Spermatogonial and meiotic chromosomes of the Armenian hamster, Cricetulus migratorius

The dwarf hamster, Cricetulus migratorius, commonly referred to as the Armenian hamster, possesses a low diploid number of 22 chromosomes. Individual chromosome types can be accurately identified in spermatogonial, meiotic and cultured somatic cells. Two distinct types of spermatogonial metaphase complements are described on the basis of staining affinities and sex chromosome polymorphism. The onset of meiosis in this species occurs as early as 15 days after birth and the initial wave of spermiogenic activity is completed by the 27th day of age. Consequently, initiation and progression of various stages of meiosis can be correlated with the age of preweanling males. In pachytene, X and Y chromosomes are isomorphic as revealed by the XY bivalent. A specific chiasma occurs at the secondary constriction or the nucleolar organizing region of the short arm of the XY bivalent. Furthermore, the sex chiasma occurs only between inner chromatids of the X and Y chromosomes. The sex bivalent of this species is compared with those of other species of hamsters in an effort to relate phylogenetic homologies.