Sex determination, sex ratios, and genetic conflict

Genetic mechanisms of sex determination are unexpectedly diverse and change rapidly during evolution. We review the role of genetic conflict as the dr iving force behind this diversity and turnover. Genetic conflict occurs whe n different components of a genetic system are subject to selection in oppo site directions. Conflict may occur between genomes (including paternal-mat ernal and parental-zygotic conflicts) or within genomes (between cytoplasmi c and nuclear genes or sex chromosomes and autosomes). The sex-determining system consists of parental sex-ratio genes, parental-effect sex determiner s, and zygotic sex determiners, which are subject to different selection pr essures because of differences in their modes of inheritance and expression . Genetic conflict theory is used to explain the evolution of several sex-d etermining mechanisms, including sex chromosome drive, cytoplasmic sex-rati o distortion, and cytoplasmic male sterility in plants. Although still limi ted, there is growing evidence that genetic conflict could be important in the evolution of sex-determining mechanisms.