# The role of mitotype variation and positive epistasis in trait differences between Saccharomyces species

## TL;DR

This study explores how differences in mitochondrial DNA contribute to the ability of a yeast species to tolerate high temperatures.

## Contribution

The study demonstrates that mitochondrial DNA variation, combined with nuclear genome compatibility, influences thermotolerance in yeast.

## Key findings

- Mitochondrial DNA from S. cerevisiae improves thermotolerance and respiration in S. paradoxus.
- Positive epistasis occurs between mitochondrial and nuclear genomes when both support thermotolerance.
- Thermotolerance benefits come with reduced performance in cooler conditions.

## Abstract

Many traits of interest in biology evolved long ago and are fixed in a particular species, distinguishing it from other sister taxa. Elucidating the mechanisms underlying such divergences across reproductive barriers has been a key challenge for evolutionary biologists. The yeast Saccharomyces cerevisiae is unique among its relatives for its ability to thrive at high temperature. The genetic determinants of the trait remain incompletely understood, and we sought to understand the role in its architecture of species variation in mitochondrial DNA. We used mitochondrial transgenesis to show that S. cerevisiae mitotypes were sufficient for a partial boost to thermotolerance and respiration in the Saccharomyces paradoxus background. These mitochondrial alleles worked best when the background also harbored a pro-thermotolerance nuclear genotype, attesting to positive epistasis between the two genomes. The benefits of S. cerevisiae alleles in terms of respiration and growth at high temperature came at the cost of worse performance in cooler conditions. Together, our results establish this system as a case in which mitoalleles drive fitness benefits in a manner compatible with, and fostered by, the nuclear genome.