The genome loading model for the origin and maintenance of sex in eukaryotes

Balanced spatiotemporal arrangements of histone H3 and H4 posttranslational modifications are necessary for meiotic prophase I chromosome organization

Dynamic nuclear architecture and chromatin organizations are the key features of the mid-prophase I in mammalian meiosis. The chromatin undergoes major changes, including meiosis-specific spatiotemporal arrangements and remodeling, the establishment of chromatin loop-axis structure, pairing, and crossing over between homologous chromosomes, any deficiencies in these events may induce genome instability, subsequently leading to failure to produce gametes and infertility. Despite the significance of chromatin structure, little is known about the location of chromatin marks and the necessity of their balance during meiosis prophase I. Here, we show a thorough cytological study of the surface-spread meiotic chromosomes of mouse spermatocytes for H3K9,14,18,23,27,36, H4K12,16 acetylation, and H3K4,9,27,36 methylation. Active acetylation and methylation marks on H3 and H4, such as H3K9ac, H3K14ac, H3K18ac, H3K36ac, H3K56ac, H4K12ac, H4K16ac, and H3K36me3 exhibited pan-nuclear localization away from heterochromatin. In comparison, repressive marks like H3K9me3 and H3K27me3 are localized to heterochromatin. Further, taking advantage of the delivery of small-molecule chemical inhibitors methotrexate (heterochromatin enhancer), heterochromatin inhibitor, anacardic acid (histone acetyltransferase inhibitor), trichostatin A (histone deacetylase inhibitor), IOX1 (JmjC demethylases inhibitor), and AZ505 (methyltransferase inhibitor) in seminiferous tubules through the rete testis route, revealed that alteration in histone modifications enhanced the centromere mislocalization, chromosome breakage, altered meiotic recombination and reduced sperm count. Specifically, IOX1 and AZ505 treatment shows severe meiotic phenotypes, including altering chromosome axis length and chromatin loop size via transcriptional regulation of meiosis-specific genes. Our findings highlight the importance of balanced chromatin modifications in meiotic prophase I chromosome organization and instability.