Wednesday, October 27, 2010

How/why did selection for lactase persistence occur and spread?

Interesting, albeit speculative, hypothesis for the origins of the selective advantage associated with lactase persistence. Basically, looks like there is evidence for increased reliance on milk products and dairy animals at the same time as a sudden cold and dry spell - about 8,000 years ago.
Link to new story in Science

...on a related note, see a post by Razib pointing to a story about "How Middle Eastern Milk Drinkers Conquered Europe".

Saturday, October 16, 2010

Selection in fruitflies - what kind of genomic signatures does it leave?

We are pretty confident that we know what signatures of selection look like for some loci in humans (skin color, lactase etc...) but how about less clear-cut/subtle cases which may represent the majority of cases? This experiment gives us some much needed insight into what signatures of selection we might expect for most instances of natural selection.

Genome-wide analysis of a long-term evolution experiment with Drosophila

Molly K. Burke, Joseph P. Dunham, Parvin Shahrestani, Kevin R. Thornton, Michael R. Rose & Anthony D. Long
Nature 2010
Abstract: Experimental evolution systems allow the genomic study of adaptation, and so far this has been done primarily in asexual systems with small genomes, such as bacteria and yeast. Here we present whole-genome resequencing data from Drosophila melanogaster populations that have experienced over 600 generations of laboratory selection for accelerated development. Flies in these selected populations develop from egg to adult ~20% faster than flies of ancestral control populations, and have evolved a number of other correlated phenotypes. On the basis of 688,520 intermediate-frequency, high-quality single nucleotide polymorphisms, we identify several dozen genomic regions that show strong allele frequency differentiation between a pooled sample of five replicate populations selected for accelerated development and pooled controls. On the basis of resequencing data from a single replicate population with accelerated development, as well as single nucleotide polymorphism data from individual flies from each replicate population, we infer little allele frequency differentiation between replicate populations within a selection treatment. Signatures of selection are qualitatively different than what has been observed in asexual species; in our sexual populations, adaptation is not associated with ‘classic’ sweeps whereby newly arising, unconditionally advantageous mutations become fixed. More parsimonious explanations include ‘incomplete’ sweep models, in which mutations have not had enough time to fix, and ‘soft’ sweep models, in which selection acts on pre-existing, common genetic variants. We conclude that, at least for life history characters such as development time, unconditionally advantageous alleles rarely arise, are associated with small net fitness gains or cannot fix because selection coefficients change over time.

Thursday, September 09, 2010

More loci to explain eye color variation, but still not great prediction

Here they do a GWAS for a continuous/refined eye phenotype based on hue and saturation. The authors find three new loci in addition to the other known loci, and are able to explain 50% of the variance in an independent (I think) sample of Dutch individuals. HERC2 alone explains 45% or so of the variance.

Regarding prediction of eye color categories:
The accuracy in predicting 3-category eye color was 0.92 for blue, 0.74 for intermediate, and 0.93 for brown,...
There is some discussion about the limited quality of the photos due in part to the un-standardized lighting conditions.
I wonder how much better prediction would have been in a more diverse sample.

Digital quantification of human eye color highlights genetic association of three new loci.
Liu F, Wollstein A, Hysi PG, Ankra-Badu GA, Spector TD, Park D, Zhu G, Larsson M, Duffy DL, Montgomery GW, Mackey DA, Walsh S, Lao O, Hofman A, Rivadeneira F, Vingerling JR, Uitterlinden AG, Martin NG, Hammond CJ, Kayser M.

