From the bare minimum : genetics and selection in populations founded by only a few parents

Question: Genetic variation is expected to control the fate of populations colonizing new environments, because the amount and nature of this variation influences adaptation. Thus, it is generally expected that the ability of populations to colonize new environments is severely compromised if the number of founding individuals is very few. Organisms: Brown trout (Salmo trutta) are native to the northern hemisphere, but have been widely introduced globally, including into the southern hemisphere. Times and places: We analysed two isolated populations of brown trout introduced in 1993 to the remote Kerguelen Islands in the sub-Antarctic region, each population being founded with the offspring of only a single mother, and with either only one or two fathers. Methods: Scale samples were collected in 2003 and 2010 and analysed using a set of 16 microsatellite markers. These data were used to calculate individual homozygosity level and variance in inbreeding. The association between age (estimated through scale reading) and homozygosity level was used to assess the potential consequences of low genetic variation for individual survival. Results: The two populations represented different outcomes. In one population, variance in inbreeding was high and a clear heterozygosity–fitness correlation (HFC) was evident: older individuals were less homozygous than younger individuals. Consistent with these results, homozygosity level in this population decreased from 2003 to 2010. In the other population, variance in inbreeding was low, no consistent HFC was detected, and no decline in homozygosity level was evident from 2003 to 2010. Low genetic variation and severe initial bottlenecks through founding effects did not prevent the establishment and success of these populations, one of which appears to be actively purging inbred individuals.