Imprinting of the paternal marsupial X chromosome by DNA methylation

Why is it that the X chromosome that comes from the male parent is inactivated in female marsupials, female mice, and even female mealy bugs, or the whole paternal chromosome complement in some weird flies? A new paper by Milton et al. now reveals DNA methylation patterns established in the male germline before meiosis in wallabies that may constitute the elusive paternal imprint.