Epigenetic Inheritance Systems (EISs) allow cells of different phenotype but identical genotype to transmit their phenotype to their offspring, even when the phenotype-inducing stimuli are absent, as is often the case. Jablonka et al. (ref. 2), name three types of EISs that may play a role in what has become known as cell memory.
Epigenetic variants exhibit spontaneous emergence and reversion. However, they can be induced by the presence of other genetic factors, and some alleles of a gene have been shown to convert the epigenetic status of the same locus on the homologous chromosome. Environmental factors are also known to influence the emergence and reversion of epigenetic factors. This produces the possibility that epigenetic variations might be produced at several loci and in several cells or organisms. If these systems would affect biological evolution, adaptive variation would occur, which is a Lamarckian form of evolution. The question then is, to what extent does epigenetic inheritance play a direct role in evolution?
Orthodox theories on biological evolution hold that the only role the environment plays is in the phase of selection: the environment determines on what grounds selection takes place and what characteristics are necessary for better reproduction opportunities. For selection to be possible, individuals within a species must differ somewhat, so that good characteristics can amplify and bad ones can be deleted from the gene pool. These differences between individuals are usually thought to arise from random mutations. The source of the variation that is necessary in Darwin's theory of evolution is the random variation in the sequence of the DNA bases that constitute the genes. The environment can influence these variations slightly (for example, radioactivity is known to influence the structure of DNA), but only in a random manner. In recent years, however, scientists are realizing the role of the environment in the story of life may have been underrated. Some forms of epigenetic inheritance may be maintained even through the production of germ cells (meiosis).
A number of experimental studies seems to indicate that epigenetic inheritance plays a part in the evolution of complex organisms. For example, Tremblay et al. (ref. 3), have shown that methylation differences between maternally and paternally inherited alleles of the mouse H19 gene are preserved. There are also numerous reports of heritable epigenetic marks in plants.
That epigenetic heredity seems to exist trangenerationally in complex organisms can be explained by allowing for minor epigenetic changes not affecting totipotency[?]. This puts some constraints on the extent to which epigenetic changes can be brought upon DNA, but it allows for EISs to play direct evolutionary roles.
1 G.W. Grimes; K.J. Aufderheide; Cellular Aspects of Pattern Formation: the Problem of Assembly. Monographs in Developmental Biology, Vol. 22. Karger, Basel (1991)
2 E. Jablonka; M. Lachmann and M.J. Lamb; Evidence, mechanisms and models for the inheritance of acquired characteristics, J. Theoret. Biol. 158: 245-268 (1992)
3 K.D. Tremblay; J.R. Saam; R.S. Ingram; S.M. Tilghman and M.S. Bartolomei; A paternal-specific methylation imprint marks the alleles of the mouse H19 gene. Nature Genet. 9: 407-413 (1995)
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