DEVELOPMENTAL PROGRESSION OF GPD EXPRESSION FROM THE INACTIVE X-CHROMOSOME OF THE VIRGINIA OPOSSUM

Metatherian (marsupial) mammals possess a non-random form of X-chromos ome inactivation in which the paternally-derived X is always the one i nactivated. To examine the progression of X-linked gene expression dur ing metatherian development, we compared relative levels of the matern ally and paternally encoded Gpd gene products in heterozygous female V irginia opossums (Didelphis virginiana) across a major portion of the developmental period. Panels of tissues obtained from fetuses, newborn s, and pouch young were examined via polyacrylamide gel electrophoresi s of the G6PD protein. As in adults, G6PD phenotypes in these developm ental stages were highly skewed in favor of the maternal allele produc t, but in some tissues there was a marked increase in paternal allele expression with advancing developmental age. However, even by 42 days of post-partum development, expression of the paternal Gpd allele had not attained the adult, tissue-specific activity pattern. Our findings indicate remarkable developmental changes in the activity of the pate rnal allele in several tissues/organs continuing well into mid pouch-l ife stages and beyond. Specifically we found that 1) a substantially r epressed paternal Gpd gene is present in the cells of female stage 29 fetuses and later developmental stages, 2) the activity state of the p aternal Gpd gene is not fixed during early embryonic development in th is species, 3) major changes in paternal Gpd expression occur in advan ced developmental stages and comprise a maturation of the gene express ion pattern during ontogeny, and 4) alterations of paternal Gpd allele activity during development occur in a tissue-specific manner. (C) 19 95 Wiley-Liss, Inc.