Absence of synapsis during pachynema of the normal sized sex chromosomes of Microtus arvalis

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Abstract
The pachytene behavior of the chromosomes of males of Microtus arvalis (Pall.) (Rodentia, Arvicolidae) was examined by electron microscopy in microspread preparations of spermatocytes. There was no synapsis between the axes of these two chromosomes during this period. Since synapsis is universally considered a preequisite for crossing over and chiasmata formation, disjunction of the sex chromosomes in this species must be achiasmatic. Unlike previously examined species with no synapsis or crossing over between the X and Y, the sex chromosomes of M. arvalis are of normal size: the X chromosome is of an "original" X size and the Y is a small acrocentric. C-band studies of M. arvalis mitotic metaphse reveal no blocks of heterochromatin on the sex chromosomes. The implications of these findings are discussed.