The class of mammals comprises three infraclasses, namely, Prototheria (monotremes, for example, duckbill platypus and echidna), Metatheria (marsupials, for example, kangaroo and opossum), and Eutheria (eutherians, for example, mouse and human). X chromosome inactivation is observed in the members of the infraclasses Metatheria and Eutheria.
Despite recent advances in our understanding of the molecular mechanisms governing X inactivation, little is known about how the X inactivation process has arisen and developed during the evolution of mammals. Marsupials belong to a branch of mammals that diverged from utherians about 180 million years ago (Woodburne et al. 2003). The marsupial X chromosome may therefore carry some features of the ancestral mammalian X chromosome. The euchromatic part of the marsupial X consists of only 3% of the total genomic euchromatin (versus 5% in eutherians), and corresponds to the long arm and the pericentric region of the human X (Graves 1996). As in eutherian mammals, dosage compensation of X-linked genes is achieved by X-chromosome inactivation. However, in marsupials X inactivation is paternally imprinted in all tissues (Sharman 1971). Moreover, chromosome silencing is incomplete, and tissue-speciﬁc (Cooper et al. 1993).
Mating behavior in (Andromeda Oxford Ltd., 1984; Andromeda Oxford Ltd., 2001; Trupin and Fadem, 1982)is strongly tied to olfaction. Males habitually mark their surroundings with a chemical mark produced by a sternal gland. This scent likely serves as an advertisement to local females and a warning to local males. When a male and a receptive female meet, a precopulatory dance of sniffing, chasing, biting, and licking ensues. At the completion of this dance, the male immobilizes the female’s hind legs and begins copulation, which lasts from 4 to 7 minutes. The majority of matings take place with the animals laying on their right sides.
Immediately after birth, newborn (Kalafut, 2005; Trupin and Fadem, 1982)crawl to their mother’s stomach and attach to a nipple. They remain attached this way for 3 to 4 weeks. After detachment the young climb on their mother and/or follow her around for another three months or more. Paternal care in is nonexistent, moreover, in captivity when fathers are confronted with their offspring, they act aggressively towards them.