Subterranean Fishes of the World

5622

2022

AIM 2022

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Low levels of dissolved oxygen (DO) are common in subterranean environments. However, the evolutionary consequences of low DO remain incompletely understood. The model species Astyanax mexicanus provides the opportunity to study the effects of life in low DO environments on an microevolutionary scale. Namely, the two closely-related morphotypes, a surface-dwelling and a derived obligate cave-dwelling morph, occupy different environments containing drastically different DO levels within the Sierra de El Abra region of northeastern Mexico. Here, we aimed to investigate potential adaptations to low DO in the cave morphotype by assessing hemoglobin levels, the primary oxygen transport protein of nearly all vertebrates. Despite rearing under normal lab conditions (ample DO), we found higher mean hemoglobin concentrations for three cave populations (Chica, Tinaja, Pachón) compared to surface fish. To examine possible underlying mechanisms that may augment hemoglobin concentration, we measured features of the red blood cells (RBC) of this species. Despite differences in hematocrit, the number of RBCs was comparable between surface and several cave morphs. However, we found that the 2-d area of cavefish RBCs was significantly larger than surface fish, implying a potential adaptation resulting in increased hemoglobin (per cell) in cavefish. To further examine possible adaptations to hypoxic caves, we conducted an RNA-sequencing analysis on whole blood. This analysis reveals patterns of differentially expressed genes shared among all three cave populations as well as patterns unique to each population, providing the opportunity to examine convergent and divergent changes to gene expression. Interestingly, we found similarities of hemoglobin expression in Pachón and Tinaja while Chica hemoglobin expression resembled surface fish. These transcriptional, protein, and cellular differences may provide adaptive advantages to life in hypoxic caves.

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