time rather than single sex-specific factors. The presence of linked sterility factors worked for TRD and TRD, and strength of their linkage in chromosomal regions might determine the type of sterility and TRD. The study of TRD systems has a potential to reveal the relationships between selfish genes and their functions for reproductive isolation.