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 th e 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 advant age 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 advanta ge if it reaches a high enough frequency. This is due to the production of XO females in matings between XX females and X'Y males; low fertility and e mbryo loss of such females reduce the fitness of the X chromosome in female s, relative to that of X'. Under some conditions, however, the enhanced rep roductive value of males, caused by the production of inviable Y0 embryos i n X0 x X'Y matings, can outweigh any advantage to X'. Inbreeding also reduc es any advantage to X'.