Subterranean Fishes of the World

6312

2023

A Thesis submitted in partial fulfillment of the requirements for the Degree of Master of Science, Department of Biology, College of Arts and Sciences, Kansas State University, Manhattan, Kansas. Copyright © Madison Nobrega 2023

Poecilia mexicana is a live-bearing fish known for its extremophilic populations. The species has been studied for their high tolerance to hydrogen sulfide (H2S), an environmental toxicant that occurs naturally in some freshwater springs in southern Mexico. Though studies often reduce this system to a presence or absence of H2S, focusing entirely on H2S is likely to generate an incomplete picture. H2S springs are complex habitats, differing in biotic and abiotic conditions besides the presence of H2S. For example, sulfidic habitats are characterized by hypoxia, low pH, different trophic regimes and predators, and strong energy constraints. How these other sources of selection impact the evolution of extremophile fish populations, and how they might interact with H2S, remains largely unstudied. My research focused on two questions within the P. mexicana system, where one question acknowledged the complexity of environment and organism, and the other consciously stripped away complexity to isolate the direct impacts of H2S. My first chapter embraced environmental and organismal complexity to investigate phenotypic differences between sulfidic and nonsulfidic populations of P. mexicana. Though H2S is commonly identified as the driving selective force between the two populations, not all divergent traits exhibit clear adaptive hypotheses related to this toxicant. I compared phenotypic traits between populations in the wild, then compared those same traits in first-generation individuals from a common-garden environment. Almost every trait showed significant divergence between wild sulfidic and nonsulfidic populations. Most traits remained different between populations reared in the lab, though many simultaneously presented evidence of plasticity. My results suggested that trait divergences between populations in different habitats were largely shaped by evolutionary change, and I developed hypotheses about the potential adaptive function of divergent traits in the context of the complex selective regimes in sulfide springs. My second chapter examined H2S responses outside of environmental and organismal context. To test what organismal responses are a direct response of H2S alone, I analyzed the gene expression response of P. mexicana to an H2S-releasing pharmaceutical compound (AP39) that targets the mitochondria. I exposed both sulfidic and nonsulfidic populations to AP39, then compared their gene expression in the liver and the gills to individuals not exposed to AP39. I found no gene expression differences between the gill tissue of AP39-exposed and unexposed individuals. While there were significant differences in gene expression in livers, none of the expressed genes were related to H2S tolerance or detoxification. I concluded that AP39 makes a poor model for H2S exposure in P. mexicana. My findings underscore the importance of context. Exposing just the mitochondria to H2S via a targeted compound did not evoke the same responses as exposing the whole organism via H2S dissolved in water, as it is found in nature. Overall, my research attests that evolutionary biologists cannot ignore environmental and organismal complexity, even in systems with strong selective pressures.

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