3000 years of solitude: extreme differentiation in the island isolates of Dalmatia, Croatia

Vitart, Veronique and Biloglav, Zrinka and Hayward, Caroline and Janićijević, Branka and Smolej-Narančić, Nina and Barać, Lovorka and Peričić, Marijana and Martinović-Klarić, Irena and Škarić-Jurić, Tatjana and Barbalić, Maja and Polašek, Ozren and Kolčić, Ivana and Carothers, Andrew and Rudan, Pavao and Hastie, Nick and Wright, Alan and Campbell, Harry and Rudan, Igor (2006) 3000 years of solitude: extreme differentiation in the island isolates of Dalmatia, Croatia. European Journal of Human Genetics, 14 (4). pp. 478-487. ISSN 1018-4813

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Abstract

Communities with increased shared ancestry represent invaluable tools for genetic studies of complex traits. '1001 Dalmatians' research program collects biomedical information for genetic epidemiological research from multiple small isolated populations ('metapopulation') in the islands of Dalmatia, Croatia. Random samples of 100 individuals from 10 small island settlements (n<2000 inhabitants) were collected in 2002 and 2003. These island communities were carefully chosen to represent a wide range of distinct and well-documented demographic histories. Here, we analysed their genetic make-up using 26 short tandem repeat (STR) markers, at least 5 cM apart. We found a very high level of differentiation between most of these island communities based on Wright's fixation indexes, even within the same island. The model-based clustering algorithm, implemented in STRUCTURE, defined six clusters with very distinct genetic signatures, four of which corresponded to single villages. The extent of background LD, assessed with eight linked markers on Xq13-21, paralleled the extent of differentiation and was also very high in most of the populations under study. For each population, demographic history was characterised and 12 'demographic history' variables were tentatively defined. Following stepwise regression, the demographic history variable that most significantly predicted the extent of LD was the proportion of locally born grandparents. Strong isolation and endogamy are likely to be the main forces maintaining this highly structured overall population.

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