Histone genes of the razor clam Solen marginatus unveil new aspects of linker histone evolution in protostomes

摘要

The association of DNA with histones results in a nucleoprotein complex called chromatin that consists of re-petitive nucleosomal subunits. Nucleosomes are joined together in the chromatin fiber by short stretches of linker DNAthat interact with a wide diversity of linker H1 histones involved in chromatin compaction and dynamics. Although thelong-term evolution of the H1 family has been the subject of different studies during the last 5 years, the lack of moleculardata on replication-independent (RI) H1 variants from protostomes has been hampering attempts to complete the evolution-ary picture of this histone family in eukaryotes, especially as it pertains to the functional specialization they impart to thechromatin structure in members of this bilaterian lineage. In an attempt to fill this gap, the present work characterizes thehistone gene complement from the razor clam Solen marginatus. Molecular evolutionary analyses reveal that the H1 genefrom this organism represents one of the few protostome RI H1 genes known to date, a notion which is further supportedby its location within the monophyletic group encompassing the RI H1 variants in the overall phylogeny of eukaryotic H1proteins. Although the detailed characterization of the nucleotide substitution patterns in RI H1 variants agrees with themodel of birth-and-death evolution under strong purifying selection, maximum-likelihood approaches unveil the presenceof adaptive selection during at least part of the evolutionary differentiation between protostomes and deuterostomes. Thepresence of increased levels of specialization in RI H1 proteins from deuterostomes as well as the significant differencesobserved in electrostatic properties between protostome and deuterostome RI H1s represent novel and important prelimi-nary results for future studies of the functional differentiation of this histone H1 lineage across bilaterians.