Developmentally arrested Austrofundulus limnaeus embryos have changes in post-translational modifications of histone H3.

Although vertebrate embryogenesis is typically a continuous and dynamic process, some embryos have evolved mechanisms to developmentally arrest. The embryos of Austrofundulus limnaeus, a killifish that resides in ephemeral ponds, routinely enter diapause II (DII), a reversible developmental arrest promoted by endogenous cues rather than environmental stress. DII, which starts at 24-26 days post-fertilization and can persist for months, is characterized by a significant decline in heart rate and an arrest of development and differentiation. Thus, A. limnaeus is a unique model to study epigenetic features associated with embryonic arrest. To investigate chromosome structures associated with mitosis or gene expression, we examined the post-translational modifications of histone H3 (phosphorylation of serine 10, mono-, di- and tri-methylation of lysine 4 or 27) in preDII, DII and postDII embryos. As seen by microscopy analysis, DII embryos have a significant decrease in the H3S10P marker for mitotic nuclei and an inner nuclear membrane localization of the H3K27me2 marker associated with silencing of gene expression. ELISA experiments reveal that the levels of methylation at H3K4 and H3K27 are significantly different between preDII, DII and postDII embryos, indicating that there are molecular differences between embryos of different chronological age and stage of development. Furthermore, in DII embryos relative to preDII embryos, there are differences in the level of H3K27me3 and H3K4me3, which may reflect critical chromatin remodeling that occurs prior to arrest of embryogenesis. This work helps lay a foundation for chromatin analysis of vertebrate embryo diapause, an intriguing yet greatly understudied phenomenon.