Selection on standing genetic variation mediates convergent evolution in extremophile fish.

Area of Science:

• Evolutionary Biology
• Genomics
• Toxicology

Background:

• Hydrogen sulfide (H₂S) is a toxicant impacting mitochondrial energy production.
• The *Poecilia mexicana* species complex exhibits independent evolution of H₂S tolerance across multiple populations.
• Phenotypic convergence in H₂S tolerance is evident, but the underlying genomic basis remains unclear.

Purpose of the Study:

• To investigate whether repeated evolution of H₂S tolerance in *Poecilia mexicana* involves similar genomic changes.
• To identify specific genes under selection related to H₂S tolerance and detoxification.

Main Methods:

• Targeted capture sequencing of candidate genes involved in H₂S processes and toxicity.
• Comparative analysis of sequence variation between sulfidic and nonsulfidic populations.
• Population genetics analyses to infer demographic history and selection.

Main Results:

• Most candidate genes showed population structure driven by demographics, not selection.
• A subset of genes displayed significant tree discordance, indicating divergent evolution.
• Two outlier genes, linked to mitochondrial H₂S detoxification, showed strong signatures of selection in all sulfidic populations.
• Long, shared haplotypes were identified in these selected regions among sulfidic populations.

Conclusions:

• Convergent phenotypic evolution of H₂S tolerance in *Poecilia mexicana* is likely driven by selection on standing genetic variation.
• Specific genes involved in mitochondrial sulfide detoxification appear to be key targets of selection.
• This study highlights the role of adaptive genetic variation in repeated evolutionary events.