Population Structure Within Italy

October 29, 2012

The good news is there's a new study on population structure in Italy. The bad news is that it's not very well done. The authors failed to sample populations from elsewhere in Southern Europe (like Iberia and the Balkans), which Italians are most related to in other studies, and they didn't collect 4-grandparental information on each individual, resulting in several outliers apparently with recent origins in different parts of the country. They also describe genetic components that are probably very old and have a wide distribution as "Northern European ancestry" and "Middle Eastern ancestry" as if they came from modern populations. Besides all that, the results aren't too surprising.

According to the study, Italians have similar proportions of the same genetic components, varying slightly from North to South, but distinct from both Northern/ Central Europe and the Middle East/North Africa. The island of Sardinia is unique in having an excess amount of one of those components.

In terms of PCA, Italians plot expectedly according to geography between the Western and Eastern Mediterranean (the authors say "France" and "the Middle East" because of the lack of Southern European reference samples), with Sardinians out to the side.

The position of the Italian population samples suggests that genetic distances between these populations and other European and Middle East populations has a good correlation with geographic distances. At the same time, Sardinia was confirmed to be a genetic "outlier".


The relative position of the samples reflected their geographic location: the close correlation between PC and geography was previously reported by several authors. When compared to other European populations, Sardinia was confirmed to be a genetic "outlier", whereas the Northern Italian population was genetically close to the French population, and the Southern Italians had some similarities with other Mediterranean populations such as those from Middle East. Unfortunately, lack of data from other relevant reference populations from the South-East Europe, e.g. from the Balkan peninsula, made it impossible to fully analyze the extent of the Eastern contribution in Italian populations.

Our main goal was to investigate the genetic structure of the Italian population considering four main macro-areas (Northern, Central, Southern Italy and Sardinia). We carried out PC analysis on the Italian samples and plotted the eigenvectors 1 and 2 in Figure 2. Most samples fell within a main cluster which seems to be indicative of Italian peninsula individuals. The first PC divided Italian populations in two clusters, one for Sardinia and the other for the remaining three Italian macro-areas. The Sardinian population is highly dispersed along the first eigenvector.

The second PC divided Italian mainland population into two clusters, with a certain degree of overlapping between Northern and Central Italy, and a separate cluster for Southern Italy, suggesting that genetic variation is generally continuous rather than discrete, at least within Italian mainland.

The overlap of Northern and Central Italy, and the gap between Central and Southern Italy, is explained by the uneven distribution of the samples.

ADMIXTURE analysis confirms that there was no clear separation between Northern and Central Italy, at least as considered as macro-areas. Additional comparison of the distribution of pair-wise identity-by-state within each of the four populations and ADMIXTURE analysis clarified that this is not an artifact of the PC analysis. However, the PC and ADMIXTURE analysis results could be due to the sparse geographical coverage of our samples, especially for the Central and Northern macro-areas. In fact, many of the individuals (N = 413) in the North Italian sample analyzed in this study were from Piedmont — a North West Italian region that has historically been affected by intense migration. At the same time, many individuals in the Central Italy macro-area (113 samples) are settled in Tuscany, an administrative region which is at the border with northern regions.

Within each macro-area, there isn't much substructure, meaning that a Sicilian, e.g., is not particularly differentiated from a Campanian or a Puglian.

A finer view of the Italian substructure, can be seen in Figure S2 where the hidden population structure within the Italian dataset is appreciable. Subjects are labeled by municipality, or in the case of the Sardinian samples, by the main linguistic area. In this figure we can appreciate the lack of clustering at the municipality level, also within Sardinia. Individuals seem to cluster within the main macro-area, but the geographic patterning is less obvious for the municipality (or in the case of Sardinia, linguistic) division, and in our opinion this pattern indicates no substructure within regions among municipalities, while the structuring between regions can be easily detected. It is also possible to appreciate a certain genetic homogeneity within Sardinia.

Di Gaetano et al. "An Overview of the Genetic Structure within the Italian Population from Genome-Wide Data". PLoS One, 2012.

Related: More Italian Population Structure, Much Better Population Structure


Blogger said...

