Fig. 7.

Comparison of amniote sex chromosome systems through prophase I and evolutionary pathway to mammalian MSCI. a Chicken and platypus are grouped due to lack of X borne retrogenes, ZW/XY self-association dynamics, low general states of late prophase I transcription and absence of sex body. Nucleolar tethering of the platypus sex chromosome chain and chromosome 6 are prominent features in late prophase I spermatocytes along with para-nucleolar repressive epigenetic mark associations. Question mark in nucleolus denotes unknown prophase I dynamics in chicken and marsupials. * Repressive epigenetic marks on the chicken ZW at late pachytene are restricted to constitutive heterochromatin and proposed to not be part of a MSCI pathway [12]. b Following monotreme divergence the mammalian lineage utilised a different sex chromosome system leading to MSCI. The emergence of therian MSCI involved a change from general low states of transcription levels through prophase I to sex chromosome specific silencing. This triggered the movement of X genes to autosomes via retrotransposition. The monotreme sex chromosome complex nucleolar tethering and persistence of chromosome 6 nucleolar association in combination with the establishment of paranucleolar repressive machinery may have been the precursors then recruited into the new therian sex chromosome MSCI system