Abstract: Segregation distortion (SD) is defined as abnormal segregation ratio of hybrid offsprings at some genetic loci deviating from the Mendelian ratio. SD results from the incompatibility among genes from different parents, which could be due to loss-of-function or gain-of-function gene interactions. The mechanism for loss-of-function SD is relatively simple: defective gene combination leads to loss of the original function and eventual cell death. The gain-of-function hybrid SD system is a multi-gene genetic system, comprising two basic components: the killer and the protector. Additional modifiers, such as enhancers and repressors, are also involved. There is a general genetic model for gain-of-function hybrid SD: haplotypes with transmission advantage possess high-activity killer+ and protector+; those with transmission disadvantage possess low-activity killer- and protector-; neutral haplotypes (wide compatibility types) possess killer- and protector+. Depending upon close linkage between the killer and the protector and the accumulation of modifiers, the SD system survived through natural selection. Although the genetic mechanisms are highly similar, different gain-of-function hybrid SD systems have distinctive molecular mechanisms. In this review, we summarize the genetic and molecular mechanisms of hybrid SD, and the relationship between hybrid SD and hybrid sterility.

Key words: segregation distortion, gene interaction, wide compatibility, reproductive isolation, hybrid sterility