The cnidarian Hydractinia echinata employs canonical and highly adapted histones to pack its DNA.
BACKGROUND
Cnidarians are a group of early branching animals including corals, jellyfish and hydroids that are renowned for their high regenerative ability, growth plasticity and longevity. Because cnidarian genomes are conventional in terms of protein-coding genes, their remarkable features are likely a consequence of epigenetic regulation. To facilitate epigenetics research in cnidarians, we analysed the histone complement of the cnidarian model organism Hydractinia echinata using phylogenomics, proteomics, transcriptomics and mRNA in situ hybridisations.


RESULTS
We find that the Hydractinia genome encodes 19 histones and analyse their spatial expression patterns, genomic loci and replication-dependency. Alongside core and other replication-independent histone variants, we find several histone replication-dependent variants, including a rare replication-dependent H3.3, a female germ cell-specific H2A.X and an unusual set of five H2B variants, four of which are male germ cell-specific. We further confirm the absence of protamines in Hydractinia.


CONCLUSIONS
Since no protamines are found in hydroids, we suggest that the novel H2B variants are pivotal for sperm DNA packaging in this class of Cnidaria. This study adds to the limited number of full histone gene complements available in animals and sets a comprehensive framework for future studies on the role of histones and their post-translational modifications in cnidarian epigenetics. Finally, it provides insight into the evolution of spermatogenesis.
Centre for Chromosome Biology, School of Natural Sciences, National University of Ireland, Galway, Ireland.
Anna
A
Török
Genetics Environment and Evolution, University College London, London, UK.
Philipp H
PH
Schiffer
Division of Intramural Research, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892 USA ; Whitney Laboratory for Marine Bioscience, University of Florida, St. Augustine, FL 32080 USA.
Christine E
CE
Schnitzler
Whitney Laboratory for Marine Bioscience, University of Florida, St. Augustine, FL 32080 USA ; Australian Research Council Centre of Excellence in Plant Cell Walls, School of Biosciences, The University of Melbourne, Parkville, VIC 3010 Australia.
Kris
K
Ford
Division of Intramural Research, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892 USA ; NIH Intramural Sequencing Center, National Human Genome Research Institute, National Institutes of Health, Rockville, MD 20852 USA.
James C
JC
Mullikin
Division of Intramural Research, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892 USA.
Andreas D
AD
Baxevanis
Australian Research Council Centre of Excellence in Plant Cell Walls, School of Biosciences, The University of Melbourne, Parkville, VIC 3010 Australia.
Antony
A
Bacic
Centre for Chromosome Biology, School of Natural Sciences, National University of Ireland, Galway, Ireland.
Uri
U
Frank
Centre for Chromosome Biology, School of Natural Sciences, National University of Ireland, Galway, Ireland.
Sebastian G
0000-0002-8026-1336
SG
Gornik
Chromatin; Cnidaria; Histone; Histone variants; Sperm-specific histones