BEHAVIOR OF XY PAIR IN MAMMALS

This chapter analyzes the behavior of the XY pair in small number of selected species, and discusses the essential problems concerning the XY pair on that basis. The leptotene stage of mammalian spermatocytes is poorly understood, and the limited available evidence suggests that at least in human and mouse spermatocytes the X and Y chromosomes are not fully heterochromatic at this stage. Although zygotene nuclei should show the pairing process, this process has seldom been observed in mammalian spermatocytes. Examination of thick and thin sections of each stage in the cycle of the seminiferous epithelium of the mouse show that after leptonema the chromosomes become strongly condensed during zygonema, and that zygotene nuclei are relatively small and strongly stained. The beginning of pachynema may be conventionally set at stage 12 in the cycle of the seminiferous epithelium of the mouse. At this stage the sex pair is already forming a single body and has no nucleolar region. The sex pair undergoes several changes during pachynema. In spreads corresponding to early pachynema, the XY body of the mouse shows a smoothly curved side facing the inner nuclear space, and a rough side corresponding to the attachment of the X and Y chromosomes to the nuclear envelope. © 1974 Academic Press Inc.