WHY IS MEIOSIS ARRESTED

In mammals and birds, females are born with all the oocytes required f or future ovulations. These oocytes are not haploid: they are arrested at the Prophase I of meiosis when they have two sister chromatids of each chromosome (i.e. the eggs are at the 4C stage). Under the assumpt ion that the low number of ovulated eggs in these species have been se lected to be of high quality, I propose three adaptive explanations fo r these two observations: (I) producing the eggs before birth may decr ease the variance in mutations and genetic damage, thereby reducing ri sk; (II) arresting the oocytes at a 4C stage may allow sister-strand r ecombination to repair genetic damage; (III) arresting the oocytes at the 4C stage may mitigate intergametic conflict, hence potential selec tion of the oocytes for ovulation will not involve conflict. Predictio ns from the Mutational, Repair, and Conflict Hypotheses are critically evaluated in the light of published data, evolutionary theory, and co mputer simulations. (C) 1998 Academic Press.