In a recent article I described the teachings of Jean Baptiste de Lamarck (1744-1829), who believed that characteristics acquired by an animal during its lifetime are passed to its offspring, and proposed that this explains evolutionary changes in animal form over time.
His theory fell from favour because no supporting experimental evidence could be found, and also because Darwin and Wallace proposed that natural selection accounts for evolutionary change - a mechanism quickly accepted as being brilliantly obvious.
Later work revealed the basis of inheritance as individual units of information called genes, chemically made of DNA. Our sex cells - egg in female, sperm in male - contain sets of genes kept pristine for passing to the next generation. Apart from cases of accidental damage, the genes in the sex cells are not affected by the external environment or by any characteristics acquired by the rest of the body during its life. But is this conventional picture entirely correct?
There have been sporadic reports over the years of cases of the inheritance of acquired characteristics. This area was reviewed by Gail Vines in New Scientist (November 28th, 1998). For example, male rats administered the drug Alloxan, which makes the body less sensitive to the hormone insulin and therefore more prone to diabetes, produce offspring, and the offspring produce offspring that are more prone to diabetes.
In other experiments mice, exposed to morphine doses which damaged the nervous system, passed an impaired nervous system on to their offspring.
Pregnant Dutch women, who starved during the second World War, not unexpectedly, produced small babies. But when these babies grew and had babies of their own, their offspring were also smaller, even though their parents had been well fed and had not been genetically manipulated.
Our bodies are composed of many tissues, each of which has its own characteristic properties - e.g. liver, muscle, nerve etc. Every cell in each of these tissues contains exactly the same total genetic information, but the inventory of genes that is active in any particular tissue is uniquely characteristic of that tissue.
Each tissue develops and operates according to its own "instruction manual" that specifies which genes are active and which genes are switched off. The instruction manual provides "epigenetic" information.
The conventionally accepted theory holds that the epigenetic instruction manual is emptied of information during the formation of sperm and egg cells. This means all genes are fully available until the embryo (formed when a sperm unites with an egg) starts to develop specific tissues. But recent evidence suggests strongly that changes in epigenetic information are sometimes passed from parent to offspring.
Epigenetic information, in part, is present as a chemical modification of DNA called methylation, which seems to mark a gene for inactivation. Recent research has shown that some genes become methylated if you transfer the nucleus (where the genes are located) from a freshly fertilised mouse embryo into the egg of a mouse of a different strain that has its nucleus removed, and then let the hybrid embryo develop in the womb of another mouse. The subsequent mouse pups are also noticeably smaller.
Analysis showed two genes were shut down in these pups, but the really surprising thing was that when these mice grew and mated, their offspring again showed the same two genes methylated and switched off. The epigenetic information acquired in one generation had passed to the next generation.
If epigenetic information is heritable it could explain effects like the smaller children and grandchildren of the starved Dutch mothers. Starvation could trigger heritable methylation of certain genes. Heritable epigenetic information could also provide an important mechanism allowing species to adapt flexibly to changes in the environment. Again, the Dutch response to starvation could illustrate this. Severe food restriction may trigger chemical inactivation of genes, thereby producing smaller people needing less resources to grow to maturity. But when times of plenty return, environmental cues may trigger further chemical changes in DNA that activate the dormant genes, producing bigger people once more.
The central dogma of molecular biology states that there is a one-way flow of information from storage in DNA to expression in protein, and that information never flows back along the line from protein (or some other agent) to be incorporated into DNA. Much work has shown that this is largely true, but maybe it is not always absolutely true. The central dogma implies that information in DNA is highly conserved. We know that this is so.
Changes in DNA occur slowly and over a long time. If this were not true we would quickly lose our identities.
But it may also be true that, under certain circumstances, information in DNA is sensitive to environmental cues, thereby facilitating quick adaptive changes.
The general mechanism of evolution is the slow process of natural selection among naturally occurring variants and not any widespread inheritance of acquired characteristics, as envisaged by Lamarck, but from time to time organisms may also be capable of a bit of fancy footwork allowing them to get around various obstacles.
Biology is now very much in the era of the gene. Genes are unquestionably of great importance, but the concept is oversold. Many would have you believe that genes are all there is to biology. This leads to such vulgar notions as a gene for homosexuality, a gene for criminal behaviour, etc.
At the other end of the spectrum we have the equally wrong idea that the environment accounts for everything. The basic reality of biology is neither genetics nor the environment, but the interaction between the two. Think of genetics as music and the environment as a dancer. Life is the dance.
William Reville is a Senior Lecturer in Biochemistry and Director of Microscopy at UCC.