A model of the evolution of the unusual sex chromosome system of Microtus oregoni.

In the creeping vole, Microtus oregoni, females are X0 and males are XY. In the female germ line, mitotic nondisjunction ensures that the products of meiosis all carry the X chromosome. Similarly, mitotic nondisjunction in the male germ line leads to the production of 0 and Y sperm. We propose that the present situation in M. oregoni has evolved by invasion of a normal XX/XY system by a mutant X chromosome, X', with a complete transmission advantage in X'X females, and a complete transmission disadvantage in X'Y males. X' is at best initially nearly neutral, but can gain a transmission advantage if it reaches a high enough frequency. This is due to the production of X0 females in matings between XX females and X'Y males; low fertility and embryo loss of such females reduce the fitness of the X chromosome in females, relative to that of X'. Under some conditions, however, the enhanced reproductive value of males, caused by the production of inviable Y0 embryos in X0 x X'Y matings, can outweigh any advantage to X'. Inbreeding also reduces any advantage to X'.