Convergent Evolution of Conserved Mitochondrial Pathways Underlies Repeated Adaptation to Extreme Environments
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Abstract
Extreme environments test the limits of life; yet, some organisms thrive in harsh conditions. Extremophile lineages inspire questions about how organisms can tolerate physiochemical stressors and whether the repeated colonization of extreme environments is facilitated by predictable and repeatable evolutionary innovations. We identified the mechanistic basis underlying convergent evolution of tolerance to hydrogen sulfide (H 2 S)—a toxicant that impairs mitochondrial function—across evolutionarily independent lineages of a fish ( Poecilia mexicana , Poeciliidae) from H 2 S-rich springs. Using comparative biochemical and physiological analyses, we found that mitochondrial function is maintained in the presence of H 2 S in sulfide spring P. mexicana , but not ancestral lineages from nonsulfidic habitats, due to convergent adaptations in the primary toxicity target and a major detoxification enzyme. Genome-wide local ancestry analyses indicated that convergent evolution of increased H 2 S tolerance in different populations is likely caused by a combination of selection on standing genetic variation and de novo mutations. At a macroevolutionary scale, H 2 S tolerance in 10 independent lineages of sulfide spring fishes across multiple genera of Poeciliidae is correlated with the convergent modification and expression changes of genes associated with H 2 S toxicity and detoxification. Our results demonstrate that the modification of highly conserved physiological pathways associated with essential mitochondrial processes mediates tolerance to physiochemical stress. In addition, the same pathways, genes, and—in some instances—codons are implicated in H 2 S adaptation in lineages that span 40 million years of evolution.
Some organisms can tolerate environments lethal for most others, but we often do not know what adaptations allow them to persist and whether the same mechanisms underly adaptation in different lineages exposed to the same stressors. Investigating fish inhabiting springs rich in toxic hydrogen sulfide (H 2 S), we show that tolerance is mediated by the modification of pathways that are inhibited by H 2 S and those that can detoxify it. Sulfide spring fishes across multiple genera have evolved similar modifications of toxicity targets and detoxification pathways, despite abundant lineage-specific variation. Our study highlights how constraints associated with the physiological consequences of a stressor limit the number of adaptive solutions and lead to repeatable evolutionary outcomes across organizational and evolutionary scales.
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Excerpt
Multiple lineages of poecilid fish have colonized one of Earth’s most extreme environments – hydrogen sulphide (H2S) springs. How, you ask? Greenway et al. reveal that convergent mitochondrial adaptations conferred H2S tolerance to each lineage.
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