The mechanism of sex determination in mammals appears highly conserved: the presence of a Y chromosome triggers the male developmental pathway, whereas the absence of a Y chromosome results in a default female phenotype. However, if the Y chromosome fails to initiate the male pathway (referred to as Y^*), XY^* females can result, as is the case in several species of South American field mice (genus Akodon). The breeding genetics in this system inherently select against the Y^* chromosome such that the frequency of XY^* females should decrease rapidly to very low frequencies. However, in natural populations of Akodon, XY^* females persist at substantial frequencies; for example, 10% of females are XY^* in A. azarae and 30% in A. boliviensis. We develop a mathematical model that considers the potential roles of three evolutionary forces in maintaining XY^* females: Y-to-Y^* chromosome transitions (mutation), chromosome segregation distortion (meiotic drive), and differential fecundity (selection). We then test the predictions of our model using data from breeding colonies of A. azarae. We conclude that any single force is inadequate to maintain XY^* females. However, a combination of segregation bias of the male and female Y chromosomes during spermatogenesis/oogenesis and increased fecundity in XY^* females could account for the observed frequencies of XY^* females.

Corresponding Editor: M. Whitlock