Mitochondrial function depends on jointly expressed gene products from two genomes, and a 2020 systematic mtDNA-replacement study in Saccharomyces cerevisiae (the full 225-genotype panel of nuclear-background × mtDNA combinations among 15 natural isolates, of which 210 are synthetic pairings and 15 recreate the original pairings) demonstrated pervasive, environment-dependent mitochondrial-nuclear (mitonuclear) coadaptation: original mitonuclear pairings outgrew synthetic ones, especially in media mimicking the strains' isolation habitats. The causal genetic determinants were not identified — mitochondrial coding-SNP association tests were null — leaving open whether the relevant variants are mtDNA intergenic or regulatory sequences affecting replication or copy number, combinations of multiple small-effect sites, or particular nuclear backgrounds. Follow-up work through 2026 has produced a dedicated mapping resource (the Mitonuclear Recombinant Collection, used to map determinants of mtDNA stability), species-level mitotype effects with positive epistasis, and confirmation of intraspecific coadaptation in laboratory cybrids (cytoplasmic hybrids, constructed strains whose mitochondria derive from a different nuclear background), but no study has mapped the determinants of the intraspecific growth and fitness coadaptation itself or causally assigned its mechanistic class. The open problem is to identify the nuclear loci and mtDNA sequence features underlying this coadaptation, determine their mechanistic basis and strain and environment specificity, and exclude laboratory-media and strain-construction artifacts.

Background

Nearly all mitochondrial proteins are nuclearly encoded, while the mitochondrial genome (mtDNA) of budding yeast Saccharomyces cerevisiae encodes only a handful of respiratory-chain subunits, ribosomal components and tRNAs, embedded in long AT-rich intergenic spacers punctuated by short mobile GC-rich palindromic elements called GC-clusters. Respiratory function therefore requires matched gene products from two genomes with very different genetics: mtDNA is cytoplasmically inherited, recombines, and cannot be handled with the meiotic-recombination mapping used for nuclear loci. 'Mitonuclear coadaptation' is the proposition that selection tailors nuclear alleles to the local mtDNA, so that mitochondrial-nuclear combinations drawn from the same population interact more favorably than synthetic combinations; 'mitonuclear epistasis' denotes the non-additive phenotypic interaction between the two genomes.

Nguyen, Sondhi, Ziesel, Paliwal and Fiumera (2020) tested this directly in S. cerevisiae by replacing mtDNAs among 15 natural isolates from diverse ecological niches, generating the full 225-genotype panel of nuclear-background × mtDNA combinations (210 synthetic pairings plus 15 recreated original pairings). mtDNA was moved between nuclear backgrounds by cytoduction: rho0 strains (cells experimentally depleted of their mtDNA) receive mitochondria through kar1-1 matings, matings in which nuclear fusion fails so that only cytoplasm, and hence mitochondria, is transferred. Growth profiling across standard and niche-mimicking media showed pervasive, environment-dependent mitonuclear epistasis, with larger effect sizes between than within population clades, and with original (coadapted) mitonuclear pairings outgrowing synthetic ones in media resembling the strains' isolation habitats. An earlier study of the same system had already detected mitonuclear epistasis for growth and sporulation traits (Paliwal, Fiumera and Fiumera 2014).

Critically, the 2020 study did not identify which sequence variants produce these interactions: association tests between mitochondrial coding SNPs and growth phenotypes found no significant hits in the small, strain-structured panels, which the authors attributed to low statistical power and relatively small phenotypic differences. Because exchanges between mitotypes with identical mitochondrial coding sequences but different intergenic sequence still showed significant mitonuclear interactions, and because the parental strains differ in mtDNA copy number, the authors proposed that causal variants may lie in mitochondrial intergenic or regulatory sequences influencing mtDNA replication or copy number; the alternative that interactions reflect combinations of multiple small-effect sites remains open. They further noted that coadapted loci are probably strain- and niche-specific (so mapping needs larger samples, broader variant sets and environment-matched assays), that laboratory media may imperfectly proxy natural habitats, and that the rho0 and kar1-1 construction protocol is potentially mutagenic, so construction-introduced mutations are not definitively excluded. A review of yeast mitogenomics (Visinoni and Delneri 2022) frames exactly this dissection of molecular determinants as the field's open frontier.

Problem Statement

Identify the genetic determinants of the mitonuclear coadaptation demonstrated among natural Saccharomyces cerevisiae isolates by mitochondrial-DNA (mtDNA) replacement (Nguyen et al. 2020), that is, the environment-dependent growth advantage of original over synthetic mitochondrial-nuclear (mitonuclear) genotype combinations. Concretely: (i) map the nuclear loci and mtDNA sequence features whose interactions cause the environment-dependent growth and fitness differences between coadapted and synthetic mitonuclear genotypes, with designs that have adequate power for epistatic effects and control the strong population structure of natural isolates; (ii) assign the mechanistic class of the mitochondrial contribution along two non-exclusive axes, sequence location (protein-coding changes versus intergenic or regulatory sequences affecting mtDNA replication or mtDNA copy number) and genetic architecture (single-site versus combinations of multiple small-effect sites), so that an outcome spanning both is reported on both axes, given that mitochondrial coding-SNP association was null while intergenic-only exchanges still produced significant interactions; (iii) establish how general the coadapted architectures are across nuclear backgrounds and ecological contexts, since the source evidence indicates they are strain- and niche-specific; and (iv) exclude the two principal artifacts, laboratory-media distortion of natural selection pressures and mutations introduced by rho0 generation (experimental depletion of mtDNA from recipient strains) or kar1-1 cytoduction (mitochondria-only transfer through matings in which nuclear fusion fails). All four components are mandatory aspects of this single determination.

References

  1. Nguyen THM, Sondhi S, Ziesel A, Paliwal S, Fiumera HL (2020). Mitochondrial-nuclear coadaptation revealed through mtDNA replacements in Saccharomyces cerevisiae. BMC Evolutionary Biology 20:128. DOI: 10.1186/s12862-020-01685-6. https://doi.org/10.1186/s12862-020-01685-6 (PMID 32977769, PMC7517635)
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