Environmental hypoxia controls the evolution of cavefish heart asymmetry

The environmental factors responsible for the evolution of novel traits and their underlying mechanisms are poorly understood. Here we use the teleost Astyanax mexicanus consisting of ancestral surface fish and derived cavefish morphotypes to address the evolution of heart asymmetry. As in other vertebrates, surface fish develop right(D)-looping heart tubes, whereas high levels of left(L)-looping heart tubes have uniquely evolved in cavefish. Cavefish cardiac L-looping is mediated by the upregulation of Sonic Hedgehog (Shh) signalling during gastrulation, which disrupts left-right organizer (LRO) function and modifies the downstream left-oriented Nodal signalling cascade. As a proxy for the original cave colonizers, we exposed contemporary surface fish to key cave-associated environmental factors, namely complete darkness, reduced electrical conductivity, low temperature, hypoxia, or combinations of these conditions. Only hypoxia induced the cavefish L-looping phenotype. Hypoxia increased expression of genes in an integrated Hypoxia Inducing Factor 1(HIF1)-Shh axis, disrupted LRO function, and modified the L-R determination program in surface fish similarly to naturally evolved changes in cavefish. In contrast to surface fish, which show high plasticity in response to hypoxia, cavefish were unable to survive acute hypoxia, suggesting evolutionary refinement and canalization of the ancestral hypoxia response. These studies establish hypoxia as the driver of heart asymmetry and reveal a mechanistic connection between the hypoxic cave environment and the evolution of a novel trait.