Chapter 8 Long-Term Evolution of Histone Families: Old Notions and New Insights into Their Mechanisms of Diversification Across

Abstract In eukaryotes and some archaebacteria, DNA is found associated with histones in a nucleoprotein complex called chromatin, which allows for a high extent of compaction of genomic DNA within the limited space of the nucleus. Early studies led to the notion that histones exhibit a conserved structural gene organization and limited protein diversity. However, research has been progres-sively accumulating to demonstrate that the structure, configuration and copy number of histone genes varies widely across organisms as a result of a long-term evolutionary process that promotes genetic variation. This genetic diversity is mirrored by the structural and functional diversity exhibited by the protein mem-bers of the different histone families that is, in most instances, concomitant with the complexity of the organism. The present chapter is aimed at providing a compre-hensive review of the most recent information on the origin of eukaryotic histone multigene families. Particular attention is paid to the structural and functional constraints acting on histones and their relevance for the progressive diversification of histone variants during evolution, especially as it pertains to histone gene organization and expression