Subterranean Fishes of the World

4294

2019

AIM 2019

45

Aquatic cave animals can face low dissolved oxygen levels relative to their surface dwelling counterparts but little is known about how their circulatory systems have adapted to respond to these hypoxic conditions. We have addressed this question in the Mexican tetra Astyanax mexicanus, a single species consisting of both surface dwelling (surface fish, SF) and cave dwelling (Pachon cavefish, CF) forms. One of the ways in which mammals adapt to hypoxia is by increasing the number of erythrocytes. Therefore, we imaged live embryos immediately after circulation commences at about 36 hours post fertilization (hpf) and observed more erythrocytes flowing from the common cardinal vein over the yolk to the heart in CF embryos than in SF embryos. To substantiate this observation, we showed upregulation of hemoglobin genes by quantitative RT-PCR (qPCR) at 24 hpf and confirmed the increase in blood cells at 48 hpf in CF relative to SF embryos by in situ hybridization with a general ß-hemoglobin probe. To understand how the increase in red blood cells is controlled, we conducted qPCR and in situ hybridization for some of the genes regulating erythrocyte determination and observed upregulated expression in the posterior blood islands of CF embryos at 26 hpf. To test the possible benefits of the increase in red blood cells, we induced sub-threshold hemolysis by treating SF and CF embryos with phenylhydrazine and found greater effects on SF than CF larval development, consistent with buffering by additional red blood cells. Lastly, we exposed SF and CF embryos to a hypoxic environment and looked at key genes involved in aerobic and anaerobic respiration and show that CF are able to cope better with hypoxia. We conclude that CF have modified circulatory system development by changes in gene expression to produce more red blood cells as an adaptation to hypoxia.

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Astyanax Interrnational Meeting