Significance of avian linker histone (H1) polymorphic variation

Most of avian histone H1 non-allelic subtypes, i.e. eight out of nine, show polymorphic heterogeneity manifested by the presence of two or three allelic variants formed as a result of amino acid deletion and substitution. In addition, some of histone H1 non-allelic subtypes exhibit various allelic complements in different bird species leading to the widening of a whole pool of histone H1 polymorphic variation. A wide range of histone H1 heterogeneity may indicate that the polymorphic variants can individually modulate some histone H1-dependent cellular processes by showing allele-specific influence on chromatin organization and function. Although, the exact way of avian histone H1 allelic variants' activity is not known, their structural separateness inferred from biochemical experiments and relationship with some characteristics of organism functioning disclosed in the genetic studies seem to confirm their importance. The aim of this review is to characterize the molecular origin of histone H1 polymorphisms and draw attention to the link between the histone H1 polymorphic variants and avian organismal features related to the physiological effects of bird individuals' living in the natural and breeding populations.