Roles of histone H3K9 methyltransferases during Drosophila spermatogenesis

Epigenetic regulation of gene expression by covalent modification of histones is important for germ line cell development. In mammals, histone H3 lysine 9 (H3K9)-specific histone methyltransferases (HMTases), such as G9a, SETDB1, and SUV39H, play critical roles, but the contribution of H3K9-specific HMTases in Drosophila remains to be clarified, especially in male sperm. Here, we performed immunocytochemical analyses with a specific antibody to dG9a, Drosophila G9a ortholog, and demonstrated localization in the cytoplasm from the growth to elongation stages of spermatogenesis. In the subsequent early canoe stage, strong dG9a signals were detected exclusively in nuclei, suggesting a regulatory role. However, mono-, di-, and trimethylated H3K9 signals were not extensively decreased in a homozygous dG9a null mutant throughout these stages. In contrast, mono- and trimethylated H3K9 signals were extensively decreased in a heterozygous DmSetdb1 mutant during spermatogenesis, and similar reduction in monomethylated H3K9 signals was observed in a homozygous Su(var)3-9 mutant. Therefore, DmSETDB1 is likely to be mainly responsible for mono- and trimethylation of H3K9 and SU(VAR)3-9 for monomethylation of H3K9 during spermatogenesis. However, the reduced methylation of H3K9 in premeiotic spermatocytes did not influence X-Y chromosome disjunction in male meiosis, suggesting that it may not be critical for spermatogenesis in Drosophila.