In this paper, Namekawa et al., studied the X-inactivation (XCI) by imprinting in marsupials, a process not described before at the molecular level for this taxa. The complication here was that, opposite to what happens in eutheria where XCI is regulated by Xist and other components of the Xic locus, marsupials lack of a XIC homolog. Using several FISH and immuno- staining experiments in Cot-1 DNA, they found that both meiotic sex chromosome inactivation (MSCI) and postmeiotic sex chromatin (PMSC) do occur in the opossum. They also found similarities in the histone epigenetic marks between eutheria and marsupials:
Namekawa SH. et al. 2007. Proc Natl Acad Sci U S A 104(23):9730-35In conclusion, our study [...] show that MSCI and PMSC occur in the marsupial and that the silencing initiated during the first meiotic prophase continues through meiosis II and into the postmeiotic period. Thus, spermatogenic events regulating transcriptional activity of the sex chromosomes are very well conserved in the marsupial and eutherian. This is in striking contrast to the absence of conservation in XIC elements that regulate XCI in the eutherian soma
[...] The state of the paternal X upon arrival in the opossum zygote requires further study. In the absence of any significant cytoplasm and the replacement of histones for protamines, how might epigenetic information be transmitted from the sperm to the zygote? [...] Our studies have highlighted three persistent marks of meiotic silencing (H3–3meK9, HP1beta, and HP1gamma) that are shared between eutherians and metatherians. Interestingly, H3–3meK9, HP1beta, and HP1gamma are also the last chromatin- associated marks to be detected before protamine- mediated compaction during mouse spermiogenesis. These marks are therefore candidates for transgenerational inheritance of epigenetic programming associated with the paternal X.
epigeneticsMore comments can be found at the same issue of PNAS.
