The genetic diversity of Quebecers of French ancestry that we observe today is largely the result of the influence that geography and geology had on the migration of the first French settlers in the lands hitherto inhabited by the First Nations. This is revealed in particular by a study conducted at McGill University by Luke Anderson-Trocmé and Simon Gravel, in collaboration with researchers from UQAC, the University of Oxford, in the United Kingdom, and universities French institutions of Nantes and Sorbonne Paris Nord.
This study first consisted of building a mathematical simulation model capable of describing and explaining the genetic diversity of the Franco-Quebec population today. To do this, the researchers used, in particular, genealogical data from more than four million marriage records — from parish registers and civil status — which were digitized and compiled in the BALSAC database, managed by Professor Hélène Vézina from UQAC.
These genealogical data dating back to the arrival of the first French immigrants in the 17e century were crossed with the genotypic data (of the complete genome) of 20,451 contemporary French Canadians — for whom the family tree was drawn up — from the Cart@gene and Genizon databases, as well as of 2,276 French people.
Given the exceptional quality of this genealogical data, which is unique in the world, it was possible to estimate the genetic relatedness between individuals in time and space, either according to where they migrated , underlined in an interview Simon Gravel, professor in the Department of Human Genetics at McGill University, who led this study. The researchers thus observed that the similarity, even the genetic similarity between individuals, was greater between those who had settled along the same river or watershed.
Chance causes that some people will have more children than others and, therefore, the mutations they carry will be found with higher frequency in the population.
The influence of geography and geology
The waterways (the St. Lawrence and then its tributaries, such as the Saguenay and the Chaudière River) were important axes of migration, because they facilitated transportation and because the French “were also facing a vast forest territory occupied by the Iroquois First Nations”, explain the authors of the study in an article published last May in the journal Science.
Geology also had a determining influence. For example, the Charlevoix astrobleme, a geological formation created by the impact of a meteorite that fell near Baie-Saint-Paul 400 million years ago, formed a pocket of fertile land where a group of French settlers from Quebec settled. But since this zone of arable land was relatively restricted, demographic pressure gradually developed which became so strong that, when the government finally opened up Saguenay–Lac-Saint-Jean (SLSJ) to colonization, several inhabitants from Charlevoix then migrated to this isolated region, which until then had been reserved for the Hudson’s Bay Company.
In the Bas-Saint-Laurent, settlers gradually settled on the shore of the St. Lawrence before migrating inland. And in Beauce, we founded a first village, from which we radiated and founded other villages, along the Chaudière River in particular, specifies Mr. Gravel. The colonization of Abitibi-Témiscamingue, which took place later, is distinguished by the fact that the Franco-Quebec settlers who settled there came from different regions of the province. “So there was no founder effect on the scale of the region as a whole,” he says.
On the other hand, “there were various founding effects elsewhere in Quebec — notably in Charlevoix, SLSJ, Bas-Saint-Laurent, Îles-de-la-Madeleine and Beauce — whose appearance we can identify temporal resolution with an exceptional and unprecedented resolution from the point of view of genetic diversity”, he points out.
The researchers attempted to elucidate the contribution of these founder effects on the gene drift observed in the different regions of Quebec. “Gene drift is defined as changes that occur randomly in the frequency of variants [ou mutations] genetics. For example, chance causes that some people will have more children than others and, therefore, the mutations they carry will be found at a higher frequency in the population. Most geneticists believe that during human evolution, changes in the genetic makeup of the human species are due to gene drift rather than natural selection,” says Gravel. The latter specifies that “natural selection favors certain variants rather than others because the individuals who carry them are better adapted. As these variants help them, they will consequently have more children, and over the generations these variants will become more and more frequent”.
Mr. Gravel gives the example of SLSJ, where we know that there was a founder effect and where certain mutations causing rare diseases are at higher frequency. His study shows that most of the gene drift observed at SLSJ would have taken place in Charlevoix, or even in Quebec. “Almost nothing happened at the SLSJ,” he underlines.
“The received idea that there is inbreeding in the Saguenay is not true at all. […] The majority of the kinship [éloigné] between individuals at SLSJ predates the colonization of this region. These are things that go back to Charlevoix or Quebec. »
“The factors that led settlers from Charlevoix who immigrated to the SLSJ to have more children, to produce more descendants in the Saguenay were purely demographic and had nothing to do with their genome. The variants increased in frequency at SLSJ not because they were beneficial, but just because the individuals who carried them had more children. But these factors have of course contributed to the increase in the frequency of certain variants, including deleterious mutations”, explains Mr. Gravel.
Simulation of the genomes of the Franco-Quebec population
The mathematical model developed by the McGill team also made it possible to simulate the entire genome of more than 1.4 million individuals whose four grandparents could be linked to genealogy. And a comparison of these simulated genomes with the real genotypic data obtained from Cart@gene revealed an excellent concordance, which confirms the quality and reliability of the simulation model. “This shows that our model is working well,” says mathematician Simon Gravel, a specialist in population genetics.
The comparison of these data of the Franco-Quebec population with the data of the various regions of the North-West (Normandy, Perche) and the West (Aunis, Poitou) of France, as well as of the Ile-de-France , where the first French settlers came from, showed that “although the Franco-Quebec population has retained genetic signatures from the regions of France from which the first immigrants came, the genetic structure of the current Franco-Quebec population is very different from the ancestral structure of these French regions”. “It results from events that took place in North America. The model observed in a given region of France has not been transposed to a particular region of Quebec,” explains Mr. Gravel.
Simon Gravel’s team at the McGill University Genome Center is currently refining its simulation model and, using it, is trying to determine the genome of the first French settlers, which will probably allow understand how each mutation — especially those that cause disease — spread in Quebec. What we will know in a future publication.