15
Feb
09

Lactase

Lactase is present in all mammals at birth but only persists throughout life in some societies. These societies typically have a history of dairy production and consumption. Lactase persistence (LP) has arisen multiple times in human history. Strong selection for LP occurred in both European and African populations which historically have relied on dairy products for a substantial portion of calories. Some African populations still rely heavily upon dairy products for a large portion of dietary needs. This represents an ongoing selective pressure for LP in these populations. The European allele for lactase persistence has been shown to differ substantially from the African alleles which also have variation within them. The presence of multiple alleles for lactase persistence in Africa indicates multiple selective events which were separate from each other. Based upon this, it is expected that ongoing selective forces would have a higher LP frequency than in European populations as the selective force on most European populations has since subsided. It would thus be expected that in groups with ongoing selection for LP would have a higher frequency of LP.

These are the frequencies which are actually found:

Northern Europe1 .75
Chinese1 .06
Southern European1 .26
Maasai2 .36
While the LP frequency of Maasai children is higher than that of Southern Europeans, it does not demonstrate the predicted frequency of approaching .75 which would be expected for a strong selective force. This means Maasai must overcome the lactose intolerance (LI) frequency by compensating with some behavioral modification relying upon less raw milk and more processed milk products or other dietary sources. This behavior could include the use of blood and processed foods into their diets instead of unprocessed milk which is high in lactose. The Maasai LP level is much lower than the predicted value and only slightly higher than Southern European populations and far lower than Northern European populations.1
While While LP frequency is not a reliable measure for dietary intake of dairy products, it may still prove useful in evolutionary phylogenies of human populations. Lactase persistence is not a requirement of societies which are dependent upon dairy products if the lactose intake is low enough in those individuals without LP alleles. The multiple alleles present do not indicate stronger selection for LP, but rather for multiple origins of lactase persistence in these cultures. Additional research could shed light on how exactly these populations get around the inability to digest lactase with dietary supplementation from processed milk products and non-dairy animal products.
1) Lactase Haplotype Diversity in the Old World.Hollox, E.J., et alAm. J. Hum. Genet. 2001. 68:160–172
2) Lactose malabsorption among Masai children of east Africa. Jackson, R.T., et al. American Journal of Clinical Nutrition

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2 Responses to “Lactase”


  1. February 17, 2009 at 11:44 pm

    The speaker I saw last Friday (Sarah Tishkoff) did mention the use of blood and dairy processing in her talk.

    The other thing that she brought up which I found interesting, but didn’t mention in my last comment, had to do with genetic markers for natural selection. Again, I didn’t take notes, so sorry if I don’t get this exactly right.

    Due to genetic drift, the genome gets kind of rearranged during meiosis and reproduction. Because of this, the stretches of DNA that are conserved between members of a given population tend to get shorter and shorter through the generations. This fact can be exploited as a sort of “biological clock.” We can estimate how long it’s been since a given population underwent a significant bottleneck by looking at the length of conserved stretches in the genome. We can also see how closely two populations are by comparing the length of their conserved stretches.

    Of course, this is all assuming no natural selection–since natural selection acts against genetic drift. So we can also exploit this fact. In other words, the parts of a population’s genome with the longest conserved stretches will be the ones subject to the strongest selection forces. As it turns out, the stretches of DNA associated with lactase persistence are the longest conserved stretches (by far) in the human genome.

  2. 2 jaredcormier
    February 18, 2009 at 12:43 am

    Well, assuming no natural selection for something as vital as lactase in mammals (you know, lactose intolerant babies didn’t do so well before the 20th century…) In my personal opinion, with just that bit of information, assuming no selective pressure for the lactase gene itself is kind of like assuming no selective pressure for hemoglobin. The coding region of the gene is HIGHLY selected for. The outlying regions aren’t as strongly selected, but the lactase gene is pretty big in itself (~200 kDa). Lactase persistence sequences, on the other hand, is a pretty short stretch in comparison, they are usually associated with promoters and enhancers which are less than 200 bases long. The coding region, on the other hand is over 1000 bases long, making the persistence regions pretty damn small. Now, if Dr. Tishkoff was talking about the regions between the promoters and enhancers ALSO being conserved, well, that would be interesting, but be pretty much useless for determining bottlenecks as they may be subject to related selective pressures due to the role in enhancer location relative to the lactase gene.


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