Question
Lake trout were introduced into Flathead Lake in western Montana in the early 20^th century. They have had a devastating effect on native fish species after they invaded other lakes in the Flathead drainage. Kalinowski et al. (2010) described genetic variation at 11 microsatellite loci in a non-native population of lake trout in Swan Lake, Montana. This population was recently established by a few migrant individuals from the nearby Flathead Lake population. The lake trout in Swan Lake had greatly reduced average heterozygosity (0.68) compared to the average heterozygosity in lake trout from Flathead Lake (0.88). Assume that all of the loss in heterozygosity was caused by the initial founding event; that is, assume that there has been no additional loss of heterozygosity because of small population size. We know that that the percentage of heterozygosity remaining after a bottleneck of a single generation is (1 - 1/2N), regardless of the number of alleles present and their frequencies. First calculate the percentage of heterozygosity that was lost during the founding event. Then determine the estimated number of founders based upon the observed reduction in heterozygosity.
Explanation / Answer
Fathead Lake and in Swan Lake are 0.88 and 0.68, respectively. Let us assume only one generation of genetic
drift (t = 1), because around 7000 lake trout were already
present in Swan Lake only 10 years (two generations)
after their discovery there. Therefore, H0 = 0.88 and H1 =
0.68 (assuming t = 1), and solving for the equation above
for N, then
N=H0/2(H0-H1)=2.2
Suggesting that there were primarily two founders that established this population.
