In 85 mammal species, the ADH7 gene has become nonfunctional at least 10 separate times during evolution, meaning these species no longer use this gene to break down ethanol. This loss occurred in...
Mechanism
Synthesis from 1 study
Animals that don’t eat much fermented fruit lose the ability to break down alcohol because their bodies no longer need it, so the gene for that job breaks down over time. In animals that do eat fermented fruit, a small tweak in the enzyme makes it work better — showing that evolution keeps or loses...
Most probable mechanism
When animals eat little or no fermented fruit or nectar, there's no need to break down alcohol, so mutations that break the alcohol-processing gene stick around and spread. Over time, this gene stops working in multiple animal groups, and they lose the ability to process alcohol in their stomachs. In some herbivores, gut bacteria take over detoxifying other plant chemicals, making the broken gene even less of a problem.
Ethanol from fermenting plant materials enters the upper gastrointestinal tract and is metabolized by the ADH IV enzyme, which is encoded by the ADH7 gene.
In species with diets low in fermenting fruits or nectars, there is no selective advantage to maintaining a functional ADH7 gene, leading to accumulation of loss-of-function mutations such as premature stop codons and frameshifts.
These mutations cause the ADH7 gene to become a pseudogene, preventing production of a functional ADH IV enzyme and eliminating ethanol oxidation in the upper GI tract.
In herbivorous lineages, detoxification of plant-derived toxic alcohols is handled by microbial communities in the hindgut or foregut, reducing reliance on ADH IV and allowing ADH7 pseudogenization without fitness cost.
Less supported by current evidence, but not ruled out
In animals that eat a lot of fermented fruit or nectar, a small change in the alcohol-processing enzyme makes it much better at breaking down alcohol, and this change evolved separately in different species as an adaptation.
A specific amino acid change at position 294 in the ADH IV enzyme alters its shape to increase catalytic efficiency for ethanol oxidation.
This change occurs independently in multiple lineages with diets rich in fermenting plant materials, providing a selective advantage for ethanol metabolism.
These substitutions are maintained by strong selection in frugivorous and nectarivorous species, contrasting with pseudogenization in species lacking such dietary exposure.
Evidence from Studies
Supporting (1)
Community contributions welcome
Genetic evidence of widespread variation in ethanol metabolism among mammals: revisiting the ‘myth' of natural intoxication
Contradicting (0)
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