title: A model of the evolution of the unusual sex chromosome system of Microtus oregoni.
abstract: 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'.
authors.affiliation: ICAPB, University of Edinburgh, Ashworth Laboratories, Edinburgh EH9 3JT, UK. Brian.Charlesworth@ed.ac.uk
authors.forename: B
authors.initials: B
authors.lastname: Charlesworth
authors.forename: N D
authors.initials: ND
authors.lastname: Dempsey
mesh_terms: D000818:Animals; D003411:Arvicolinae / Q000235:genetics*; D005075:Biological Evolution; D005260:Female; D005828:Genetics, Population; D008297:Male; D008957:Models, Genetic; D009154:Mutation; D012730:Sex Chromosomes; D014960:X Chromosome; D014998:Y Chromosome