Wednesday, June 25, 2008

Female brain: hundreds of genetic differences

Björn Reinius and six co-authors (Sweden) have studied sexual differences in gene expression in the brains of humans, macaques, and marmosets. Marmosets (the right picture) were chosen because males and females look almost the same, unlike the other two species.

They focused on the occipital cortex (or visual cortex in the occipital lobe, if you wish), a graphics card inserted into the rearmost region of the brain. They found that the degree of brain differences is nearly proportional to the amount of general morphological differences: there are dozens of differences in gene expression in the marmosets but hundreds of differences in humans and macaques.
An evolutionarily conserved sexual signature in the primate brain (full scientific paper)
Generally, it was shown that the sexual signature is conserved during the evolution - so these differences between the sexes are at least tens of millions of years old, as you could have heard from your humble correspondent for years.

Another result that is well-known from non-genetics research has been confirmed, too. Specific aspects of female brains are more heavily evolutionarily constrained than the specific male features as well as the sexually neutral properties of the brain.

I guess that there could be a very general explanation of this fact (XX is inherited from the general "melting pot" while the way to inherit XY may depend on the context, and similar comments could apply even if the sexually specific gene is carried by non-sexual chromosomes). However, the authors use their different-variance observation to predict that the brain could have been important in the evolution of sex in primates.

Do these differences show up in the functioning of the brains?

This question is usually answered in the last sentence of the text. However, your choice of the source may matter. ;-) The last sentence of the abstract of the actual paper in PLoS genetics says the obvious thing:
Genes within sexual expression profiles may underlie important functional differences between the sexes, with possible importance during primate evolution.
They also argue that such insights could be helpful to treat sexually specific psychiatric diseases.

On the other hand, the last paragraph in popular presentations of the paper, e.g. in ScienceDaily, says the following:
Lead author Björn Reinius notes that the study does not determine whether these differences in gene expression are in any way functionally significant. Such questions remain to be answered by future studies.
Can you spot the difference? I find it disgraceful for the journalists-activists-liars to flagrantly deform the results of scientific research and to contaminate it by their own ideology. They should be ashamed and people on the street should be spitting on these journalists.

Björn Reinius is no idiot so he, of course, knows that a very large portion of differences in gene expression is guaranteed to be functionally significant. If it were not the case, the brain would be the first living object that is invariant under changes of the gene expression. So what remains for the future research is not to find out "whether" they are significant but "how" they are significant.

ScienceDaily has misinterpreted the very essence of the paper and why it's important and new. There are dozens of papers that show huge anatomical differences between male and female brains. But the goal of Reinius et al. was nothing else than to study the functional differences. And gene expression is the first example of functional issues.

Also, they decided that if the differences are functionally important, they should be evolutionarily conserved. Much like in the case of the very existence of gene expression dimorphism, they have also confirmed the other prediction in their published paper, not in a future paper: they are evolutionarily conserved. Could the journalists kindly notice what the paper is actually about?

Hat tip: Tom Weidig

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