The Role of X-Chromosome Inactivation d (Drosophila/allocycly/chromosome evolution/male sterili

Inactivation of the single X chromosome in the primary spermatocytes of species with hetero- gametic males is postulated as a basic control mechanism on the chromosomal level that is required for normal spermatogenesis. This view is supported by (a) cytological observations of X-chromosome allocycly in the primary spermatocytes of all male-heterogametic organisms that were adequately examined, (b) autoradiographic evidence of early cessation of transcription by the X chromosome in the mouse and three species of grasshopper, and (c) the male sterility of animals with certain X-chromosome rearrangements that cannot be attributed to misfunction of specific genes. X-chromosome inactivation during spermatogenesis is proposed as the ideal system for studies of genetic control at the chromosomal level. Sex chromosomes provide a striking example of differentia- tion of the chromosomal complement and inferentially of chro- mosomal function during evolution. It seems reasonable to suppose that natural selection has resulted in a particular distribution of genes between the sex chromosomes and the autosomes that is related to the role of these genes in the development and expression of sexuality. For example, male- fertility genes are found on the Y chromosome (Y) in Droso- phila, whereas male-determining genes are found on mammal- ian Y chromosomes. Furthermore, sexual differentiation might depend on males differing in constitution from females (e.g. dosage) with respect to some genes but not others; Bridges' and Goldschmidt's genie balance theories of sex determination (1, 2) are based on such a requirement. If accompanying sex chromosome evolution there were the concomitant evolution of a chromosomal mechanism permitting coordinate control of sex-linked gene function, then the potential of functioning in coordination with other sex-linked genes might favor the location of certain loci on the sex chromosome. That such control could have developed is evident from other forms of control operating at the chromosomal level. For example, coordinate control of chromosomal activity is evident in all eukaryotic organisms at mitosis; furthermore, asynchro- nous control of different chromosomes or chromosome seg- ments may be inferred from their allocycly (differences in chromosome condensation) during the cell cycle. Inactivation of one X chromosome (X) in somatic cells of female mammals (3) or of the entire paternally inherited chromosome set in certain coccids (4), as well as the physical elimination of chromosomal material from cell lines (5, 6), are other examples of gene inactivation at the chromosomal level. We conclude that the evolution of both genie and chromosomal processes