Friday, June 05, 2009

The evolution of Icelanders   posted by Razib @ 6/05/2009 01:17:00 AM

Iceland has long been of some interest because of its peculiar demographic history and their genetic consequences. So a new paper in PLoS Genetics is of interest, The Impact of Divergence Time on the Nature of Population Structure: An Example from Iceland:
The Icelandic population has been sampled in many disease association studies, providing a strong motivation to understand the structure of this population and its ramifications for disease gene mapping. Previous work using 40 microsatellites showed that the Icelandic population is relatively homogeneous, but exhibits subtle population structure that can bias disease association statistics. Here, we show that regional geographic ancestries of individuals from Iceland can be distinguished using 292,289 autosomal single-nucleotide polymorphisms (SNPs). We further show that subpopulation differences are due to genetic drift since the settlement of Iceland 1100 years ago, and not to varying contributions from different ancestral populations. A consequence of the recent origin of Icelandic population structure is that allele frequency differences follow a null distribution devoid of outliers, so that the risk of false positive associations due to stratification is minimal. Our results highlight an important distinction between population differences attributable to recent drift and those arising from more ancient divergence, which has implications both for association studies and for efforts to detect natural selection using population differentiation.

Figure 3 is a PCA map which shows how individuals from different regions of Iceland sort out. The Scottish and Norwegian populations are there two, and they don't vary much along the components of variation which Icelanders sort out along, the conclusion being that the Iceland variation isn't due to different ancestral proportions. They further calculate that if the ancestral Iceland populations were like the modern Scottish and Norwegian ones, Icelanders are ~35% Scottish and ~65% Norwegian. Most of the differences between Icelanders and continental Europeans is no doubt due to drift because of their very small population size, no migration due to their isolation and the a few specific bottleneck events. But a section on natural selection in Icelanders is interesting:
We found eight SNPs, representing two chromosomal regions, for which the evidence of unusual population differentiation was genomewide-significant...Six of the SNPs lie in or near the TLR (toll-like receptor) genes TLR10 and TLR1, while the other two lie inside the NADSYN1 (NAD synthesase 1) gene....

Toll-like receptors were pinpointed in a recent paper as likely possibilities for localized adaptation.

Labels: ,