|dc.description.abstract||Background: Histone variants alter the composition of nucleosomes and play crucial roles in transcription,
chromosome segregation, DNA repair, and sperm compaction. Modification of metazoan histone variant lineages
occurs on a background of genome architecture that shows global similarities from sponges to vertebrates, but
the urochordate, Oikopleura dioica, a member of the sister group to vertebrates, exhibits profound modification of
this ancestral architecture.
Results: We show that a histone complement of 47 gene loci encodes 31 histone variants, grouped in distinct sets
of developmental expression profiles throughout the life cycle. A particularly diverse array of 15 male-specific
histone variants was uncovered, including a testes-specific H4t, the first metazoan H4 sequence variant reported.
Universal histone variants H3.3, CenH3, and H2A.Z are present but O. dioica lacks homologs of macroH2A and
H2AX. The genome encodes many H2A and H2B variants and the repertoire of H2A.Z isoforms is expanded
through alternative splicing, incrementally regulating the number of acetylatable lysine residues in the functionally
important N-terminal “charge patch”. Mass spectrometry identified 40 acetylation, methylation and ubiquitylation
posttranslational modifications (PTMs) and showed that hallmark PTMs of “active” and “repressive” chromatin were
present in O. dioica. No obvious reduction in silent heterochromatic marks was observed despite high gene density
in this extraordinarily compacted chordate genome.
Conclusions: These results show that histone gene complements and their organization differ considerably even
over modest phylogenetic distances. Substantial innovation among all core and linker histone variants has evolved
in concert with adaptation of specific life history traits in this rapidly evolving chordate lineage.
Keywords: histone complement, DNA repair, urochordate, posttranslational modification, endocycle, gametogenesis,
testes, H2A.Z, alternative splicing||en