The pairing of chromosomes during meiosis requires histone modifications, such as histone H3 lysine 9 di- and tri-methylation (H3K9me2 and H3K9me3, respectively), at pericentric heterochromatin (PCH) regions. But how do these epigenetic marks control chromosome interactions? Haruhiko Koseki and colleagues demonstrate that heterochromatin protein 1γ (HP1γ) regulates chromosome interactions by recognising histone methylation marks during meiosis in mice (see ). The researchers show that, in meiotic spermatocytes, H3K9me2 by the G9a histone methyltransferase requires pre-existing H3K9me3 marks, which are deposited by the Suv39h histone methyltransferase. They further show that HP1γ recognizes H3K9me3 marks and localises to PCH regions in an H3K9me3-dependent manner, where it then recruits G9a. Importantly, the loss of HP1γ results in defective spermatogenesis, aberrant centromere clustering and impaired homologous chromosome pairing. The authors thus propose that HP1γ acts as an important link between the cascade of H3K9me3 and H3K9me2 modifications, acting to align homologous chromosomes and facilitate their pairing during meiosis.
