Higher Levels of Neanderthal Ancestry in East Asians Than in Europeans (Wall et al. 2013)

The title seems to say it all; such a conclusion was also arrived at by Meyer et al. (high coverage Denisova paper). However, the extent of this ancestry appears to be differently estimated in the new paper:

By using the high coverage Denisova genome, we are able to show that the admixture rate into East Asians is 40% higher than into Europeans.

Of course, the interesting question is why East Asians have this excess of Neandertal ancestry, given that Neandertals were a west Eurasian-distributed species (for the most part). Similarly, we would not have expected Australo-Melanesians to possess higher Denisovan admixture, and yet they do. Some models of multiregional evolution assumed regional continuity with pre-existing archaic populations in different parts of the world (e.g., Europeans with Neandertals), but clearly much more interesting things were taking place in deep prehistory.

Of particular interest is this conclusion:

In particular, at least some Neanderthal-modern human admixture must postdate the separation of the ancestors of modern European and modern East Asian populations.

The publication of Tianyuan has shown that by ~40kya, differentiation of Asians from Europeans was already on its way, and this is a date close to the disappearance of the Neandertals, the date of which is contested, but one can imagine that already-differentiated Eurasians may have encountered some lingering Neandertal groups.

Genetics doi: 10.1534/genetics.112.148213

Higher Levels of Neanderthal Ancestry in East Asians Than in Europeans

Jeffrey D. Wall et al.

Neanderthals were a group of archaic hominins that occupied most of Europe and parts of Western Asia from roughly 30-300 thousand years ago (Kya). They coexisted with modern humans during part of this time. Previous genetic analyses that compared a draft sequence of the Neanderthal genome with genomes of several modern humans concluded that Neanderthals made a small (1-4%) contribution to the gene pools of all non-African populations. This observation was consistent with a single episode of admixture from Neanderthals into the ancestors of all non-Africans when the two groups coexisted in the Middle East 50-80 Kya. We examined the relationship between Neanderthals and modern humans in greater detail by applying two complementary methods to the published draft Neanderthal genome and an expanded set of high-coverage modern human genome sequences. We find that, consistent with the recent finding of Meyer et al. (2012), Neanderthals contributed more DNA to modern East Asians than to modern Europeans. Furthermore we find that the Maasai of East Africa have a small but significant fraction of Neanderthal DNA. Because our analysis is of several genomic samples from each modern human population considered, we are able to document the extent of variation in Neanderthal ancestry within and among populations. Our results combined with those previously published show that a more complex model of admixture between Neanderthals and modern humans is necessary to account for the different levels of Neanderthal ancestry among human populations. In particular, at least some Neanderthal-modern human admixture must postdate the separation of the ancestors of modern European and modern East Asian populations.

Link

IBD length distribution and demographic history (Palamara)

Another interesting new paper in AJHG deals with the problem of inferring the demographic history of a population from the length distribution of IBD segments. I wonder how admixture (which is likely to have occurred in both chosen real-world examples used in the paper, the Ashkenazi Jews and the Maasai) may affect the accuracy of the reconstructed demographic history.

In any case, the conclusions are worth mentioning in themselves. For the Ashkenazi:

We obtained an improved fit for a population composed of ~2,300 ancestors 200 generations before the present; this population exponentially expanded to reach ~45,000 individuals 34 generations ago. After a severe founder event, the population was reduced to ~270 individuals, which then expanded rapidly during 33 generations (rate r ~ 0.29) and reached a modern population of ~4,300,000 individuals.

And, for the Maasai:

Optimizing a model of exponential expansion and contraction (Figure 1A), we obtained a good fit to the observed IBD frequency spectrum (Figure 6), suggesting that an ancestral population of ~23,500 individuals decreased to ~500 current individuals during the course of 23 generations (r ~ -0.17). We note that this result might not be driven by an actual gradual population contraction in the MKK individuals, but it most likely reflects the societal structure of this seminomadic population. ... We thus used the village model to analyze the MKK demography and relied on coalescent simulations to retrieve its parameters: migration rate, size, and number of villages that provide a good fit for the empirical distribution of IBD segments.We observed a compatible fit for this model, in which 44 villages of 485 individuals each intermix with a migration rate of 0.13 individuals per generation (Figure 6).

If I understand this correctly, it appears that Maasai (MKK) individuals share long IBD segments not because their population has contracted (and hence they're all descended from a limited number of founders, as is the case for Ashkenazi Jews), but rather because their social structure follows the "village model" in which people share shallow ancestry (and hence long IBD) with other people in their "village" and exchange genes with other "villages".

The American Journal of Human Genetics, 25 October 2012 doi:10.1016/j.ajhg.2012.08.030

Length Distributions of Identity by Descent Reveal Fine-Scale Demographic History

Pier Francesco Palamara et al.

