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Genetic diversity and structure of eastern Adriatic islanders revealed by mitochondrial DNA analysis

High-resolution mitochondrial DNA (mtDNA) phylogenetic analysis was performed on 873 samples from 11 island populations of the eastern Adriatic (Dugi otok, Pašman, Ugljan, Pag, Vrgada, Hvar, Brač, Korčula, Vis, Lastovo and Mljet). MtDNA haplogroup classification was determined by sequencing HVS-I of the control region and by screening diagnostic coding region SNPs for mtDNA (sub)haplogroups. A high diversity of haplogroups and haplotypes was observed in the overall sample. Altogether, 228 haplotypes were recorded and affiliated to 14 basic and at least 48 of their subhaplogroups. In total sample, all typical European mtDNA haplogroups and corresponding frequency distributions were observed. However, the frequencies of haplogroups significantly differ across populations, including the complete absence of certain haplogroups due to evolutionary forces. Recorded non-European and rare European haplogroups suggest infrequent, but still considerable gene flow from long-distance migrations. Gene diversity index exhibited positive trend in relation to population size, but negative trend to the distance from the mainland. Yet contrary to expectations, the gene diversity index was the highest in the population of the island of Hvar (0.978), while it was the lowest in the island of Mljet population (0.857). Altogether, 38 haplotypes were identified as potential founders for at least one of the populations. The most prominent example of the founder haplotype was recorded in the Mljet population where single haplotype accounted for 30.9% of the population sample. A number of haplotypes unique for this region was observed. Genetic structure analyses revealed more homogeneity among the northern group of island populations, than among the southern islands ; as well as their genetic similarity to the coast. The island of Mljet was the greatest genetic outlier, followed by Vis and Hvar. Population differentiation was also confirmed on the intra-island level and is owed both to genetic drift and to distinctive phylogeographical affiliations reflecting different gene flows. The relatively reduced level of genetic diversity, observed in some of the populations, may be useful in future studies of genetic determinants for complex diseases. The phylogeographic approach to observed genetic diversity and structure confirms the dynamic gene flow in this region throughout history by genetically distant founders, followed by a long-term isolation of previously formed groups. This has enabled founder effect and genetic drift to become the main forces in shaping the mtDNA gene pool of the contemporary Adriatic island isolates. This study contributes to the research of microevolutionary processes in isolated populations.

mtDNA; haplogroups; genetic diversity; genetic structure; genetic isolate; island population; founder effect

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