Geostatistical Analysis of Genetic Diversity in the Present Male Argentine Population

Abstract

The geographical study of genetic diversity has a long history in fields related to population genetics. Its relevance has been already acknowledged in the 1940's and numerous tools have been developed since then. Most of these tools are based on principles and assumptions of classical population genetics and search for one or some particular spatial patterns fitting the observed data. The increasing availability of genetic markers and genotypes motivated the development of new tools in the last decades. Very powerful are the individual-based methods, which jointly analyze genetic and geographical information. Nevertheless, either methods are still based on quite simplistic assumptions or do not provide enough flexibility to analyze the complex spatial structure of a modern human group. Special challenges to the analysis are the presence of admixture –which implies the existence of genetically differentiated subgroups within geographically delimited groups- and guaranteeing individual anonymity –an utmost relevant reason why individual-based analyses in modern humans are almost always unfeasible.<br /> This work presents a novel approach, a ‘Genetic Geostatistical Framework’, for the spatial analysis of genetic diversity with special focus on present-day populations, which are as a rule highly admixed. The proposed methodological approach is based on the integration of genetic statistics and spatial analysis, performed in the framework of Geographic Information Systems (GIS).<br /> The underlying model defines the spatial pattern of genetic diversity of an admixed population as the spatial overlap of several groups of genetically similar individuals, each group accounting for an independent spatial frequency distribution. This is comparable to the idea of decomposing the genetic structure of an admixed population into genetic layers, each one represented by the spatial frequency distribution of a group.<br /> This new Genetic Geostatistical Framework provides a flexible environment to precisely quantify and map inquires at population level related to the spatial distribution and frequency of alleles, haplotypes, and groups of closely related haplotypes. A novel feature is the identification, quantification, and mapping of the most frequent alleles or groups of closely related haplotypes per tract of land. The resulting maps partition the region into sub-regions, each one delimited by the area where one group of genetically closer individuals is the most predominant. This assessment can be extended to map areas delimited by groups with the second highest spatial frequency and so forth.<br /> In the case study the spatial diversity of the male population in central and northern Argentina was investigated. 145 samples were collected in 10 provinces and genotyped for highly polymorphic forensic Y-chromosomal haplotypes corresponding to seven short tandem repeat polymorphisms (Y-STR), also known as microsatellites –DYS19, DYS389I/II, DYS390, DYS391, DYS392, DYS393. Spatial analysis of the distribution of the most frequent alleles showed a clear differentiation among three regions. The northwest differed genetically in relation to central and littoral Argentina. The Argentinean Chaco presented a lower differentiation of the rest of the Argentine territory. The genetic geostatistical analysis identified four major groups of Y-chromosomal lineages, represented by groups of similar Y-STR haplotypes. Results showed that two genetically similar European lineages alternately dominate most of Argentina's territory. A third group was localized in the northwest, which included haplotypes representing South-American lineages. The fourth group was preponderantly localized in central Argentina, where it was the second in frequency. This last group included worldwide dispersed haplotypes. These results indicate a widespread European paternal ancestry throughout Argentina’s territory, substantial Amerindian proportion in the most northwestern area, and a noteworthy contribution of paternal lineages incoming from diverse worldwide origins. These findings are in agreement with ethno-historical, genetic, and demographic studies. It is noteworthy that a substantial genetic differentiation between Argentine territories, as it has been measured in this work, is of foremost relevance for statistical inferences in forensic studies based on Y-STR haplotypes. Additionally, a genetic geostatistical analysis was performed using published data of three populations inhabiting distant Argentine territories. The congruity between these and previously reported results, which were obtained with widely acknowledged population genetics methods, further demonstrates the reliability of this new method.<br /> Findings of the spatial genetic diversity analysis of the georeferenced forensic Argentine data show that integrating genetic statistics and geostatistics within a GIS framework is a powerful instrument to address a wide range of spatial genetic inquiries at population level. These may include the assessment of the spatial distribution and frequency of alleles, haplotypes, or groups of genetically similar individuals as well as the identification, quantification and mapping of the spatial coverage of the most frequent alleles or groups of related haplotypes per tract of land. In addition, this approach provides a flexible environment to adapt the analysis to various chromosomal levels and geographical scales and resolution. Results may be presented in form of summary statistics or charts. As well, patterns of genetic variation can be spatially quantified and precisely displayed in form of maps at the desired scale and resolution. The method has been primarily designed and tested on forensic Y-STR data. However, it will be straightforward to adapt it to other marker types or to increase substantially the number of loci. Since this new method aggregates genotypes spatially, violation of individual anonymity is also not at risk. In summary, it can be stated that this novel method offers an appropriate framework for detail investigation of spatial genetic diversity on the basis of genotypes accounting for a geographic reference.