Specific patterns of histone marks accompany X chromosome inactivation in a marsupial.
The inactivation of one of the two X chromosomes in female placental mammals represents a remarkable example of epigenetic silencing. X inactivation occurs also in marsupial mammals, but is phenotypically different, being incomplete, tissue-specific and paternal. Paternal X inactivation occurs also in the extraembryonic cells of rodents, suggesting that imprinted X inactivation represents a simpler ancestral mechanism. This evolved into a complex and random process in placental mammals under the control of the XIST gene, involving notably variant and modified histones. Molecular mechanisms of X inactivation in marsupials are poorly known, but occur in the absence of an XIST homologue. We analysed the specific pattern of histone modifications using immunofluorescence on metaphasic chromosomes of a model kangaroo, the tammar wallaby. We found that all active marks are excluded from the inactive X in marsupials, as in placental mammals, so this represents a common feature of X inactivation throughout mammals. However, we were unable to demonstrate the accumulation of inactive histone marks, suggesting some fundamental differences in the molecular mechanism of X inactivation between marsupial and placental mammals. A better understanding of the epigenetic mechanisms underlying X inactivation in marsupials will provide important insights into the evolution of this complex process.
Koina
Edda
E
Research School of Biological Sciences, The Australian National University, Canberra, ACT, 0200, Australia.
Chaumeil
Julie
J
Greaves
Ian K
IK
Tremethick
David J
DJ
Graves
Jennifer A Marshall
JA
D000818:Animals; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism; D006657:Histones / Q000235:genetics / Q000378:metabolism*; D017404:In Situ Hybridization, Fluorescence; D008394:Marsupialia / Q000235:genetics* / Q000378:metabolism; D014960:X Chromosome / Q000235:genetics / Q000378:metabolism*; D049951:X Chromosome Inactivation / Q000235:genetics*
D006657:Histones