The theory of heritability of acquired characteristics, also known as Lamarckism after the French thinker JB Lamarck who was the first to explicitly defend the theory (1809), has great appeal from a number of perspectives, not just biologically, but also due to its philosophical, ethical, political and social connotations. Perhaps this explains why the theory has constantly surfaced over time, despite always eventually being dismissed on the grounds of insufficient support from scientific data and experiment. However, the theory is now enjoying a “comeback” in relation to discoveries made in epigenetics. So, can a relationship between the two be proved?
Epigenetics: genes and environmental factors
Epigenetics is the scientific study of changes that take place in the genomic expression of living beings as a fundamental result of certain modifications, but not mutations, to the DNA molecules comprising said genes and the proteins that cover them, mainly histones. These modifications chiefly consist of adding or removing certain chemical tags to or from DNA or histones, such as methyl, acetyl and phosphoric acid groups.
Such modifications can be caused by environmental factors. For example, an environmental change, such as an increase in temperature, produces a series of epigenetic modifications in the organ cells and somatic tissues of organisms, which generate alterations in the expression of certain genes (heat-shocks in this case), subsequently allowing organisms to resist the original environmental change.
Specifically, what happens is that these epigenetic modifications change the structure and arrangement of certain genes, which allows (or prevents) the transcription mechanisms (RNA polymers) from accessing the information contained within said genes, leading them to be enabled or disabled. In other words, they either begin to produce RNA messengers or they don’t, which eventually create (or don’t create) corresponding proteins, and it is these that shape the short-term response in living beings to environmental change.
Epigenetics and Lamarckism
Recent discoveries in the field of epigenetics have revolutionized our understanding of the roles played by various environmental factors in regulating genetic expression in organisms: food, temperature, chemical and physical components, atmosphere, diet, alcohol, tobacco, dugs, hormones, toxins and in humans even types of behavior and stress. These discoveries underscore just how important lifestyles and environmental factors are to the functioning of genes and organisms. These phenomena are particularly relevant in the field of medicine, as some diseases can be triggered by errors in genes and enzymatic systems, which can modify DNA or histones, helping us to read such modifications and correct them.
But in the context of Lamarckism the key question is whether the epigenetic effects experienced by an organism caused by environmental factors can somehow be passed on to subsequent generations. In other words, can they be inherited and last for generations, and therefore is so-called transgenerational epigenetic inheritance possible?
Indeed, a growing number of voices are arguing that in certain cases epigenetic modifications experienced by living beings can be inherited. Thus, in the specific case of humans, it is thought that the effects had by famine on the genes of a specific generation in Europe may have been handed down to subsequent generations. There is also debate over the transmission of epigenetic effects had by certain chemical products, such as fungicides. While in humans there is even the possibility that epigenetic impacts on genes had by certain stress types, such as being incarcerated in concentration camps, could also be inherited.
But this research field faces a fundamental problem and a huge misunderstanding. The problem for epigenetic modifications experienced by organisms, particularly mammals, in a specific generation being transferred to the following and subsequent generations, is that the modifications that the gametes of organisms (both male and female) can transmit are largely lost during embryonic development, with others taking their place.
While the misconception is that what is often thought to be inheritance of epigenetic modifications is nothing of the sort. Instead they are the result of actions on embryotic somatic cell genes had by environmental factors affecting the mothers, which then “transfer” to the embryos. In these cases, epigenetic effects will only be seen in the following generation, the so-called F1. And if these epigenetic modifications on embryos generated by the mother come in the reproductive cells of their gonads, the effects of such modifications may also be seen in the descendants of the embryos, F2, but no further. Therefore, in both cases these would be epigenetic modifications not inherited from the previous generation, but that take place at the outset of the embryo’s life.
Two criteria must both be met to identify true transgenerational epigenetic inheritance:
- The effects of epigenetic modifications must persist until the third generation at least.
- The effects must also occur in the absence of the environmental factor that initially determined the epigenetic modification.
At present very little evidence has been found in complex organisms, such as mammals, of the two criteria that would spell true transgenerational epigenetic inheritance. The only possible examples of both conditions being met have been found among plants and non-mammal organisms, and they are very few.
Therefore, this time in relation to epigenetics, it seems that Lamarckism can once again be dismissed, or at least put “on hold”.
Manuel Ruiz Rejón
Granada University, Autónoma de Madrid University