Friday, October 17, 2008

Lineages in the Cantabrian Coast

This article comes to us via the Basqueresearch page:

Study confirms importance of lineages in the Cantabrian coast of the Bay of Biscay in the European genetic map

A PhD thesis at the University of the Basque Country (UPV/EHU) involved a detailed study of the maternal lines (mitochondrial DNA) of three autochthonous human populations in the Cantabrian coast area of the Bay of Biscay and which aimed to clarify the role of these populations in the postglacial recolonisation in Europe. The study, carried out by Dr. Sergio Cardoso Martín, confirmed the importance of the proposed H1 and V lines as well as considering lines J1c, U5b and T2b as significantly important demographic landmarks in the history of the evolution of European human populations.

One of the aspects that has sparked most interest in recent years in the field of Population Genetics is the postglacial repopulation of Europe that happened about 15,000 years ago. A great amount of this research has focused on the analysis of mitochondrial DNA or maternal lines, using them as a tool to assess the impact of large human migrations in the make-up of the genetic heritage of current European populations.

With his thesis entitled, “Diversity of the mitochondrial genome in autochthonous populations in the Cantabrian coast area: Traces of postglacial recolonisation in Europe”, Dr. Sergio Cardoso Martín explored the involvement of the Franco-Cantabrian populations of the Franco-Cantabrian refuge during the postglacial recolonisation of Europe, by analysing the variability of mitochondrial DNA. The Franco-Cantabrian refuge, extending from the south-east of France to the eastern end of the Cantabrian coast, is considered to have been the main settlement for groups of humans who arrived from the north of Europe during the last glacier age, in order to escape from the extremely adverse weather conditions.

Sergio Cardoso Martín has a degree in biochemistry and is currently working in postdoctoral research. His PhD thesis was directed by Dr.Marian Martínez De Pancorbo of the Department of Zoology and Animal Cell Biology at the Faculty of Pharmacy (UPV/EHU) and of the General Genomic Research Service: DNA Bank, and by Dr. Miguel Ángel Alfonso Sánchez, of the General Genomic Research Service: DNA Bank.

Looking for genetic markers

In order to carry out this research, an analysis was undertaken of a sample made up of 194 individuals belonging to three autochthonous populations from the Cantabrian coast: from the Arratia and Goierri valleys in the Basque Country, the Valley of Baztan in Navarre and from the Pas valley in Cantabria. The HVI and HVII segments from the mitochondrial DNA control region of the 194 participating individuals were sequenced. Moreover, the complete mitochondrial DNA for 43 of the individuals was sequenced with a twin objective: to reconstruct the development of the various maternal lines from the Franco-Cantabrian refuge area and to confirm the results of the analysis of the sequenced segments. The results of this analysis enabled the classification of the individuals under study into genetic families known as haplogroups. The frequency of the various haplogroups was also analysed - their spatial distribution throughout Europe and the age of the most recent common ancestor to the various lines was estimated.

Reduced genetic diversity

Most of the 194 individuals analysed presented mitochondrial haplogroups characteristic of the European populations. Additionally, very infrequent haplogroups were found and exceptionally individuals, carriers of a haplogroup of African origin, were found in the Pas Valley.

The most frequent haplogroup amongst our samples from the Basque Country and Navarre turned out to be H, and, more concretely, the subhaplogroup H1. Also notable amongst these two populations was the high frequency of the J1c line, and particularly in the case of the north of Navarre, lines U5b and T2b also registered notable frequencies. In the Pas Valley, on the other hand, the greatest frequency corresponded to haplogroup V. The adverse climate and the orography of the terrain would have favoured a marked isolation of the populations and, in consequence, a local genetic microdifferentiation that is reflected today in the predominance of some or other maternal lines in each of the analysed populations.

Apart from presenting caracteristic haplogroups, as regards diversity, the research showed that the three autochthonous populations studied were characterised by reduced values for genetic diversity with respect to other European populations, even with respect to the rest of the Iberian Peninsula. The populations of the Basque Country and the Pas Valley, together with that of Galicia, demonstrate the lowest diversity of lines within the European context. The population of the north of Navarre showed values within the range of European populations taken as reference. The author of this study has put forward these differences in mitochondrial genome diversity as being related to the lower isolation of northern Navarre, given the powerful influences of the presence of Romans, Arabs and Jews in the region, as well as its location on the pilgrim route to Santiago de Compostela.

The comparison undertaken with other European populations showed that certain mitochondrial haplogroups presented their maximum level of frequency in the area of the Franco-Cantábrico refuge. It can be deduced from these findings that the definitory mutations of these haplogrupos could have originated in the refuge zone, or otherwise could have been transported by humans who retreated from the north of Europe, thus substantially increasing their frequency subsequently as a consequence of the genetic drift, a phenomenon known as the “founder effect”.

Genetic markers of recolonisation in Europe

The study enabled the confirmation of the importance of lines H1 and V – the most abundant amongst the sample of analysed individuals – as genetic markers of the postglacial recolonisation from the refuges of southeast Europe. Also, the study’s findings showed that the T2b, J1c and U5b haplogroups constitute “paleolithic maternal lines”, well conserved to date and with relevant frequencies in the Franco-Cantabrian refuge area. This is why it is recommendable to include the area in future studies aimed at finding genetic tracks of the postglacial repopulation of Europe and at the evaluation of the impact of this grand demographic event in the reconstruction of the genetic patrimony of contemporary European populations.

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