Knocking out genes to reveal drivers of natural selection on phenotypic traits: a study of the fitness consequences of albinism.

Conclusions about the adaptive significance of phenotypic traits typically rely on correlations between the trait and fitness, but pleiotropic effects of a single trait on fitness and covariation among traits can confound such comparisons. For example, a trait may have several benefits or costs which may be affected by its correlation to some other trait. We overcame this barrier by using CRISPR-Cas9 in captive cane toads (Rhinella marina) to create gene-knockout albinos. Using this approach, we could evaluate direct effects of a single allele on fitness by comparing rates of survival, growth and development of albino versus pigmented siblings. Contrary to the prevailing view that albinism is rare solely due to reduced crypsis (increased vulnerability to predation), we found that albino tadpoles and terrestrial-phase toads were competitively inferior to their pigmented siblings even in the absence of predation. Visual impairment appears to explain this cost in the terrestrial life-stage, as albino toads had lower foraging success, were less accurate when striking at prey, and needed higher light levels to forage successfully. Our findings suggest that competitive inferiority may contribute to selection against albinism in the wild and demonstrate the utility of gene knockouts for experimental evolutionary biology.