Data-driven studies of identity by descent (IBD) were recently enabled by high-resolution genomic data from large cohorts and scalable algorithms for IBD detection. Yet, haplotype sharing currently represents an underutilized source of information for population-genetics research. We present analytical results on the relationship between haplotype sharing across purportedly unrelated individuals and a population’s demographic history. We express the distribution of IBD sharing across pairs of individuals for segments of arbitrary length as a function of the population’s demography, and we derive an inference procedure to reconstruct such demographic history. The accuracy of the proposed reconstruction methodology was extensively tested on simulated data. We applied this methodology to two densely typed data sets: 500 Ashkenazi Jewish (AJ) individuals and 56 Kenyan Maasai (MKK) individuals (HapMap 3 data set). Reconstructing the demographic history of the AJ cohort, we recovered two subsequent population expansions, separated by a severe founder event, consistent with previous analysis of lower-throughput genetic data and historical accounts of AJ history. In the MKK cohort, high levels of cryptic relatedness were detected. The spectrum of IBD sharing is consistent with a demographic model in which several small-sized demes intermix through high migration rates and result in enrichment of shared long-range haplotypes. This scenario of historically structured demographies might explain the unexpected abundance of runs of homozygosity within several populations.

Link

The shape of things to come

(Last Update: Oct 22)

Here is a teaser from my ongoing ADMIXTURE experiments. I have assembled a nice set of Eurasian populations, and this time I will continue increasing K until the Bayes Information Criterion maxes out.

I had previously used BIC to successfully choose K in my craniometric analysis of world populations. I am now at K=7 and it keeps on rising! Either it will stop or my computer will burn out doing Quasi-Newton iterations.

Onto the teaser: I'll only say that "People" are the 12 Genomes Unzipped individuals; I will leave their individual proportions a mystery - for now.

In color:

Sampled populations, # of individuals, ancestral components:

UPDATE (Oct 21):

Here are the K=8 results. I've added some more populations. The next step is to integrate HapMap data with HGDP-CEPH and the Behar et al. dataset. Stay tuned.

I've decided to present the data as population averages, as this is much more readable, especially as the number of individuals grows.

In this run, one of the clusters (purple) became associated with north Mongoloids (Yakuts, and partially Mongols and Daur), whereas Han are mostly in the green component. Notice that Yakut, Uzbeks, Chuvash, and Turks (all Turkic populations) are predominantly in the north cluster as far as their Mongoloid component is concerned, as expected.

UPDATE II (Oct 22):

Here are results for K=9. The addition of Maasai (MKK) has revealed the east African component which they share with Ethiopians; the latter also have a significant West Asian component.

For lack of a better term, I decided to call the lime component "Sardinian", as it is dominant in that population, but it is clearly reflective of something much broader.

ADMIXTURE on African HapMap populations

Here is the result of running ADMIXTURE on the three African HapMap-3 populations, using about 440K SNPs, including Tuscans as a non-African group.

The Tuscans are in purple and show no trace of African admixture. All the other populations are separated: red: Luhya (Bantu); green: Maasai (Nilotes); Yoruba (Niger-Congo).

The two east African groups show asymmetrical affinities: the Maasai have some Luhya red, while the Luhya have little Maasai green, while they have substantial West African turqoise, consistent with the origin of their Bantu language.

Related: ADMIXTURE on HapMap-3 populations

ADMIXTURE on HapMap 3 populations

Here is the result of running ADMIXTURE on about ~50K markers from the HapMap 3 populations. I'll annotate the final K=9 run; the rest are given at the end:

I list the distinctive colors for the populations, Left-to-Right. The minor components are easy enough to pick up and as expected:

ASW (A): African ancestry in Southwest USA [Sub-Saharan blue]

CEU (C): Utah residents with Northern and Western European ancestry from the CEPH collection [European yellow]

CHB (H): Han Chinese in Beijing, China [East Asian orange]
CHD (D): Chinese in Metropolitan Denver, Colorado [East Asian orange]

GIH (G): Gujarati Indians in Houston, Texas [South Asian purple]

JPT (J): Japanese in Tokyo, Japan [East Asian light green]

LWK (L): Luhya in Webuye, Kenya [East African bright green]

MEX (M): Mexican ancestry in Los Angeles, California [Amerindian pink]

MKK (K): Maasai in Kinyawa, Kenya [East African light blue]

TSI (T): Tuscan in Italy [European yellow]

YRI (Y): Yoruban in Ibadan, Nigeria (West Africa) [Sub-Saharan blue]

The rest of the runs for K=3 to K=8 are below:

K=3: Notice: Asian red+Caucasoid blue on Gujarati Indians and Mexicans. At this level of resolution, these two populations look similar. Notice presence of blue in East Africans but not Yorubans (green at the end).

K=4: East Africans get their own (yellow) cluster. Notice the diminution of the Sub-Saharan (purple) element, relative to the previous figure. This is due to the fact that the East African element is intermediate between Caucasoids and Sub-Saharans and "eats up" the other two elements, although residual Caucasoid red and Sub-Saharan purple remains. The tripartite origin of Mexicans is especially visible in this plot, with components being in order of European, East Eurasian, Sub-Saharan.
K=5: Gujarati Indians now get their own (purple) cluster. Here the difference between Luhya (mostly Sub-Saharan blue + East African yellow) and Maasai (the reverse) is quite striking. The former are Bantu speakers and thus not indigenous to east Africa.
K=6: Mexicans get their own cluster (blue) reflecting their Amerindian, rather than East Asian ancestry which could not be resolved in the previous figures.
K=7: the Luhya get their own cluster, splitting off from the Maasai. There are now 3 components in Africa centered on Yorubans (light blue), Luhya (very light green) and Maasai (red).
K=8: A low-frequency element appears in Maasai that is hard to interpret; it is preserved in the next and final K=9 run (shown at the beginning of the post), in which the Japanese and Chinese are split off.