Epigenetics in the red meat industry

Summary

During the life of an animal chemical changes occur in the chromosomes that affect the expression of genes and hence the phenotype of the cell. These changes can be passed on during mitosis so that the daughter cells have the same chemical changes or epigenetic marks as the parent cell. Generally, the epigenetic marks are wiped clean in the process of forming sperm and eggs and a new zygote so that the new one-cell embryo has the potential to form all the different cell types in the body. However, occasionally some epigenetic marks are not wiped clean and are passed on from parent to offspring. These changes are called epimutations to distinguish them from ordinary mutations, which are a change in the sequence of DNA bases. It is the inheritance across generations of epigenetic marks or epimutations and their exploitation by the sheep and cattle industries that is the main topic of this review. 
The inheritance of epigenetic marks across generations is difficult to prove. There are few well documented cases, mainly using inbred strains of mice. The epimutations are unstable and revert to wild type after a few generations. Although, there are no known cases in sheep or cattle, it is likely that inherited epimutations occur in these species but it is unlikely that they explain a large part of the inherited or genetic variation. There is limited evidence in mice and rats that an environmental treatment can cause a change in the epigenetic marks of an animal and that this change can be passed on the next generation. If substantiated, this is a mechanism for the inheritance of acquired characteristics.
There are other phenomena that are related to inheritance of epigenetic marks. At a small proportion of genes, around one hundred in mice and humans, the allele inherited from either the sire or dam is not expressed. This is called imprinting and involves an epigenetic mark that is generated in the formation of the sperm or egg but which lasts for only one generation. In these specific cases, epigenetic marks, although not part of the DNA sequence, are part of the chromosome and can be inherited with it. There is some evidence for the transmission of information from parent to offspring independent of chromosomes. 
In the case of the mother, there are numerous potential mechanisms by which she could affect her offspring (ie maternal effects). However, it is less obvious how the sire could affect his offspring other than through the chromosomes. One possibility for such a paternal effect is RNA transmitted in the sperm. If inherited epimutations occur in sheep and cattle, they will already be utilised to some extent by existing genetic improvement programs. It would be possible to modify the statistical models used in the calculation of EBVs to better recognise the variance controlled by epimutations, but it would probably have, at best, a small effect on the rate on genetic gain achieved. 
The inheritance of epigenetic marks caused by the environment experienced by the sire offers a new opportunity in sheep and cattle breeding. However, at present we do not know if this occurs or, if it does, what environmental treatment might have a beneficial effect. We conclude that there are no immediate opportunities for exploitation of epigenetic inheritance across generations in the sheep or cattle industries. However, this is an area of very active research in basic biology and MLA should maintain a watch on developments in our knowledge to see if opportunities arise in the future. We list some possible research projects in the report.