October 23, 2012

Blood groups A and B inherited from simian ancestors

A new study has found that blood group A and B alleles have been stable (albeit in likely dynamic equilibrium) in the primate family since... always.

Laure Ségurel et al., The ABO blood group is a trans-species polymorphism in primates. PNAS 2012. Pay per view (6 months embargo) ··> LINK [doi:]

Abstract

The ABO histo-blood group, the critical determinant of transfusion incompatibility, was the first genetic polymorphism discovered in humans. Remarkably, ABO antigens are also polymorphic in many other primates, with the same two amino acid changes responsible for A and B specificity in all species sequenced to date. Whether this recurrence of A and B antigens is the result of an ancient polymorphism maintained across species or due to numerous, more recent instances of convergent evolution has been debated for decades, with a current consensus in support of convergent evolution. We show instead that genetic variation data in humans and gibbons as well as in Old World monkeys are inconsistent with a model of convergent evolution and support the hypothesis of an ancient, multiallelic polymorphism of which some alleles are shared by descent among species. These results demonstrate that the A and B blood groups result from a trans-species polymorphism among distantly related species and has remained under balancing selection for tens of millions of years—to date, the only such example in hominoids and Old World monkeys outside of the major histocompatibility complex.

Razib has some more details on the matter (being PPV, I haven't read it). Still he wonders what kind of disease or otherwise evolutionary pressure may have been so virulent as to keep the whole order of primates (or at the very least all simians) on our toes all these millions of years.

The answer may well be known already: it seems that type A blood protects against the plague, while type B protects against smallpox, both great historical killers of those without enough defenses. However they may favor other less important health problems like blood clots or cancer, enough to exert a mild pressure in favor of a return to the basic type zero ("O"), which would have evolved by loss of function once and again. 

Whatever the case I find fascinating that these immune mechanisms may be so extremely persistent and I wonder if the bacterian mechanisms they confront may be more generic than just an specific disease.

See also: maps of distribution of major blood types.


Update: a pre-print copy of the paper is available at arXiv.

12 comments:

  1. I think you mean catarrhines (excludes New World monkeys), not simians.

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    1. I meant to exclude lemurs, lorises and tarsiers, not New World Monkeys, which are indeed part of the ABO antigen group system, and according to the paper, fully part of this genetic inheritance pattern. However in them it seems that the antigens manifest elsewhere and not in the red blood cells.

      I updated with a link to the pre-pub paper, so you can read the details there.

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    2. My mistake! I was just going by the abstract, which makes no mention of New World monkeys.

      Delete
    3. You could not know. I actually did not know myself until I read the pre-pub paper, so I was speaking without a clear knowledge, admittedly.

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  2. What does this imply about groups with only one blood type? Certain American-Indian groups are all "O", I believe.

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    1. Very strong founder effect. It is correct that the Amerindian populations are nearly 100% O, while Na Dene and Inuits (or North American natives in general, depending who you read) instead have some strong A (B is very rare).

      They are probably less well protected against epidemic diseases like smallpox, which we know affected them severely in the conquest period.

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  3. Also, Maju, do you have any information on Rh- rates by nationality or ethnicity? One of the links in this post says that Basques are 35% Rh- (though it is unclear if this means "pure ethnic-Basques" or all residents of the Pais Vasco).

    What about other specific regions, nations, or subnationalities of Europe? (Scandinavians Norwegians? Saami? etc.)

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    1. Probably means pure-blooded Basques. But you would have to go to the particular study to find out.

      "What about other specific regions, nations, or subnationalities of Europe? (Scandinavians Norwegians? Saami? etc.)"

      Rh⁻ is essentially a "European marker", being most concentrated in the sub-Pyrenean region, with a Basque center but it is fairly common in all the continent and parts of North Africa and West Asia. You can see a map at this entry.

      It is often considered a "harmful" phenotype but in fact both are equally harmful: Rh⁻ for women (as mothers) with Rh⁺ couples, and Rh⁺ for men (as fathers) with Rh⁻ couples. What makes Rh⁻ "weak" is its recessive character vs. the Rh⁺ allele and its minority status in all populations, including Basques. IF Rh⁻, for whatever reason, would be widespread, then there'd be another perception because it's always the minority who is perceived with precaution primarily.

      I have no idea what evolutionary role it may have.

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  4. Only the first combination is causing problems. This is analogous to Dienekes' idea about
    Lacking the Rhesus factor is not the ancestral situation. Some natural selection (pathogen?) helped spreading Rh⁻. Rh⁻ was probably even fixed in the genome of the ancestors of Basque. In the last thousands of years these "old Europeans" started to mix with "new Europeans" that were Rh⁺. The interbreeding did not effect the Rh⁺ new Europeans but the old Europeans.

    There are 4 possible combinations, 2x interbreeding and 2x intrabreeding:
    1. Rh⁻ mother with Rh⁺ father (disadvantage)
    2. Rh⁺ mother with Rh⁻ father (neutral)
    3. Rh⁺ mother with Rh⁺ father (neutral)
    4. Rh⁻ mother with Rh⁻ father (neutral)

    After the last ice age, old European mtDNA might have been reduced more than Old European Y chromosomes in Europe due to Rh incompatibility.

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    1. I don't understand well what you mean but, even in case #1, the disadvantage is minor (13% risk and never for the first born). Also the balance is kept because the disadvantage is for both the Rh⁻ mother and the Rh⁺ father equally.

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  5. Sorry, one piece of my comment is missing.
    I think that thousands of years ago Rh⁺ and Rh⁻ were 2 different groups of humans.

    The situation is analogous to Dienekes' idea here:
    http://dienekes.blogspot.com/2012/10/neandertal-modern-hybrid-babies-and.html

    The balance for Rh⁻ and Rh⁺ might be the same, however, in case #1 mtDNA from Rh⁻ people are spreading less good the more Rh⁺ is in the same location. Y chromosome from Rh⁺ people are spreading less good the more Rh⁻ people are in the same location.

    By mixing of Rh⁺ and Rh⁻ people, the ancestral mtDNA diversity of the original Rh⁻ people was reduced due to this incompatibility.

    Rh⁻/Rh⁺ mixing caused fertility problems; even "13% risk" is a disadvantage in term of evolution.

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    1. Dienekes' speculation on Neanderthal baby heads has no scientific support. There is some info on baby Neanderthals (whose heads were similar to that of H. sapiens) but he's not even bothering with it. It sounds to me to total rant, never mind that nobody respectable uses "H. sapiens neanderthalensis" anymore (a clear sign of pushing for multiregionalism).

      "I think that thousands of years ago Rh⁺ and Rh⁻ were 2 different groups of humans".

      It might be but we lack the evidence.

      "Rh⁻ people are spreading less good the more Rh⁺ is in the same location".

      Same vice versa. Everything else equal, the optimal reproductive situation is all homozygous for either allele, same for Rh⁻ as for Rh⁺ (no difference, except that arguably the reproductive cost is higher for females - but NOT in mongamous cultures/species).

      In any case we don't know if the small cost of heterozygosity may be, as happens with other similar polymorphisms like sickle cell or the recently discovered malaria protection polymorphism from South Asian tribals, may be compensated by some sort of advantage.

      Just imagine that Rr heterozygous children might have a markedly better health, being protected against, say, smallpox or pneumonia or whatever. We don't know that yet but it is a serious possibility. If so, the cost (loss of 1 baby every 9 pregnancies, never the first one, nothing too radical considering how common perinatal mortality used to be before modern medicine) would be quite low compared with the benefits overall. You shouldn't really consider these genes in isolation most probably but there's probably a dynamic equilibrium involved, surely an immunological one.

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