# Mechanisms Maintaining Mitochondrial DNA Polymorphisms: The Role of Mito-Nuclear Interactions, Sex-Specific Selection, and Genotype-by-Environment Interactions in Drosophila subobscura

## TL;DR

This study explores how mitochondrial DNA variation is maintained in fruit flies through interactions with nuclear genes, environmental factors, and sex-specific selection.

## Contribution

The research provides new insights into mito-nuclear interactions and genotype-by-environment effects in maintaining mtDNA polymorphisms.

## Key findings

- Mito-nuclear interactions are more important as units of selection for mtDNA variability.
- Genotype-by-environment interactions show weak but present effects on mtDNA haplotype frequencies.
- No strong evidence of sex-specific selection was found between the two main mtDNA haplotypes.

## Abstract

Drosophila subobscura is an interesting model to study forces that shape and maintain sympatric mitochondrial DNA (mtDNA) variation, due to the widespread presence of the two main, almost equally frequent haplotypes. Experimental setups using different life-history components enable us to study the adaptive significance of mtDNA variation and its effects on fitness while also trying to disentangle the role of different balancing selection mechanisms that operate in order to promote stable variation in natural populations. Constructing mito-nuclear experimental lines using backcrossing enables us to discern whether mito-nuclear interactions play a role in maintaining the aforementioned mtDNA variation. Since studies examining the maintenance of intrapopulation mitochondrial variability are scarce, our experimental results significantly contribute to this field of research.

Experimental mito-nuclear introgression lines (MNILs) were established by backcrossing isofemale lines of D. subobscura originating from the same populations. MNILs were subjected to a series of life-history experiments designed to test the fitness of the bearers of different combinations of two main mtDNA haplotypes on their own nuclear background, as well as on the background of the opposite haplotype. By having 11 replicas of the four mito-nuclear combinations, we could test not only the adaptive significance of the differences between the two main haplotypes but also the influence of additional variation present within each of the 11 combinations on fitness. Testing the fitness of individuals of both sexes enabled us to examine if sex-specific selection has a role in maintaining the frequencies of the two mtDNA haplotypes in nature. Conducting the fitness assays on two different temperatures enabled us to test whether different temperatures favor specific mtDNA haplotypes or mito-nuclear genotypes and consequently promote stable sympatric mtDNA variation. The results show weak signature of genotype-by-environment interactions, and no sex-specific selection regarding differences between the two main haplotypes. However, individual models across different life-history components showed these two mechanisms at play in promoting mtDNA variability present in specific mito-nuclear crosses. Our models show that mito-nuclear interactions are, in fact, more important as units of selection.