Subterranean Fishes of the World

5623

2022

AIM 2022

29

Subterranean waters of the Mexican Sierra de El Abra have reduced levels of dissolved oxygen. The effects of hypoxia on fish in surface waters is well known but the effects of hypoxia in caves are poorly understood. We compared Astyanax mexicanus surface (SF) and cave (CF) morphs to understand how CF cope with hypoxia and showed that more erythrocytes develop in CF than SF. In embryos of previously-studied teleost species, hematopoiesis occurs in the anterior and posterior lateral mesoderm (LM), but erythrocytes are formed only in the posterior LM. In Astyanax SF and CF embryos, erythropoiesis occurs in both the anterior and posterior LM, which could be a preadaptation for colonization of hypoxic caves. Hematopoiesis and erythropoiesis were expanded in both the anterior and posterior LM in CF compared to SF embryos, which may be responsible for more CF erythrocytes. Reciprocal crosses between SF and CF showed that CF erythrocyte increase is under maternal control. To determine whether erythrocytes are required for normal development, we induced hemolytic anemia with phenylhydrazine. Lowering erythrocyte numbers reduced growth and caused axial defects, indicating that erythrocytes are required for normal development. The importance of more erythrocytes in an oxygen deficient environment was tested by analyzing growth and molecular responses in SF and CF larvae subjected to hypoxia in the laboratory. Hypoxia reduced erythrocyte number and postanal tail and eye growth in SF but not CF, activated the hypoxia-inducible factor1 (hif1) genes, and increased the expression of some HIF1 downstream target genes. Quantitative analysis showed that some hif1 genes are constitutively upregulated to similar levels in normoxic CF as in hypoxic SF. These results show that cavefish cope with hypoxia by constitutive hif1 overexpression and increasing erythrocyte development.

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