Subterranean Fishes of the World

4272

2019

AIM 2019

22

Cave dwellers are equipped with a suite of troglomorphic traits associated with survival in the highly specialized dark environment. A widely accepted model for the evolution of cave animals posits cave colonization by surface dwelling ancestors followed by the acquisition of adaptations over many generations until fully troglomorphic forms are achieved. However, the speed of troglomorphic evolution in some species is difficult to reconcile with this conventional viewpoint, suggesting the importance of alternative mechanisms operating over shorter timescales. To obtain insight into these mechanisms, we exposed Astyanax mexicanus surface fish (SF) to completely dark (D/D) conditions as embryos or larvae and compared them to siblings placed on a standard photoperiod (L/D SF). We identified multiple traits that were altered in D/D SF compared to L/D SF adults raised in complete darkness for up to 2 years. These changes include differences in the expression of approximately 350 genes related to sensory systems, endocrine signaling, circadian rhythms, the circulatory system, and numerous metabolic changes, especially in lipids. In addition, on a phenotypic level we find alterations in size, weight, metabolic rates, starvation resistance, neurotransmitter levels, and hormone levels. Remarkably, most of these alterations are associated with known adaptations to the cave environment that change in the direction of the cavefish phenotype. Therefore, contrary to the widely-accepted scenario, our results indicate that the appearance of some cave-associated traits can occur within a single generation by phenotypic plasticity. The results suggest that phenotypic plasticity followed by genetic assimilation may be an important mechanism for the rapid evolution of cave-related traits during the colonization of A. mexicanus to dark cave environments.

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