Evolution of mental illness
I usually only discuss papers with original data, but I'm going to make an exception this week.
"Battle of the sexes may set the brain" was published in Nature by Christopher Badcock and Bernard Crespi. It's labeled "Opinion" but it is based on facts as well as theory. Their central hypothesis is that mental illness in humans is often the result of conflicts between genes inherited from the mother and father.
Imagine a gene -- let's call it IGF2, since that's it's name -- that causes a fetus to grow faster, resulting in somewhat higher birth weight. Babies with slightly higher birth weight tend to be healthier, up to a point, but they may endanger the health of the mother, at least a little. So most women shut down the copy of IGF2 in their eggs. Therefore, most babies get an active copy from their father and an inactive copy from their mother. Occasionally, though, the mother's copy is also active in the fetus, resulting in larger-than-normal babies. It turns out that these babies have a higher risk of autism.
Their reasoning was as follows. Over most of our evolutionary history, males had the opportunity to have children with more than one female. Therefore, mutations in males that increase the growth and survival of one of their children would be favored by natural selection. Most such mutations, however, would spend half their time (over generations) in females. Their, selection would also favor growth and survival of children, but not at the expense of other current or future children. So genes like IGF2 get shut down by most mothers but not by fathers.
The authors suggest that this and other undiscovered genes may have effects that increase the lifetime reproductive success of one parent at the expense of the other. Mothers would (usually) shut down genes they were passing to their children that made one child demand excessive resources, either as a faster-growing fetus or as an overly demanding child. Fathers would activate those genes and deactivate any genes with the opposite effect. So autism may be an example of excessive male influence on a child's genes.
The theory does not assume that autism is actually beneficial to anyone. Rather, it is the result of too many doses of male influence, which (in smaller doses) would benefit the child at the expense of the mother and her other children. They suggest that some other mental illnesses, including depression, represent the opposite extreme: excessive maternal damping-down of "selfish genes." Children with Prader-Willi syndrome are undemanding but prone to depression when they get older. This trait is linked to a maternal effect on a particular part of chromosome 15.
These ideas are far from proven, but they seem worth testing. In fact, the data on the association between autism and having two active copies of IGF2 was apparently inspired by an earlier paper by these authors.
Mothers and fathers have a shared interest in the survival of their offspring, but their interests are not quite identical. This is true of plants as well as animals, of course. Do corn plants have genes expressed in pollen that make one seed grow at the expense of others? If so, does the mother plant have ways of countering these selfish genes?
Comments
Wow, this is really interesting stuff. I'll have to download that article.
Posted by: VWXYNot? | September 16, 2008 11:40 PM