HP1β and HP1γ, but not HP1α, decorate the entire XY body during human male meiosis

During meiosis in male mammals, the X and Y chromosomes become heterochromatic and transcriptionally silent, and form the XY body. Although the HP1 proteins are known to be involved in the packaging of chromosomal DNA into repressive heterochromatin domains, their involvement in facultative heterochromatinization has not been precisely determined. Here, we analyse, for the first time in humans, the subcellular distribution of the heterochromatin protein HP1α, HP1β and HP1γ isoforms, in male pachytene spermatocytes, and the XY body facultative heterochromatin in particular. Our results demonstrate that HP1β and HP1γ, but not the HP1α isoforms, decorate the entire XY body in half the pachytene nuclei observed. In some nuclei, the XY body appears to be only partially labelled. In these cases, the HP1β and HP1γ signals are adjacent to the Yq12 constitutive heterochromatin and signal appears to originate in this region before spreading over the entire XY body. This distribution suggests that HP1β and HP1γ proteins, which are components of the constitutive heterochromatin, may also be involved in the facultative heterochromatinization of the XY body. Nevertheless, their absence from the early pachytene substage, even though the XY body is already condensed, suggests that these proteins are not involved in the initiation of this process.