The traditional Darwinian view of evolution holds that evolution occurs through the selection of the most successful members of a group. Each member of the group is stable over its lifetime. This view was later modified to include the idea that DNA is the stable carrier of this information throughout the lifetime of an organism. But cells do hold other forms of genetic information or so-called epigenetic modifications -- that generally take the form of modifications to DNA such as methylation which influence their transcription. There is some evidence that these modifications are transmissible to the next generation. If that is true, one could argue for a more Lamarkian view of evolution.
(Does everyone remember high school biology? Lamarck was the guy who argued that evolution took place do to accumulating changes to the an organism that occured over the organism's lifetime -- the giraffe has a long neck because it stretched its neck to reach fruit that was further and further away. Darwin, on the other hand, argued that the inheritance was unchanging over the organisms life, but that evolution instead resulted from the selection of a variable stock of stable organisms.)
Anyway, Eric Richards from Washington University in St. Louis has an excellent review on the possibility of epigenetics as a means of inheritance. Money quote:
A growing body of evidence indicates that epigenetic states can be influenced by the environment. For example, prolonged cold-temperature treatments in plants can lead to both chromatin and DNA methylation changes at specific genomic loci. Treatment with DNA damaging agents that are used in traditional chemical mutagenesis protocols can also change epigenetic states. Indeed, some of the best-studied inherited epialleles in plants were derived originally from chemical mutagenesis experiments. Some of the most striking examples of environmental modulation of epigenetic states are derived from the recent animal literature. One class of examples involves the alteration of DNA methylation through dietary regimes that alter single carbon metabolism in rodents. In the case of the mouse Avy epiallele (Table 1), dietary supplements (for example, folic acid and vitamin B12) that increase the abundance of the central methyl donor metabolite S-adenosylmethionine elevate DNA methylation of the upstream IAP element and suppress Agouti overexpression Another intriguing example of an environmentally induced, mitotically stable epiallele was recently described in rats: nurturing maternal behaviour leads to postnatal remodelling of the epigenetic state of the hippocampal glucocorticoid receptor gene (GR; also known as nuclear receptor subfamily 3, group C, member 1 (NR3C1)), creating a hypomethylated epiallele that persists into adulthood. Mothers that show poor nurturing behaviour rear individuals with an alternative silent GR gene epiallele. The preceding examples show that epigenotypes can respond to an organism's physical, nutritional and even behavioural environment. Although these examples do not involve meiotic transmission of the environmentally induced epialleles, this is not always the case. A recent publication reported that treatment of gestating female rats with industrial chemicals that disrupt endocrine function can lead to male fertility defects in subsequent generations (F1 to F4), which are correlated with widespread alterations in DNA methylation. This study resembles an earlier report demonstrating transgenerational effects on gene expression, DNA methylation and growth efficiency that is induced by nuclear transplantation in mice. (Citations were removed. Emphasis mine.)
For the uninitiated here is a press release on the article that much more comprehensible. Money quote from that:
Still, recent studies in mice and rats have fueled the controversy. Richards cited "a whole new world called nutritional epigenomics," where researchers are trying to influence epigenetic information by of all things diet. In a study with mice hybrids, researchers provided pregnant moms with varying levels of folate and B vitamins, to affect DNA methylation.
"The idea was : If you pump these pregnant moms up with these dietary supplements, you might be able to skew the DNA methylation patterns, and thus skew the way the mice come out at the end of the day, and it works,'" Richards says. "In this particular instance that says what you're getting fed in the womb influences your phenotype - physical and physiological attributes. "
Another study showed that early grooming and nurturing of rat pups by rat moms affects methylation of a glucocorticoid receptor gene in the hippocampus in the brain. If the pups get lots of nurturing the glucocorticoid gene gets turned on and expressed early at a critical period, providing pups the beneficial outcome to handle stress later in life. Not enough nurturing and grooming, and the gene never gets turned on. Richards says that whole mechanism appears to be the result of changes in DNA methylation associated with changes in DNA packaging.
"These studies do not demonstrate inheritance between generations, but they do show that the early nutritional environment in the mice and early behavioral environment in the rat studies can change the DNA packaging on the genome, and that that is 'remembered' in the cell divisions that make the rest of the organism, " Richards says. "But this is not from one generation to another. No one has shown that yet.
This is interesting work, particularly the bit on behavioral inheritance. I would be very interested in any evidence demonstrating clearly that there is behavioral evolution on a molecular level.
Cool. I'll have a look at it when I start writing a post about this paper. And you should seriously read some Susan Oyama, Peter Godfrey-Smith and Eva Jablonka...
Richards says that whole mechanism appears to be the result of changes in DNA methylation associated with changes in DNA packaging.
"Anyway, Eric Richards from Washington University in St. Louis has an excellent review on the possibility of epigenetics as a means of inheritance"
The full text of Richards' paper, "Inherited epigenetic variation - revisiting soft inheritance") is currently available (without subscription) as a pdf file here
John Latter / Jorolat