Academic journal article Genetics

Relaxed Selection during a Recent Human Expansion

Academic journal article Genetics

Relaxed Selection during a Recent Human Expansion

Article excerpt

(ProQuest: ... denotes formulae omitted.)

THE impact of recent demographic changes or single bottlenecks on the overall fitness of populations is still highly debated (Lohmueller et al. 2008; Fu et al. 2014; Lohmueller 2014; Simons et al. 2014; Do et al. 2015; Henn et al. 2015, 2016; Gravel 2016). Simulation and theoretical approaches suggest that populations on expanding wave fronts experience strong genetic drift (Peischl et al. 2013, 2015), leading to relaxed selection (Gravel 2016) resulting in the build-up of expansion load (Peischl et al. 2013). The main reason for this expansion load is that low population densities and strong genetic drift at the wave front promote the genetic surfing of both neutral and selected variants (Peischl et al. 2013). This relatively inefficient selection on the wave front leads to the preservation of many new mutations, unless very deleterious (Peischl et al. 2013), and it shifts the site frequency spectrum (SFS) of standing variants, thus increasing the contribution of (partially) recessive variants to mutation load (Peischl and Excoffier 2015). After a range expansion, both a decrease of diversity and an increase in the recessive mutation load with distance from the source is expected (Kirkpatrick and Jarne 2000; Peischl and Excoffier 2015). This pattern has recently been shown to occur in non-African human populations, where a gradient of recessive load has been observed between North Africa and the Americas (Henn et al. 2016). Whereas the bottleneck out of Africa that started ~50,000 years ago (e.g., Gravel et al. 2011) must have created a mutation load, the exact dynamics of this load increase due to the expansion process are still unknown. It is also unclear if a much more recent expansion could have had a significant impact on the genetic load of populations.

The settlement of Quebec can be considered as a series of demographic and spatial expansions following initial bottlenecks. The majority of the 6.5 million French Canadians living in Quebec are the descendants of ~8500 founder immigrants of mostly French origin (Charbonneau et al. 2000; Laberge et al 2005). This French immigration started with the founding of a few settlements along the Saint Laurence river at the beginning of the 17th century (Charbonneau et al. 2000). Most new settlements were restricted to the Saint Laurence Valley until the 19th century, after which remote territories began to be colonized. Bottlenecks and serial founder effects occurring during range expansions are thought to have profoundly affected patterns of genetic diversity, leading to large frequency differences when compared to the French source population (Laberge et al. 2005). Even though the French Canadian population has expanded 700-fold in ~300 years, its genetic diversity is actually not what is expected in a single panmictic, exponentially growing population, as allele frequencies have drifted much more than expected in a fast-growing population (Heyer 1995,1999). It has been shown that genetic surfing (Klopfstein et al. 2006; Peischl et al. 2016) has occurred during the recent colonization of the Saguenay-Lac St-Jean area (Moreau et al 2011), and that the fertility of women on the wave front was 25% higher than of those living in the core of the settlement, giving them more opportunity to transmit their genes to later generations. In addition, female fertility was found to be heritable on the front but not in the core (Moreau et al. 2011), a property that further contributes to lowering of the effective size of the front population (Austerlitz and Heyer 1998; Sibert et al. 2002) and the enhancement of drift on the wave front. Social transmission of fertility (Austerlitz and Heyer 1998) and genetic surfing during range expansions, or a combination of both (Moreau et al. 2011), have been proposed to explain a rapid increase of some low-frequency variants. Thus, it seems that differences in allele frequencies between French Canadians and continental Europeans are due to a mixture of the random sampling of initial immigrants (founder effect) and of strong genetic drift having occurred in Quebec after the initial settlement, resulting in a genetically and geographically structured population of French Canadians (Bhereretai 2011). …

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