HPRT ACTIVITY IN EMBRYOS OF A SOUTH-AMERICAN OPOSSUM MONODELPHIS-DOMESTICA

Marsupial females show preferential paternal X-inactivation. However, the time at which X-inactivation occurs in early development has not y et been determined. A double microassay which measures the activities of X-linked hypoxanthine phosphoribosyl transferase (HPRT) and the aut osomally-coded adenine phosphoribosyl transferase (APRT) from the same sample was performed on a collection of embryos from a South American opossum Monodelphis domestica. The embryos ranged in age from the 2-c ell stage to the bilaminar blastocyst stage. The results indicate that their embryonic HPRT and APRT are not expressed until just before the unilaminar blastocyst stage in M. domestica. This is at a later stage of development than that in the mouse where embryonic HPRT and APRT e xpression first occurs at the 4-8-cell stage. It is concluded that HPR T is an uninformative enzyme for assessing X chromosome activity in cl eaving embryos of M. domestica. The widespread distribution of HPRT:AP RT ratios after the unilaminar blastocyst stage also makes it difficul t to draw conclusions about the state of X chromosome activity in earl y marsupial development.