Multiple Sex Chromosomes in the Marsupials

DURING the past thirty years, some twenty species of marsupials have been examined cytologically. In all of them, a typical XY sex-chromosome mechanism has been reported. We have recently re-investigated the potoroo ( Potorous tridactylus ), a common species of rat-kangaroo in Tasmania. In the spermatogonial mitoses, the male shows thirteen chromosomes, whereas the female shows twelve in ovarian mitoses (Figs. 1 and 2). The chromosomes of the female can easily be arranged in homologous pairs, one pair (the X -chromosomes) being conspicuous by their long, nearly median, centromere constriction. In the male, ten chromosomes can be paired, but three have no similar partners. These are the X , with a long submedian constriction, the smallest of the set ( Y 1 ), and an acrocentric ( Y 2 ). The last two chromosomes are not represented in the female. In accord with these observations, five bivalents held together by from one to five chiasmata and a trivalent are formed at the first division of meiosis in the male (Fig. 3). The trivalent is made up of the X , pairing in one of its arms with Y 1 , and in the other arm with Y 2 . Orientation on the spindle is always such that Y 1 and Y 2 are directed to one pole, and the X to the other pole. Two types of sperm are produced, a male-determining type with seven chromosomes, and a female-determining with six. The sex-determining mechanism is, thus, of the XY 1 Y 2 : XX type.