Maternal-offspring conflict leads to the evolution of dominant zygotic sex determination.

Sex determination in many species involves interactions among maternally expressed genes (eg, mRNA's and proteins placed into the egg) and zygotically expressed genes. Recent studies have proposed that conflicting selective pressures can occur between maternally and zygotically expressed sex determining loci and that these may play a role in shaping the evolution of sex determining systems. Here we show that such genetic conflict occurs under very general circumstances. Whenever sex ratio among progeny in a family affects the fitness of either progeny in that family or maternal fitness, then maternal-zygotic genetic conflict occurs. Furthermore, we show that this conflict typically results in a "positive feedback loop" that leads to the evolution of a dominant zygotic sex determining locus. When males more negatively effect fitness within the family, a male heterogametic (XY male) sex determining system evolves, whereas when females more negatively effect fitness in the family, a female heterogametic (ZW female) system evolves. Individuals with the dominant sex allele are one sex, and the opposite sex is determined by maternally-expressed genes in individuals without the dominant sex allele. Results therefore suggest that maternal-zygotic conflict could play a role in the early evolution of chromosomal sex determining systems. Predictions are made concerning the patterns of expression of maternal and zygotic sex determining genes expected to result from conflict over sex determination.