Multiple Sex-Chromosomes In The Marsupial, Potorous
G. B. Sharnian and H. N. Barber
Department of Botany, University of Tasmania
Since 1922 the cytology of twenty-seven species of marsupials has been investigated (table i). Chromosome numbers in this group are, in general, much lower than those of eutherian mammals and the large size of the chromosomes makes the group favourable for cytological studies. In most of the species studied a typical XY sex-determination mechanism has been reported. Koller (1936) showed that the pairing segments of the X and Y chromosomes are terminal or nearly so, and that the centromeres are included in the differential segments. The first meiotic division is therefore reductional. In a preliminary account (Sharman, McIntosh and Barber, 1950) we have shown that in the macropod marsupial Potorous tridactylus three chromosomes in the male are concerned with sex determination and form a trivalent at meiosis. The female has one chromosome less than the male. There appears to be a comparable case in another macropod marsupial Wallabia bicolor Macropus ualabatus) studied by Agar (1923). Agar considered that the XY bivalent in this species was "possibly sometimes independent but more often attached to one of the autosomes ". Agar's work has been reinterpreted by Darlington (1937) and Matthey (1949) who considered that a multiple system exists in this marsupial. An examination of Agar's drawings shows that, almost certainly, the same type of multiple system is found in Wallabia as in Potorous, the chromosome number of the former species being the lower by two (Sharman, McIntosh and Barber, 1950). It is probable that a similar mechanism may exist in at least one eutherian mammal, Bovey (1949) having described a trivalent at meiosis in the male of an insectivore, Sorex araneus. The number of chromosomes in the female of this species is unknown but Bovey considers that the system is most likely of the XY1Y2 : XX type.