PLoS Genetics 2010 May 6;6:e1000934.
Abstract: Previous studies have successfully identified genetic variants in several genes associated with human iris (eye) color; however, they all used simplified categorical trait information. Here, we quantified continuous eye color variation into hue and saturation values using high-resolution digital full-eye photographs and conducted a genome-wide association study on 5,951 Dutch Europeans from the Rotterdam Study. Three new regions, 1q42.3, 17q25.3, and 21q22.13, were highlighted meeting the criterion for genome-wide statistically significant association. The latter two loci were replicated in 2,261 individuals from the UK and in 1,282 from Australia. The LYST gene at 1q42.3 and the DSCR9 gene at 21q22.13 serve as promising functional candidates. A model for predicting quantitative eye colors explained over 50% of trait variance in the Rotterdam Study. Over all our data exemplify that fine phenotyping is a useful strategy for finding genes involved in human complex traits.

Monday, April 26, 2010

Diet, disease, and pigment variation in humans

Some interesting hypotheses about the relationships between skin pigmentation, vitamin D, and immune response in Europe.

Diet, disease and pigment variation in humans.
Med Hypotheses. 2010 Apr 19. [Epub ahead of print]
Abstract: There are several hypotheses which explain the de-pigmentation of humans. The most prominent environmental explanation is that reduced endogenous vitamin D production due to diminished radiation at higher latitudes had a deleterious impact on fitness. This drove de-pigmentation as an adaptive response. A model of natural selection explains the high correlations found between low vitamin D levels and ill health, as vitamin D's role in immune response has clear evolutionary implications. But recent genomic techniques have highlighted the likelihood that extreme de-pigmentation in Eurasia is a feature of the last 10,000years, not the Upper Pleistocene, when modern humans first settled northern Eurasia. Additionally the data imply two independent selection events in eastern and western Eurasia. Therefore new parameters must be added to the model of natural selection so as to explain the relatively recent and parallel adaptive responses. I propose a model of gene-culture co-evolution whereby the spread of agriculture both reduced dietary vitamin D sources and led to more powerful selection on immune response because of the rise of infectious diseases with greater population densities. This model explains the persistence of relatively dark-skinned peoples at relatively high latitudes and the existence of relatively light-skinned populations at low latitudes. It also reinforces the importance of vitamin D as a micronutrient because of the evidence of extremely powerful fitness implications in the recent human past of pigmentation. Copyright © 2010. Published by Elsevier Ltd.

Thursday, March 11, 2010

Does Usain Bolt lack the ACTN3 R577X polymorphism?

...that's the question that pops into my mind after reading the title and abstract. I wonder if they genotyped the elite of the elite (Bolt, Powell) or just the "elite"? It looks like they did:
Forty-six of these international athletes had won medals at major international competition or held sprint world records.
Nevertheless, another negative finding on the association between the ACTN3 fast twitch muscle fiber, "sprint" genotype and sprint phenotypes. In fact, two of the elite-elite Jamaican sprinters were homozygous for the slow-twitch variant! They acknowledge that the power to detect a difference here is pretty low because of the very low frequency of the ACTN3 R5777X polymorphism in West Africans... so much for personalized prediction of sports ability, at least for now. I say just look at your big toe... does it extend out farther than your second toe? if so, you could probably be pretty fast.
Given these results, Usain probably does lack the ACTN3 polymorphism, but on the other hand, he could be the one of the two out of the 46 elite-elite sprinters who is homozygous for it.

P.S. - gotta love the senior author's name!

