The Mediterranean is the birthplace of ancient peoples and cultures, but has it acted as a bridge or a barrier in the genetic history of northern and southern populations? Gene flow and population structure on the north and south shores of the Mediterranean form the basis of the work published recently by the Human Population Genetics research group, directed by Pedro Moral of the Department of Animal Biology at the University of Barcelona.
The research, published in two separate articles in BMC Evolutionary Biology and the American Journal of Physical Anthropology, covers one of the widest geographical areas and most diverse population samples studied to date in the Mediterranean region and reveals differences in the genetic structure of the populations inhabiting the north and south shores.
"The genetic history of populations will only be fully understood when we have a more complete understanding of the architecture and variation of the human genome. The Mediterranean, in particular, is a highly complex area, and studying the genetic variability of its populations is a major scientific challenge," explains Pedro Moral, director of the project. The first article, published in the journal BMC Evolutionary Biology, reveals new data about the genetic structure of human populations on both Mediterranean shores and gene flow across the Sahara desert. In this study, the group tracked the genetic footprint of a set of polymorphisms located in and around the genomic regions of the coagulation factors FVII and FXII, which are associated with the prediction of risk factors for cardiovascular diseases. The population sample consisted of 687 individuals from countries in the Mediterranean Basin (Spain, France, Greece, Turkey, Morocco, Algeria and Tunisia) and from non-Mediterranean groups (Ivory Coast and Bolivia).
North and South: reconstructing a shared history
What role has the Mediterranean played in the genetic evolution of human populations? There are still many mysteries to be unravelled, and the north and south shores, originally populated in Palaeolithic times, show different genetic histories. "Every genetic marker tells us a different story about human populations, and the choice of marker is often the subject of scientific debate. From an experimental perspective, the Mediterranean region is a good scenario in which to validate the application of new methodologies in population studies," explains Georgios Athanasiadis, first author of the article. This new study confirms the results of previous research with other genetic markers and suggests a scenario with discrete but significant differences between the genetic structures of northern and southern Mediterranean populations. Sub-Saharan gene flow could account for some of this divergence between the two shores, since it is more intense in North Africa than in southern Europe. Surprisingly, the group also found that the functional mutations considered in the study do not appear to have a clear selective influence on the broader Mediterranean population. According to Pedro Moral, "It would seem that selection is not responsible for the current variation of these markers in the populations of the Mediterranean region."
Reconstructing the shared history of the shores of the Mare Nostrum and the gene flow between populations in the Mediterranean region is also the focus of the second article, published in the American Journal of Physical Anthropology. The study, which focuses on a larger sample from a wider geographical area, analyzes the genetic footprint of different populations through the study of genetic markers corresponding to different mutations (Alu, STR and Alu/STR combinations) in a sample of 1,831 individuals from Mediterranean countries (Spain, France, Greece, Turkey, Morocco, Algeria and Egypt) and other countries used as reference populations (Germany and Ivory Coast).
For Emili González-Pérez, first author of the article, "in the Mediterranean area, each episode has left an imprint on the genome that we can read through different types of genetic markers." As reference markers to determine the degree of genetic differentiation, the team used Alu elements -- neutral, stable inserts capable of detecting historical signals in lines of evolution -- and microsatellites or short tandem repeats (STRs), whose greater mutation rates make them indicators of more recent signals in the complex scenario of the Mediterranean region. The combined Alu/STR systems (haplotypes) are an innovative and crucial element in the detection of specific genetic combinations characteristic of Mediterranean human populations that could create new opportunities for population studies in the region.
According to González-Pérez, "the results confirm the genetic differentiation between the two shores but they also suggest that the differential features of the Mediterranean region form part of a population group with a unique shared history. Through the combined use of markers we have been able to date certain gene combinations to between thirty and forty thousand years ago, which matches the period of settlement across the entire Mediterranean region of Palaeolithic populations. This was followed by a series of complex episodes of gene flow including at least one Neolithic wave and multiple interactions in more recent times. All of this confirms that the Mediterranean region has a long and complex history and that its patterns of human settlement are certain to be equally complex."
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