Epigenetic inheritance

Epigenetic inheritance is the transmission of information from a cell or multicellular organism to its descendants without that information being encoded in the nucleotide sequence of the genes. Epigenetic inheritance occurs in the development of multicellular organisms: dividing fibroblasts for instance give rise to new fibroblasts even though their genome is identical to that of all other cells. Epigenetic transmission of traits also occurs from one generation to the next in some organisms, though it is comparatively rare. It has first been observed in maize. The study of epigenetic inheritance is known as epigenetics.

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.

1. Steady-state systems. Some metabolic patterns are self-perpetuating. Sometimes a gene, after being turned on, transcribes a product (either directly or indirectly) that maintains the activity of that gene. Descendants of the cell in which the gene was turned on will inherit this activity, even if the original stimulus for gene-activation is no longer present. Also, diffusion of the gene's product to other cells can make the (heritable) characteristic spread.
2. Structural inheritance systems. In ciliates such as Tetrahymena and Paramecium, genetically identical cells show heritable differences in the patterns of ciliary rows on their cell surface. Experimentally altered patterns can be transmitted to daughter cells. It seems existing structures act as templates for new structures. The mechanisms of such inheritance are unclear, but reasons exist to assume that multicellular organisms also use existing cell structures to assemble new ones. (ref. 3)
3. Chromatin-marking systems. Proteins or chemical groups that are attached to DNA and modify its activity are called chromatin marks. These marks are copied with the DNA. For example, several cytosines in eukaryotic DNA are methylated. The number and pattern of such methylated cytosines influences the functional state of the gene: low levels of methylation correspond to high potential activity while high levels correspond to low activity. While there are random changes in the methylation pattern, there are also very specific ones, induced by environmental factors. After DNA replication, maintenance methylases make sure the methylation pattern of the parental DNA is copied to the daughter strand.

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 transgenerationally 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.

References

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)