X chromosome activity in oocytes of kangaroo pouch young.

Abstract

IN female eutherian mammals, inactivation of one of the two X chromosomes occurs randomly from cell to cell in somatic tissues1 but in oocytes (meiotic germ cells) X inactivation does not occur for at least three loci. Oocytes of adult XX mice possess twice as much activity as those of XO mice for the sex-linked enzyme, glucose-6-phosphate dehydrogenase (G6PD)2, hypoxanthine guanine phosphoribosyl transferase (HGPRT)3, and phosphoglycerate kinase (PGK)4. Furthermore, oocytes of adult5 and foetal6 human females, heterozygous for G6PD allelic genes, show activity for both fast and slow electrophoretic forms of the enzyme. In female kangaroos (Marsuipialia: Macropodinae) inactivation of the paternally derived X chromosome occurs in cells of various somatic tissues7 but the question of X chromosome activity in oocytes has not been resolved. We have now examined G6PD expression in ovaries of pouch young known to be heterozygous for the fast and slow forms of the enzyme, and show that only a single X chromosome—the maternally derived one—is active in ovarian cells.