Characterisation of histone variants in the cnidarian Hydractinia
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Hydractinia is a marine hydrozoan cnidarian that has been a model organism for developmental biology for over a century. Very few studies have been carried out on chromatin biology in cnidarians. To date, there is no comprehensive knowledge on histone repertoire even in well characterised cnidarian model organisms. Here I present a short evolutionary history and comparative analysis of sperm nuclear basic proteins (SNBPs) in marine invertebrates based on some of the most recent publications in the field. I also present a comprehensive analysis of the histone complement of Hydractinia along with a number of interesting histone variants, such as a rare, replication-dependent H3.3 and a female germ cell-specific H2A.X. Surprisingly, we also found an unusual set of four SPK[K/R]-repeat rich H2B variants (H2B.3-6), that are male germ cell-specific. No protamines were found in Hydractinia, and we confirmed the presence of nucleosomes in mature sperm. Thus, we suggested that Hydractinia exclusively uses histones to pack sperm DNA and that this process is aided by these novel N-terminal SPK[K/R]-containing H2B variants. In order to prove this, I studied in detail the expression pattern and function of H2B.3-6 through misexpression techniques. I also studied the effect of SPK[K/R]-repeat contaning N-terminal tails of H2B variants on chromatin accessibility by in vitro and in vivo assays. Here I show that H2B.3-6 replace canonical H2B at early stages of spermatogenesis and exclusively expressed throughout male gametogenesis, but they do not increase the level of chromatin compaction in sperm. However, nuclease accessibility assays reveal different levels of chromatin accessibility, indicating that the chromatin is less accessible in sperm than in somatic cells. Given this observation, we anticipated that the N-terminal tails of these H2B variants are bound to linker DNA, thus limiting accessibility and shutting down gene expression in sperm. Biological importance of the presence of H2B variants in sperm will also be discussed.