Abstract
Extreme environmental features can drive the evolution of extreme phenotypes. Over the course of evolution, certain environmental changes may be so drastic that they lead to extinction. Conversely, if an organism adapts to harsh environmental changes, the adaptations may permit expansion of a novel niche. The interaction between environmental stressors and adaptive changes is well-illustrated by the blind Mexican cavefish, Astyanaxmexicanus, which has recurrently adapted to the stark subterranean environment. The transition from terrestrial rivers and streams (occupied by extant surface morphs of the same species) to the cave has been accompanied by the resorption of eyes, diminished pigmentation and reduced metabolism in cave-dwelling morphs. The principal features of caves most often associated with evolution of these common cave features are the absence of light and limited nutrition. However, a putatively essential cave feature that has received less attention is the frequently low concentration of oxygen within natural karst environments. Here, we review the potential role of limited oxygen as a critical environmental feature of caves in the Sierra de El Abra. Additionally, we review evidence that Astyanax cavefish may have evolved adaptive features enabling them to thrive in lower oxygen compared to their surface-dwelling counterparts.
Highlights
Extreme environmental changes can have dramatic consequences on organismal form and physiology
This review has two principal aims: First, we examine the available evidence of low oxygen in the caves of the Sierra de El Abra
Over the last few decades, the blind Mexican cavefish Astyanax mexicanus has been utilized to investigate diverse adaptive phenotypes, such as vision loss, sleep reduction and changes in nutrient metabolism evolving in cave organisms [57]
Summary
Extreme environmental changes can have dramatic consequences on organismal form and physiology. The freshwater teleost species Astyanax mexicanus is a ”natural” model system that has provided novel insight to interactions between environment and phenotype. The cavefish model system, Astyanax mexicanus, presents the opportunity to examine interspecific differences in oxygen metabolism in two morphs of the same species This powerful comparative paradigm has enabled broad insights to the developmental, genetic and morphological differences that underlie adaptation to the cave environment. We propose that reduced oxygen is a critical driver of cave-associated phenotypes, and that Astyanax cavefish have adapted to the low oxygen concentrations in the cave environment. We review key factors determining dissolved oxygen concentrations in natural bodies of water, including how these factors may differentially influence the cave and surface environments of the Sierra de El Abra. The caves of the Sierra de El Abra have likely experienced reduced level(s) of oxygen, in a manner consistently lower than terrestrial environments, across vast geologic and evolutionary timescales
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