Genetic activity of sex chromosomes in germinal cells

Abstract

If I adhere strictly to the title proposed for me and speak only of the genetic activity of the sex chromosomes in germ cells, there is very little to say. The evidence is necessarily indirect and includes, first, examples of differential behaviour of germ cells of different sex chromosome constitution in situations where competitive proliferation is a possibility, as in some mosaics and chimaeras; and secondly, exceptional species in which the sex chromosome constitution is normally different in germ cells and soma. The species concerned are all mammals. An instance of the first kind is provided by observations made on a 39,X /41,XYY mosaic mouse discovered by chance in the course of an irradiation experiment (Evans, Ford & Searle 1969). All the spermatogonia and spermatocytes examined contained 41 chromosomes, including two Y chromosomes, whereas bone marrow (the only other tissue examined) was mosaic, the probability of difference being due to sampling error being very low. The question, then, was whether the failure to detect mosaicism among the germ cells was a consequence of chance exclusion of the 39, X cell type from the germ line during development, or of differential proliferation and/or survival of 41,XYY germ cells in the testicular environment. The latter interpretation was favoured on the grounds: (1) A 39,X /41,XYY mosaic is likely to have originated by non-disjunction of the Y chromosome at the first cleavage division of a 40,XY zygote, since other theoretically possible modes of origin would require the combination of rare events or other implausible assumptions. (2) Primordial germ cells of the constitution 39, X are capable of reaching the developing gonad and subsequently forming functional oocytes as evidenced by the fertility of 39, X female mice (Russell, Russell & Gower 1959). (3) Nearly all half-and-half coat colour mosaic mutants are also germ cell mosaics (Russell 1964), implying that when two distinct cell lines are present very early in development both lines are likely to be represented among the germ cells