Evidence, mechanisms and models for the inheritance of acquired characters

Several different types of epigenetic inheritance system enable alternative functional states to be maintained in cell lineages that have identical DNA sequences. Both random and guided (directed) epigenetic variations can be transmitted by these systems, and lead to heritable modifications in cell structure and function. Although it is usually assumed that epigenetic inheritance does not occur between generations, both old and new experimental evidence suggest, and in some cases show explicitly, that epigenetic variations can be transmitted from parents to progeny. Simple models of epigenetic inheritance in asexual and sexual organisms are presented. These show that in populations of asexual unicellular organisms, the distinctive properties of induced epigenetic variations mean that the variations may be retained for many generations after the inducing stimulus is removed, even in the absence of selection. The models also show that the epigenetic systems enable some types of acquired character to be inherited in sexual, as well as asexual, organisms. The importance of epigenetic inheritance systems in the evolution of multicellularity is discussed.