Health, Facial Attractiveness, and HLA Diversity in Humans

Finally, the second installment of the X Vials... my PLoS pick of the week. But before I get to the paper, I should first explain that I'm a huge fan of PLoS (Public Library of Science) and open access publication. If you haven't heard of PLoS and don't know what I'm talking about, then go to http://www.plos.org/oa/index.html and read up. The short of it is this-- PLoS is a largely internet-based non-profit publisher of original, peer-reviewed scientific literature that is freely accessible to anyone who can access the internet. There is no journal access fee, as is true of most other scholarly journals that restrict access only to those individuals and institutions that can afford it. If you are an academic who has never stepped foot outside a large research university, then you probably don't know exactly how frustrating it can be when you don't have access to the primary literature you need. I spent a whole nine months in such a situation-- it wasn't fun.

So cheers to PLoS for having the vision to start a whole collection of scholarly journals that don't restrict access to select groups of people while excluding those less fortunate! You can download a PLoS paper from any computer, from any institution, in any country, and read to your heart's content. It's a great thing.

My PLoS pick this week is from PLoS ONE, Common HLA Alleles Associated with Health, but Not with Facial Attractiveness, by Coetzee et al. (2007). In this study, Coetzee et al. investigate the relationship between HLA (human leucocyte antigen) diversity, health, and facial attractiveness in 59 African women. To read the original article click here: www.plosone.org/article/fetchArticle.action?articleURI=info%3Adoi%2F10.1371%2Fjournal.pone.0000640

I was drawn to this paper because I've been interested in the link between genetic diversity at the major histocompatibility complex (which has its own acronym, MHC, and includes the HLA genes) and sexual selection. Coevolution with disease is one of the leading theories explaining the origin and maintenance of sexual reproduction (aka the Red Queen theory). Parasites and pathogens are good at evolving ways to circumvent our immune defenses. At a molecular level, the pathogen is evolving new surface binding proteins that disguise it's true identity and enable it to pass through the host's immune system. It's analogous to a hacker finding the right password to bypass internet security and access your bank account. So, just as we are all advised to change our internet passwords from time to time to prevent security breeches, our immune systems need to change their locks (antigen-presenting proteins) to keep the pathogens at bay. This process seems to require lots of genetic diversity and recombination of alleles (i.e. sexual reproduction). The MHC is in fact one of the most diverse gene regions of the mammalian genome.

Something so important to an individual's survival, and especially the survival of their future offspring, is bound to play a role in the selection of mates-- and there is evidence supporting this idea in humans and other vertebrates (mammals, birds, and fish). The question is, how do individuals detect another individual's MHC or HLA diversity and what exactly is being selected (maximum HLA diversity in offspring)?

Coetzee et al. focused their study on humans, testing the idea that facial attractiveness correlates positively with HLA diversity and perceived health. While some studies have reported positive correlations between HLA diversity and facial attractiveness (usually more symmetrical faces are judged as more attractive and this may be a useful proxy for sizing up a mate's HLA diversity) other studies have failed to find such a relationship. Coetzee et al. provide an additional study to try and clear up the apparent controversy, and do so using a population with higher expected pathogen loads than previous human populations studied.

As you can glean from their title, they found no evidence for any relationship between HLA diversity and facial attractiveness. Women who were homozygous at their HLA loci (two were examined) were not less likely to be considered healthy and attractive (by a cohort of males of the same ethnic group) than heterozygous women. They even looked at genetic distance, another way to measure diversity, and found no pattern. They did find, however, that women with common HLA alleles considered themselves healthier and reported fewer illnesses than women lacking those common alleles (however this did not affect male perceptions of female facial attractiveness or health).

Before making my own general conclusions, I like to think about the limitations of a study--they all have them. Coetzee et al. point out that they are dealing with a small sample size, which means limited statistical power to detect differences between groups. So maybe there is a pattern there, but they can't detect it. I will add the following-- this study only measured the HLA diversity of the female participants, not the males who ranked female facial attractiveness. My guess is that the HLA diversity of the males involved in the study also matters-- in other words, it's not diversity alone that is being sexually selected, but the particular kind of HLA diversity (alleles) possessed by a mate and how that diversity compares to your own. There's evidence now from multiple sources (mice, humans, sturgeon) suggesting negative-assortative mating with respect to MHC diversity (although not all studies have found this pattern). Translation-- individuals may prefer to mate with partners that have different MHC signatures than their own. Such a preference may lead to novel combinations of HLA genes in offspring that confer fitness advantages.

In my opinion, the question could be addressed more closely by looking for patterns at the individual level and testing whether an individual's HLA sequence predicts the HLA sequences of individuals that they find attractive. So you need genetic information from both sexes and ideally one would want to include an analysis of female preferences as well (anisogamy = choosier females, so if there is a pattern of MHC-based mating preferences, we would reasonably expect those preferences to be stronger in females).

Another point to make: MHC-HLA diversity is probably better known for its effect on individual body odor than facial attractiveness, and this is another mechanism by which individuals can assess each other's MHC-HLA signature (studies in humans and mice support this idea). So, I would also venture to say that facial attractiveness by itself may not be the strongest correlate of an individual's HLA signature. Body odor, or some combination of body odor and physical features, may be a more reliable HLA cue-- this puts a different spin on the perfume industry doesn't it?

So my take-home message from this paper is the following: females with common HLA alleles perceive themselves as healthy and report fewer illnesses than women with rare HLA alleles. Evidence to support a link between facial attractiveness, perceived health, and HLA diversity was not found, but keep in mind the study's limitations. There are still many interesting questions on this topic that remain to be answered-- good news for those interested in deciphering the behavioral and molecular mechanisms of mate choice (and of course eventually we want to link this back to what's going on with those parasites and pathogens that seem to be responsible for this whole sex thing in the first place).

That's all for now. For more info on MHC-dependent mating, there's a nice literature out there. Unfortunately, most of it is published in for-profit journals, so I can't link you to the papers here. For that, you are on your own!