ACTN3 and ACE Genotypes in Elite Jamaican and US Sprinters
ABSTRACT The angiotensin-converting enzyme (ACE) and the [alpha]-actinin-3 (ACTN3) genes are two of the most studied "performance genes" and both have been associated with sprint/power phenotypes and elite performance. Purpose: To investigate the association between the ACE and the ACTN3 genotypes and sprint athlete status in elite Jamaican and US African American sprinters. Methods: The ACTN3 R577X and the ACE I/D and A22982G (rs4363) genotype distributions of elite Jamaican (J-A; N = 116) and US sprinters (US-A; N = 114) were compared with controls from the Jamaican (J-C; N = 311) and US African American (US-C; N = 191) populations. Frequency differences between groups were assessed by exact test. Results: For ACTN3, the XX genotype was found to be at very low frequency in both athlete and control cohorts (J-C = 2%, J-A = 3%, US-C = 4%, US-A = 2%). Athletes did not differ from controls in ACTN3 genotype distribution (J, P = 0.87; US, P = 0.58). Similarly, neither US nor Jamaican athletes differed from controls in genotype at ACE I/D (J, P = 0.44; US, P = 0.37). Jamaican athletes did not differ from controls for A22982G genotype (P = 0.28), although US sprinters did (P = 0.029), displaying an excess of heterozygotes relative to controls but no excess of GG homozygotes (US-C = 22%, US-A = 18%). Conclusions: Given that ACTN3 XX genotype is negatively associated with elite sprint athlete status, the underlying low frequency in these populations eliminates the possibility of replicating this association in Jamaican and US African American sprinters. The finding of no excess in ACE DD or GG genotypes in elite sprint athletes relative to controls suggests that ACE genotype is not a determinant of elite sprint athlete status.

More than one molecular way to adaptively change a phenotype

The authors report on their finding of different mutations in the same gene MC1R affecting different molecular pathways on the way to lighter pigmentation in lizards. There are several likely examples of molecularly divergent phenotypic convergence in humans, some of which may originate in the same gene: MC1R - skin color in Europeans and E. Asians, LCT - lactase persistence in Europe, Middle East, and E. Africa, and probably, adaptation to high altitude in E. Africa, Andes, and Himalayas.

The cool thing about this paper is that they use cell culture to find that even though the mutations are in the same gene, they result in lighter pigmentation through different molecular pathways.

Molecular and functional basis of phenotypic convergence in white lizards at White Sands
Erica Bree Rosenblum, Holger Römpler, Torsten Schöneberg, and Hopi E. Hoekstra
PNAS February 2, 2010 vol. 107 no. 5 2113-2117
Abstract: There are many striking examples of phenotypic convergence in nature, in some cases associated with changes in the same genes. But even mutations in the same gene may have different biochemical properties and thus different evolutionary consequences. Here we dissect the molecular mechanism of convergent evolution in three lizard species with blanched coloration on the gypsum dunes of White Sands, New Mexico. These White Sands forms have rapidly evolved cryptic coloration in the last few thousand years, presumably to avoid predation. We use cell-based assays to demonstrate that independent mutations in the same gene underlie the convergent blanched phenotypes in two of the three species. Although the same gene contributes to light phenotypes in these White Sands populations, the specific molecular mechanisms leading to reduced melanin production are different. In one case, mutations affect receptor signaling and in the other, the ability of the receptor to integrate into the melanocyte membrane. These functional differences have important ramifications at the organismal level. Derived alleles in the two species show opposite dominance patterns, which in turn affect their visibility to selection and the spatial distribution of alleles across habitats. Our results demonstrate that even when the same gene is responsible for phenotypic convergence, differences in molecular mechanism can have dramatic consequences on trait expression and ultimately the adaptive trajectory.

Tuesday, March 02, 2010

Chinese prefer darker-skinned pigs and MC1R tells part of the story

I'm not yet able to get full text access to this paper, but after looking into the related literature, I was suprised to see that there is quite a lot out there. Given that variation in MC1R was selected upon to lighten skin in Europeans, I was somewhat surprised to see the opposite happen in pigs.
Spaniards also like black pigs.
By the way, I'm not too fond of the term "artificial" in this type of context.

