Comparative studies on sex chromosomes in different species

InLocusta migratoria (XO),Mus musculus, Rattus norvegicus, Mesocricetus auratus, Cricetulus griseus andHomo sapiens typical sex vesicle structures are visible in early meiotic prophase stages up to pachynema. The structures include whole sex chromosomes or parts thereof. The heterologous parts and the solitary X chromosome ofLocusta pass diplonema, diakinesis and first metaphase nearly in mitotic shape. Entirely heterologous sex chromosomes are kept together by a unilateral and achiasmatic end connection. The sex vesicle is interpreted as a special structure of allocyclic sex chromosomes or parts of them, corresponding in early meiotic stages to the chromocenters of mitotic interphase nuclei. The formation of the sex vesicle is independent of the orthoploidy of nuclei and of the DNA ratio between autosomes and sex chromosomes. Heteropycnotic behaviour of sex chromosomes in spermatids is interpreted as a condition capable of blocking genetic activity, like in the Barr bodies of female somatic nuclei, giving equal chances of fertilization to both types of gametes.Based on a paper read at the XI International Congress of Genetics, of which an abstract has appeared in the congress proceedings, Genetics Today, Vol. 1, p. 299 (1963).