The teleost Astyanax mexicanus is a single species with surface dwelling (surface fish) and cave dwelling (cavefish) morphs. Constructive and regressive traits have evolved in cavefish as adaptations for survival in perpetual darkness. In addition to darkness, cavefish must cope with de-oxygenated aquatic environments. Blood cell quantification and expression of hematopoietic marker genes indicated that cavefish have more erythrocytes than surface fish, and that this increase has a developmental basis. In contrast to zebrafish and other teleost embryos, in which erythrocyte formation is restricted to the posterior lateral mesoderm, the anterior as well as the posterior lateral mesoderm is involved in red blood cell formation in Astyanax embryos, and both of these hematopoietic domains are expanded in cavefish embryos. Erythroid development in the anterior mesoderm may be a pre-adaptation for surface fish to successfully colonize hypoxic cave environments. We also show that cavefish are less sensitive to phenylhydrazine-induced erythrocyte ablation than surface fish, suggesting a functional advantage of increased red blood cells. By mimicking a hypoxic cave environment in the laboratory, we further demonstrate that cavefish respond to hypoxia differently than surface fish. Surface fish with fewer red blood cells use overall metabolic depression to counteract hypoxia, whereas cavefish with larger numbers of erythrocytes respond to hypoxia by switching to anaerobic metabolism. These results suggest that cavefish may have adapted to hypoxic environments by enhancing the capacity to form erythrocytes and reprogramming metabolism.
Summary
Astyanax mexicanus cavefish adapt to life in hypoxic cave environments by evolving the capacity to increase red blood cell numbers during early development and reprogramming metabolism to favor anaerobic processes.
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