Artificial selection of the melanocortin receptor 1 gene in Chinese domestic pigs during domestication.
Li J, Yang H, Li JR, Li HP, Ning T, Pan XR, Shi P, Zhang YP
Heredity. 2010 Feb 24. [Epub ahead of print]
Abstract: Black coat colour is common in Chinese indigenous domestic pigs, but not among their wild ancestors, and it is thus presumed to be a 'domestication trait.' To determine whether artificial interference contributes to morphological diversification, we examined nucleotide variation from 157 Chinese domestic pigs and 40 wild boars in the melanocortin receptor 1 (MC1R) gene, which has a key role in the coat pigmentation of Sus scrofa. Compared with a pseudogene GPIP, our results showed that the joint effects of demography and selection have resulted in markedly low genetic diversity of MC1R in Chinese domestic pigs. Coalescent simulation and selection tests further suggest that the fixation of two non-synonymous substitutions associated with black colour is the result of artificial selection. In contrast, a much higher genetic diversity and only a single non-synonymous substitution were found among the wild boars, suggesting a strong functional constraint. Moreover, our conclusion is consistent with the preference for black colour in the ancient Chinese sacrificial culture. This case provides an interesting example of a molecular evaluation of artificial livestock selection and its associated cultural impact in ancient China.

Sunday, February 28, 2010

Predicting hair, eye, and skin color from a small set of SNPs

They examined the association between 75 SNPs in 24 genes and skin, eye and hair color among 789 people of various ethnic backgrounds. Since this is for forensic purposes, they were looking for a small set of SNP markers (i.e. 3) that could reliably predict these pigmentation phenotypes, independent of ethnic origin. Their sample consisted mostly of individuals of European descent, but a decent number of several other ethnic groups.

Hair color:

SLC45A2, SLC24A5 and MC1R - R squared: 76.3% (one SNP per gene listed)
Skin color:
SLC24A5, SLC45A2, ASIP - R squared: 45.7% ... interaction term of ASIP and SLC45A2 increased r-squared to 49.6% (one SNP per gene listed)
Eye color:
HERC2, SLC24A5, SLC25A2 - R squared: 76.4%... (HERC2 appears to be doing the vast majority of the explaining)

The obvious remaining question from all this is how high does the proportion of variance explained go if you use information from all markers together. Anyway, it appears that, as they mention, five SNPs in five genes account for much of the variation.
Given that most subjects were Eur, it would have been nice to see the extent to which they were driving the results, by for example, doing the same analysis only on them. In other words how different would the results be if most subjects were African or Native American etc...?
I did not know that HERC2 is adjacent (5' side) to OCA2, and contains a promoter region for OCA2.

Predicting Phenotype from Genotype: Normal Pigmentation
Valenzuela RK, Henderson MS, Walsh MH, Garrison NA, Kelch JT, Cohen-Barak O, Erickson DT, John Meaney F, Bruce Walsh J, Cheng KC, Ito S, Wakamatsu K, Frudakis T, Thomas M, Brilliant MH.
J Forensic Sci. 2010 Feb 11. [Epub ahead of print]

Abstract:Genetic information in forensic studies is largely limited to CODIS data and the ability to match samples and assign them to an individual. However, there are circumstances, in which a given DNA sample does not match anyone in the CODIS database, and no other information about the donor is available. In this study, we determined 75 SNPs in 24 genes (previously implicated in human or animal pigmentation studies) for the analysis of single- and multi-locus associations with hair, skin, and eye color in 789 individuals of various ethnic backgrounds. Using multiple linear regression modeling, five SNPs in five genes were found to account for large proportions of pigmentation variation in hair, skin, and eyes in our across-population analyses. Thus, these models may be of predictive value to determine an individual's pigmentation type from a forensic sample, independent of ethnic origin.

Monday, February 15, 2010

Predicting lactase persistence from genetic data ... not yet!

...especially in Africa, SE Europe, and parts of Asia.
It seems like their genetic information consists of the four SNPs that are so far known to be associated with lactase persistence.

