Monster Seminar JAM - Ecological and genetic mechanisms of divergence in threespine stickleback
Dr. Kerry Marchinko, Zoology Department, University of British Columbia
Although divergent natural selection is demonstrated frequently in nature, we often have little knowledge of the underlying ecological and genetic mechanisms responsible for evolutionary divergence. Populations of threespine stickleback often show remarkably parallel divergence in the amount of external bony armour: freshwater populations have fewer lateral plates than their marine ancestors. Despite over 80 years of research, the mechanisms causing divergence in armour between marine and freshwater stickleback remain unknown. By coupling modern ecological and molecular genetic approaches, Ive shown that the maintenance of armour diversity within populations, and the success of reduced lateral plate phenotypes in freshwater habitats, may be explained by low heterozygote fitness (underdominance) at a single gene, Ectodysplasin. Using a natural lake population of threespine stickleback polymorphic for lateral plate morphotype, I documented localised heterozygote disadvantage at the Eda gene. Stable isotopes of Carbon and Nitrogen (a cumulative measure of long term diet) revealed that individuals that differ in genotype at Eda also differ in longterm diet and habitat use. Taken together, it appears that lateral plate armour polymorphism is associated with habitat use and is maintained despite the reduced fitness of heterozygotes. Theory indicates that underdominance is the starting point of many evolutionary trajectories including the modification of dominance, allelic fixation, and assortative mating. Remarkably, populations within the adaptive radiation of threespine stickleback exhibit the entire range of evolutionary outcomes predicted by theory: the sweep and fixation of single Eda allele, stable polymorphism and modification of dominance at Eda, and assortative mating. These empirical results demonstrate that major evolutionary trajectories may be driven by relatively simple evolutionary mechanisms. Underdominance in natural populations may be fundamental towards explaining biological diversification when genes of large effect underlie the genetics of adaptive traits.
Date and Time:
March 5, 2009,
11:00 am - 12:00 pm