Dual mechanism of chromatin remodeling in the common shrew sex trivalent (XY (1)Y (2))

Abstract. Here we focus on the XY(1)Y(2) condition in male common shrew Sorex araneus Linnaeus, 1758, applying electron microscopy and immunocytochemistry for a comprehensive analysis of structure, synapsis and behaviour of the sex trivalent in pachytene spermatocytes. The pachytene sex trivalent consists of three distinct parts: short and long synaptic SC fragments (between the X and Y(1) and between the X and Y(2), respectively) and a long asynaptic region of the X in-between. Chromatin inactivation was revealed in the XY(1) synaptic region, the asynaptic region of the X and a very small asynaptic part of the Y(2). This inactive part of the sex trivalent, that we named the ‘head’, forms a typical sex body and is located at the periphery of the meiotic nucleus at mid pachytene. The second part or ‘tail’, a long region of synapsis between the X and Y(2) chromosomes, is directed from the periphery into the nucleus. Based on the distribution patterns of four proteins involved in chromatin inactivation, we propose a model of meiotic silencing in shrew sex chromosomes. Thus, we conclude that pachytene sex chromosomes are structurally and functionally two different chromatin domains with specific nuclear topology: the peripheral inactivated ‘true’ sex chromosome regions (part of the X and the Y(1)) and more centrally located transcriptionally active autosomal segments (part of the X and the Y(2)).