A worldwide correlation of lactase persistence phenotype and genotypes

Itan Y, Jones BL, Ingram CJ, Swallow DM, Thomas MG
BMC Evol Biol. 2010 Feb 9;10(1):36. [Epub ahead of print]
ABSTRACT: BACKGROUND: The ability of adult humans to digest the milk sugar lactose - lactase persistence - is a dominant Mendelian trait that has been a subject of extensive genetic, medical and evolutionary research. Lactase persistence is common in people of European ancestry as well as some African, Middle Eastern and Southern Asian groups, but is rare or absent elsewhere in the world. The recent identification of independent nucleotide changes that are strongly associated with lactase persistence in different populations worldwide has led to the possibility of genetic tests for the trait. However, it is highly unlikely that all lactase persistence-associated variants are known. Using an extensive database of lactase persistence phenotype frequencies, together with information on how those data were collected and data on the frequencies of lactase persistence variants, we present a global summary of the extent to which current genetic knowledge can explain lactase persistence phenotype frequency. RESULTS: We used surface interpolation of Old World lactase persistence genotype and phenotype frequency estimates obtained from all available literature and perform a comparison between predicted and observed trait frequencies in continuous space. By accommodating additional data on sample numbers and known false negative and false positive rates for the various lactase persistence phenotype tests (blood glucose and breath hydrogen), we also apply a Monte Carlo method to estimate the probability that known lactase persistence-associated allele frequencies can explain observed trait frequencies in different regions. CONCLUSION: Lactase persistence genotype data is currently insufficient to explain lactase persistence phenotype frequency in much of western and southern Africa, southeastern Europe, the Middle East and parts of central and southern Asia. We suggest that further studies of genetic variation in these regions should reveal additional nucleotide variants that are associated with lactase persistence.

Sunday, January 24, 2010

Signatures of selection in dog breeds

10 breeds, 21,000 SNPs, breed-specific Fst.

Their most notable hits:
HMGA2 and IGF1R (size)
SILV, MITF (coat color and texture)
CDH9, DRD5, HTR2A (behavior)
SOX9 (skeletal morphology)
FTO, SLC2A9, SLC5A2 (physiology)

Unfortunately they only use Fst to detect evidence of selection. It would have been interesting to try out some haplotype or other tests of selection, but they were probably constrained by the total number of SNPs they had.

Most interestingly:
Similar to analyses of selection in natural populations (23), we find that genes involved in immunity and defense are also significantly overrepresented in the 155 candidate selection
regions. This is somewhat surprising, as natural and artificial selection would not necessarily be expected a priori to act on similar classes of genes, and suggests that immune related genes are pervasive targets of selection because of their critical role in pathogen defense or propensity for pleiotropic effects (24).
We note that as an initial foray into comparative selection mapping, of the 1,506 genes located in putatively selected regions in dogs, 169 overlap with genes located in well-supported selected regions in humans (10). Although this result should be interpreted with caution, as the specific targets of selection are generally not known with certainty in either dogs or humans, it does raise the intriguing possibility that recent selection has influenced common loci in both the human and dog lineages.
Tracking footprints of artificial selection in the dog genome
Joshua M. Akey, Alison L. Ruhe, Dayna T. Akey, Aaron K. Wong, Caitlin F. Connelly, Jennifer Madeoy, Thomas J. Nicholas, and Mark W. Neff
PNAS January 19, 2010 vol. 107 no. 3 1160-1165
Abstract: The size, shape, and behavior of the modern domesticated dog has been sculpted by artificial selection for at least 14,000 years. The genetic substrates of selective breeding, however, remain largely unknown. Here, we describe a genome-wide scan for selection in 275 dogs from 10 phenotypically diverse breeds that were genotyped for over 21,000 autosomal SNPs. We identified 155 genomic regions that possess strong signatures of recent selection and contain candidate genes for phenotypes that vary most conspicuously among breeds, including size, coat color and texture, behavior, skeletal morphology, and physiology. In addition, we demonstrate a significant association between HAS2 and skin wrinkling in the Shar-Pei, and provide evidence that regulatory evolution has played a prominent role in the phenotypic diversification of modern dog breeds. Our results provide a first-generation map of selection in the dog, illustrate how such maps can rapidly inform the genetic basis of canine phenotypic variation, and provide a framework for delineating the mechanistic basis of how artificial selection promotes rapid and pronounced phenotypic evolution.

