Hydrozoan sperm-specific SPKK motif-containing histone H2B variants stabilise chromatin with limited compaction

Abstract

Many animals achieve sperm chromatin compaction and stabilisation by replacing canonical histones with sperm nuclear basic proteins (SNBPs) such as protamines during spermatogenesis. Hydrozoan cnidarians and echinoid sea urchins lack protamines and have evolved a distinctive family of sperm-specific histone H2Bs (spH2Bs) with extended N termini rich in SPK(K/R) motifs. Echinoid sperm packaging is regulated by spH2Bs. Their sperm is negatively buoyant and fertilises on the sea floor. Hydroid cnidarians undertake broadcast spawning but their sperm properties are poorly characterised. We show that Hydractinia echinata and H. symbiolongicarpus sperm chromatin possesses higher stability than somatic chromatin, with reduced accessibility to transposase Tn5 integration and to endonucleases in vitro. In contrast, nuclear dimensions are only moderately reduced in mature Hydractinia sperm. Ectopic expression of spH2B in the background of H2B.1 knockdown results in downregulation of global transcription and cell cycle arrest in embryos, without altering their nuclear density. Taken together, SPKK-containing spH2B variants act to stabilise chromatin and silence transcription in Hydractinia sperm with only limited chromatin compaction. We suggest that spH2Bs could contribute to sperm buoyancy as a reproductive adaptation.

Running the gauntlet: challenges to genome integrity in spermiogenesis

“…As if those were not dangerous enough, spermiogenesis truly tests the limits of genome integrity and quality control as protamines repackage DNA. Though not all organisms use SNBPs, such as zebrafish [ 35 ], cnidarians (e.g., jellyfish, corals) [ 36 ], and echinoids (e.g., sea urchins) [ 37 ], genome remodeling from the nucleosome-based architecture found in all other somatic and germline cell types to a hypercondensed protamine-based architecture is one of the most unique features of sperm cells in many animals. This dramatic reorganization sequentially replaces histones with sperm-specific SNBPs, which hypercompact the DNA from 10X in humans to 200X in Drosophila [ 8 , 38 , 39 ].…”

TheHydractiniacell atlas reveals cellular and molecular principles of cnidarian coloniality

“…sperm and spermatogenesis (6, 34): Clusters 6 and 34 were determined to be sperm and spermatogenesis due to expression of Piwi1 and Piwi2 (germline marker genes and expression of spermatogenesis specific markers including; known marker Histone H2B.3/4 (LOC130657957), verified by RNA in-situ hybridization 66 , and sperm specific antigen 16 (HSymV2.0_g12.22020 LOC130622507), Histone H2B.6 (LOC130636493), Tubulin alpha 1 chain (LOC130641471), and Myosin 4-like (LOC130621975). neurones (16, 26, 27, 29, 33): This group of clusters was identified as neuronal due to expression ELAV orthologs (Elav1; LOC130630563, Elav2; LOC130612043 and Elav3; LOC130630562) 27,67 .…”

“…sperm and spermatogenesis (6, 34): Clusters 6 and 34 were determined to be sperm and spermatogenesis due to expression of Piwi1 and Piwi2 (germline marker genes and expression of spermatogenesis specific markers including; known marker Histone H2B.3/4 (LOC130657957), verified by RNA in-situ hybridization 67 , and sperm specific antigen 16 (HSymV2.0_g12.22020 LOC130622507), Histone H2B.6 (LOC130636493), Tubulin alpha 1 chain (LOC130641471), and Myosin 4-like (LOC130621975).…”

The protamines of the noble false widow spiderSteatoda nobilisprovide an example of liquid-liquid phase separation chromatin transitions during spermiogenesis

“…SNBPs have been characterized in the phylum Porifera (Ausió et al, 1997) and the classes Schiphozoa (jellyfish Aurelia aurita, and Thaumatoscypha hexaradiatus), Hydrozoa (Catablema sp. ; Mitrocoma cellularia (Rocchini et al, 1996), Hydractinia echinata (Torok et al, 2023) ) and Anthozoa [anemones, Urticina (=Tealia) crassicornis, Anthopleura xanthogrammica and Metridium senile] of the phylum Cnidaria (Rocchini et al, 1996) (Rocchini et al, 1995b). SNBPs have also been analyzed in the phylum Annelida (Chaetopterus varipedatus) (Fioretti et al, 2012) and quite extensively in the phylum Mollusca (Casas et al, 1993;Subirana, 1973) and in the phylum Chordata (Lewis et al, 2003;Saperas and Ausio, 2013).…”