Chance News 87

MHC dating and mating

Wikipedia says

Immunology is a branch of biomedical science that covers the study of all aspects of the immune system in all organisms.[1] It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders (autoimmune diseases, hypersensitivities, immune deficiency, transplant rejection); the physical, chemical and physiological characteristics of the components of the immune system in vitro, in situ, and in vivo. Immunology has applications in several disciplines of science, and as such is further divided.

Within one of those divisions is the study of MHC, the major histocompatibility complex. From Chance News 39: MHC which referenced Wikipedia,

The major histocompatibility complex (MHC) is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system, autoimmunity, and reproductive success.

It has been suggested that MHC plays a role in the selection of potential mates, via olfaction. MHC genes make molecules that enable the immune system to recognise invaders; generally, the more diverse the MHC genes of the parents, the stronger the immune system of the offspring. It would obviously be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with.

It has been further proposed that despite humans having a poor sense of smell compared to other organisms, the strength and pleasantness of sweat can influence mate selection. Statistically proving all of this--that is, by means of olfaction we somehow sense and select mates who are different in MHC-- via a convincing clinical trial is a challenge. Nevertheless, several things are in its favor. For one, unlike a medical trial, it is quite inexpensive to have subjects smelling a series of odoriferous T-shirts of contributors. For another, the lay media are sure to publicize the study with lame and juvenile headlines. The previously mentioned Chance News post describes several clinical trials and lay media reaction.

The latest manifestation of the linkage between MHC and mate selection is illustrated by this newspaper article which appeared throughout the country. Pheromone parties would appear to be the next big thing, replacing online dating. The only thing needed to make a match is a smelly T-shirt in a freezer bag. The article refers to Prof. Martha McClintock, founder of the Institute for Mind and Biology at the University of Chicago. "Humans can pick up this incredibly small chemical difference with their noses. It is like an initial screen.”

Discussion

1. MHC in humans is often called HLA, human leukocyte antigen.

2. An expert in the field informed Chance News that the cost of MHC serological typing is about $100 per person but is “not as accurate as molecular methods.” Previous T-shirt studies mentioned in Chance News involved about 100 people, thus an outlay of about $10,000.

3. The expert referred to in #2 says he serologically typed his former girlfriends and, as is often alleged, indeed, each was MHC dissimilar to him. No T-shirt was involved. He is “convinced it [olfaction] is a real biological phenomenon” because “it makes sense” and “the large amount of non-human data that supports it.” However, he further adds,

I would not recommend HLA testing for couples, because there are too many other sociological and physical variable that play into human ideas of attractiveness and mate preference. And from a strictly biological advantage standpoint, it doesn’t really matter anymore (see below).

The alleles that provide a selective advantage for pathogen protection are presumably the alleles that are most common, especially when maintained as a haplotype, which is the A-B-C-DR complex on one chromosome. For instance, A1, B8, DR17 haplotype is found in approx. 5% of Caucasians. But now that medical science allows for survival from infections via antimicrobics (antibiotics), there really is no such thing as immunologic evolution, because an infection that would have killed someone in childhood without the appropriate HLA 100 years ago can be saved, and go on to breed and pass the genes to their offspring. Science has, in essence, negated the Darwinian evolution process, particularly when it comes to the immune system.

4. Chance News also contacted several research immunologists to obtain their opinion on the T-shirt phenomenon. One had never heard of it and the other was very skeptical.

5. The following table of T-shirt smelling is taken from a review article by Roberts where “ns” stands for not [statistically] significant.

The following table regarding marital choice is from the same article. How does this compare and contrast with the T-shirt table above?

6. A study not mentioned in the above table regarding marital (mating) choice is Is Mate Choice in Humans MHC-Dependent?.

In this study, we tested the existence of MHC-disassortative mating in humans by directly measuring the genetic similarity at the MHC level between spouses. These data were extracted from the HapMap II dataset, which includes 30 European American couples from Utah and 30 African couples from the Yoruba population in Nigeria.

For the 30 African couples, the authors conclude

African spouses show no significant pattern of similarity/dissimilarity across the MHC region (relatedness coefficient, R = 0.015, p = 0.23), whereas across the genome, they are more similar than random pairs of individuals (genome-wide R = 0.00185, p<10−3).

For the 30 Utah couples, the authors conclude

On the other hand, the sampled European American couples are significantly more MHC-dissimilar than random pairs of individuals (R = −0.043, p = 0.015), and this pattern of dissimilarity is extreme when compared to the rest of the genome, both globally (genome-wide R= −0.00016, p = 0.739) and when broken into windows having the same length and recombination rate as the MHC (only nine genomic regions exhibit a higher level of genetic dissimilarity between spouses than does the MHC). This study thus supports the hypothesis that the MHC influences mate choice in some human populations.

Comment on the last sentence especially with regard to the size of the sample(s) and the strength of the conclusion.

7. Even if you are not of Yoruba ancestry nor reside in Utah, ask your parents if they are willing to have a serological MHC test to determine if they contributed to your genetic wellbeing.

8. Chance News was unable to find any MHC studies, T-shirt or otherwise, regarding the reproductive strategy or mate selection for homosexuals.

Submitted by Paul Alper