Tuesday, January 19, 2010

More on the genetics of European farmers vs. hunter-gatherers

Continuing on the theme from the last post, here is a paper that just came out in PLoS Biology. It seems to come to the same conclusion as the Science one that used ancient mtDNA...sort of. The results conflict since the Bramanti one finds discontinuity in mtDNA lineages, while this paper doesn't. So the Bramanti paper finds little evidence for continuity in mtDNA lineages, while this one finds that there is some continuity in mtDNA lineages, at least compared to the Y-chromosome. The two papers use different sources of data and different methods, which could somehow explain the discrepancy.
Razib's already all over this.

A Predominantly Neolithic Origin for European Paternal Lineages
Patricia Balaresque, Georgina R. Bowden, Susan M. Adams, Ho-Yee Leung, Turi E. King, Zoë H. Rosser, Jane Goodwin, Jean-Paul Moisan, Christelle Richard, Ann Millward, Andrew G. Demaine, Guido Barbujani, Carlo Previderè, Ian J. Wilson, Chris Tyler-Smith, Mark A. Jobling
PLoS Biology 8(1): e1000285.
Abstract: The relative contributions to modern European populations of Paleolithic hunter-gatherers and Neolithic farmers from the Near East have been intensely debated. Haplogroup R1b1b2 (R-M269) is the commonest European Y-chromosomal lineage, increasing in frequency from east to west, and carried by 110 million European men. Previous studies suggested a Paleolithic origin, but here we show that the geographical distribution of its microsatellite diversity is best explained by spread from a single source in the Near East via Anatolia during the Neolithic. Taken with evidence on the origins of other haplogroups, this indicates that most European Y chromosomes originate in the Neolithic expansion. This reinterpretation makes Europe a prime example of how technological and cultural change is linked with the expansion of a Y-chromosomal lineage, and the contrast of this pattern with that shown by maternally inherited mitochondrial DNA suggests a unique role for males in the transition.

Saturday, January 09, 2010

Genetics of hunter-gatherers and early farmers in Europe

I'm back, and will try to be more regular about posting. This paper came out a few months ago in Science and I've been meaning to look at it for a while.
They examine the Fst in mtDNA sequence between hunter-gatherer samples (13,400 to 2,300 ago) in Central Europe, from more recent individuals (and modern Europeans). I guess they must use archaeological or other evidence to determine their subsistence pattern.
They get an Fst of 0.163, which is indeed remarkably high - although do we really know what to expect when comparing populations over time? To answer this, they perform some simulations, and reject the hypothesis that this Fst could have been due to a process of population continuity.
Along with their discussion of the haplotype differences betwee the hunter-gatherers and farmers, this result is pretty interesting and suggests a migration of early farmers into central Europe and replacement of hunter-gatherers.

Genetic discontinuity between local hunter-gatherers and central Europe's first farmers.
Bramanti B, Thomas MG, Haak W, Unterlaender M, Jores P, Tambets K, Antanaitis-Jacobs I, Haidle MN, Jankauskas R, Kind CJ, Lueth F, Terberger T, Hiller J, Matsumura S, Forster P, Burger J.
Science 2009 Oct 2;326(5949):137-40.
Abstract: After the domestication of animals and crops in the Near East some 11,000 years ago, farming had reached much of central Europe by 7500 years before the present. The extent to which these early European farmers were immigrants or descendants of resident hunter-gatherers who had adopted farming has been widely debated. We compared new mitochondrial DNA (mtDNA) sequences from late European hunter-gatherer skeletons with those from early farmers and from modern Europeans. We find large genetic differences between all three groups that cannot be explained by population continuity alone. Most (82%) of the ancient hunter-gatherers share mtDNA types that are relatively rare in central Europeans today. Together, these analyses provide persuasive evidence that the first farmers were not the descendants of local hunter-gatherers but immigrated into central Europe at the onset of the Neolithic.
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