Italians are one race regardless of their varying elements (varying shades of skin, eye color, and hair color). The differences between the north and south are strongly exaggerated and the attempts to turn them into two separate races are ridiculous. Germans have the same varying elements amongst themselves (varying shades of skin, some with blonde hair, others with dark brown hair, etc.)

Blogger said...

Having different racial sub-types amongst a people does not necessarily indicate a completely different race. Amongst the Germans, for example, you can find the Apline type, Mediterranean type, and Nordic type, but most people (including Germans) consider the Germanic peoples to belong to the same race; they are classified as one race.

Also, being of a certain racial subtype does not necessarily mean closer racial commonality with those in other nations of the same subtype. For example, amongst the Slavic Poles and Ukrainians you can find the Nordic type, but obviously Nordic Slavs are not the same as Nordic Germans or Scandinavians, and Slavs of different subtypes are more closely related to each other than to Germans of similar subtypes.

In Scandinavia you can find both the Nordic and Mediterranean types, but Mediterranean Swedes are obviously not the same as Mediterranean Spaniards or Greeks. In Britain you can also find the Nordic type and Mediterranean type, but clearly British Mediterraneans are not the same as Italian Mediterraneans.

justinian said...

Good points Blogger. Another example of what you're saying is the fact that Greeks are extremely homogeneous in genetics, but relatively more variant in superficial phenotype.

The relatively greater variance in superficial phenotype is very little if any importance; while the homogeneity in genetics is of profound importance, its the very thing that together with culture makes the Greeks a real people.

justinian said...

Given that Otzi the Iceman was shown to be more like modern Sardinians than other modern Italians; could this result indicate that the ancient Southern Europeans were significantly less like Northern Europeans than today's Southern Europeans, while also indicating today's Southern Europeans are not significantly more like Middle Easterners than their ancient ancestors?

Onur Dincer said...

Another example of what you're saying is the fact that Greeks are extremely homogeneous in genetics

Greeks are not a genetically well studied population. From the limited genetic analyses of Greeks we have it appears that there is significant regional variation among Greeks. Excluding Cypriot Greeks, there are at least two genetic clusters of Greeks: the Balkan Greek cluster and the Anatolian Greek cluster, but likely more clusters in reality. Anatolian Greeks are especially purely studied as of today. Most genetic studies of non-Cypriot Greeks are those of Balkan Greeks.

justinian said...

There can be substructure within a very homogeneous population.

Onur Dincer said...

Greeks as a whole are not a genetically very homogeneous population. The substructure within Greeks is great.

justinian said...

"Greeks as a whole are not a genetically very homogeneous population."

Do you have a source on that, Onur?

"The substructure within Greeks is great."

Great compared to what?

Onur Dincer said...

Justinian, as an example to what I am saying (there are many other examples), the Dodecad Project of Dienekes Pontikos, who is a Greek himself, contains samples from:

1- Greeks with all known ancestry from the Greek mainland

2- Greeks with partial known ancestry from the Greek mainland and partial known ancestry from Anatolia and/or the Aegean islands (but apparently, and unfortunately, no Greeks with all known ancestry from Anatolia and/or the Aegean islands)

3- Cypriot Greeks

In Dienekes' analyses, these three groups of Greeks are grouped in separate clusters within the context of Anatolia and the Balkans and do not form a distinct Greek cluster (instead, they cluster with non-Greeks of their respective regions), thus refuting your thesis of the genetic homogenity of Greeks. BTW, Anatolian Greeks may be found to have substructures - specific to the sub-regions of Anatolia - within themselves, but more - both in number and variety - sampling of people with Anatolian Greek ancestry, and especially those with all known ancestry from Anatolia, is needed in order to show that.




Blogger said...

The sooner people (especially Nordicists) realize that "Nordic" is not a singular race, the better. People are under this delusion that all Nordic types in every country around the world belong to the same race, when the fact is those of the Nordic type among the Slavs, Germans, Scandinavians, and British do not even cluster together. Nordic Slavs cluster with Non-Nordic Slavs, but do not cluster with Nordics from Scandinavia or Britain. Why? Obviously because "Nordic" is not a race. It's a type that can be found among many different peoples within the Caucasian race. Just because a person is Nordic does not mean he has some sort of affinity with all other Nordics.