The processes that animals use to respond to differences in X chromosome dosage have been most intensively studied in three very different groups of organisms: (i) mammals, including marsupials; (ii) the nematode Caenorhabditis elegans; and (iii) the fruit fly, Drosophila melanogaster. The dosage compensation mechanisms in these species differ dramatically: eutherian ( placental) mammals randomly inactivate one of their two X chromosomes in females to form a Barr body (4); female marsupials display non random X inactivation (5); XX hermaphrodite nematodes halve the expression from each X (6); and male Drosophila increase the transcription of their single X approximately twofold (7, 8). In each case however, the regulation of chromatin composition and structure in one sex is central to the dosage compensation mechanism.

Park, Y. and M. I. Kuroda. 2001 Epigenetic aspects of X-chromosome dosage compensation. Science 293(5532): 1083-5.