"Americans" are not homogeneous

Via Dienekes, another ASHG 2008 abstract. This research seems to reproduce earlier findings of separate, distinct clusters among "European Americans":

Ethnicity-Confirmed Genetic Structure in New Hampshire.

Genetic population structure is known to result from shared ancestry. Though there have been several studies of genetic structure within and among different geographic regions and ethnic groups, little is known of the genetic structure of highly admixed US populations or whether the structure is concordant with self-reported ancestry. In this study, 1529 single nucleotide polymorphisms (SNPs) from 864 healthy control individuals from New Hampshire were measured as part of a bladder cancer epidemiology study. The SNPs were from approximately 500 cancer susceptibility genes scattered throughout the genome. Of these, 960 Tag SNPs were used to cluster individuals using the Structure algorithm for between 2 and 5 subpopulations. Subtle genetic structure was found, suggesting the appropriate number of subpopulations to be either 4 or 5 (FSTs 4 populations: 0.0377, 0.0399, 0.0363, 0.0340; 5 populations: 0.0452, 0.0536, 0.0585, 0.0534, 0.0521). We coded the individuals self-reported ancestries in a genotype fashion (i.e. 0= not reporting that ancestry, 1= reporting part that ancestry, 2= reporting only that ancestry) and conducted a Spearmans rank correlation between each ancestry and the structure q value, which represents the proportion of an individual that originated from a certain genetic subpopulation. Those of Russian, Polish and Lithuanian ancestry most consistently clustered together. The ancestry results support either 4 or 5 subpopulations. In order to investigate linkage disequilibrium (LD), the complete set of SNPs from the 7 most densely genotyped genes were used to make haploview plots between the different groups. The results vary by gene, though for one gene in particular, GHR, the results are very different for 4 subpopulations. These results suggest that despite New Hampshires admixture and presumed homogeneity, there are 4 or 5 distinct genetic subgroups within the population that can be linked to self-reported ancestry and display differences in patterns of LD.

Previous research has identified at least three clusters among "European Americans", corresponding to Northern and Central European, Southern European, and Ashkenazi Jewish ancestry groups. I'm curious exactly which 4 or 5 subgroups have been identified here. From the context, it's not immediately clear whether the Russian/Polish/Lithuanian cluster consists of ethnic E. Euros or Ashkenazi Jews.