Fig. 1: Population structure, linguistics, and patronym distribution in Northwestern France.

a fineSTRUCTURE clustering of 3234 samples from Brittany and the downstream Loire basin at k = 3. WBR stands for Western Brittany, EBP for Eastern Brittany and Pays-de-la-Loire and SLO for South Loire. b Distribution of the Breton language over time⁹³. Loth line 1, attested by the border between Celtic place names in –ac (on the west side) and Romance place names in –é (on the east side), represents the westernmost boundary between Armorican Gaulish and Gallo-Romance spoken varieties (25). Loth line 2 represents the Celto-Romance linguistic border in the 16th century CE. Grey lines encircle political territorial divisions called départements. Only départements belonging to the regions of Brittany and Pays-de-la-Loire are coloured. In the inset, the départements are abbreviated as FIN (Finistère), COT (Côtes-d’Armor), MOR (Morbihan), ILL (Ille-et-Vilaine), MAY (Mayenne), SAR (Sarthe), MAI (Maine-et-Loire), LOI (Loire-Atlantique), VEN (Vendée). c Neighbour Joining tree from pairwise Arccos distances (68) computed on the distribution of surnames between pairs of arrondissements from Northwestern France. The support of the tree nodes was obtained by bootstrapping 1000 times the distance matrices. d 18 genetic clusters identified using total variation distances (TVD) from the fineSTRUCTURE results (see Material and Methods for details). The TVD-based tree was cut at k = 39 and to ease visualisation we merged clusters with <5 individuals into the closest cluster (>= 31 individuals) resulting in a tree with 18 clusters. All the clusters have at least 31 individuals. The TVD-based tree is sample size independent and is more informative on cluster genetic affinities³¹. The tree was built using the clustree and ggtree packages of R-statistical software⁸⁹ and the map with the following R libraries: sp as in (a, b). In (a) and (c), individuals are coloured according to their cluster assignment. Individuals’ coordinates correspond to the most common grandparents’ birthplace.