Testing for population differences in evolutionary responses to pesticide pollution in brown trout (Salmo trutta)

Pesticides are often toxic to non-target organisms, especially to those
living in rivers that drain agricultural land. The brown trout (Salmo
trutta) is a keystone species in many such rivers, and natural populations
have hence been chronically exposed to pesticides over multiple
generations. The introduction of pesticides decades ago could have induced
evolutionary responses within these populations. Such a response would be
predicted to reduce the toxicity over time but also deplete any additive
genetic variance for the tolerance to the pesticides. If so, populations
are now expected to differ in their susceptibility and in the variance for
the tolerance depending on the pesticides they have been exposed to. We
sampled breeders from seven natural populations that differ in their
habitats and that show significant genetic differentiation. We stripped
them for their gametes and produced 118 families by in vitro
fertilization. We then raised 20 embryos per family singly in
experimentally controlled conditions and exposed them to one of two
ecologically relevant concentrations of either the herbicide S-metolachlor
or the insecticide diazinon. Both pesticides affected embryo and larval
development at all concentrations. We found no statistically significant
additive genetic variance for tolerance to these stressors within or
between populations. Tolerance to the pesticides could also not be linked
to variation in carotenoid content of the eggs. However, pesticide
tolerance was linked to egg size, with smaller eggs being more tolerant to
the pesticides than larger eggs. We conclude that an evolutionary response
to these pesticides is currently unlikely, and that (i) continuous
selection in the past has either depleted genetic variance in all the
populations we studied, or (ii) that exposure to the pesticides never
induced an evolutionary response. The observed toxicity selects against
large eggs that are typically spawned by larger and older females.