Hybrids between species are usually unviable or sterile. One possible mechanism causing reproductive isolation is incompatibility between genes from different species. These "speciation" genes are interacting components that cannot function properly when mixed with alleles from other species. To test whether such genes exist in two closely related yeast species, we constructed hybrid lines in which one or two chromosomes were derived from Saccharomyces bayanus, and the rest were from Saccharomyces cerevisiae. We found that the hybrid line with Chromosome 13 substitution was completely sterile and identified Aep2, a mitochondrial protein encoded on Chromosome 13, to cause the sporulation defect as S. bayanus AEP2 is incompatible with S. cerevisiae mitochondria. This is caused by the inability of S. bayanus Aep2 protein to regulate the translation of the S. cerevisiae OLI1 mRNA. We speculate that AEP2 and OLI1 have evolved during the adaptation of S. bayanus to nonfermentable carbon sources, thereby driving speciation.
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"Deleterious effects of hybridization on fitness resulting from mitonuclear epistatic interactions have been recently described in a number of species. Together, these studies have established requirements for demonstrating that mitonuclear epistasis causes interpopulation hybrid dysfunction (e.g., Breeuwer and Werren 1995; Tiffin et al. 2001; Levin 2003; Sackton et al. 2003; Bolnick and Near 2005; Zeyl et al. 2005; Ellison and Burton 2006; Fishman and Willis 2006; Dowling et al. 2007; Ellison and Burton 2008a, 2008b; Ellison et al. 2008; Lee et al. 2008; Niehuis et al. 2008; Gagnaire et al. 2012; Gibson et al. 2013; Hoekstra et al. 2013; Meiklejohn et al. 2013; Gershoni et al. 2014). First, genetic variation must exist between the nuclear genomes and the mitochondrial genomes of the two populations. "
[Show abstract][Hide abstract] ABSTRACT: In order to identify the earliest genetic changes that precipitate species formation, it is useful to study genetic incompatibilities that cause only mild dysfunction when incompatible alleles are combined in an inter-population hybrid. Such hybridization within the nematode species Caenorhabditis briggsae has been suggested to result in selection against certain combinations of nuclear and mitochondrial alleles, raising the possibility that mitochondrial-nuclear (mitonuclear) epistasis reduces hybrid fitness. To test this hypothesis, cytoplasmic-nuclear hybrids (cybrids) were created to purposefully disrupt any epistatic interactions. Experimental analysis of the cybrids suggests that mitonuclear discord can result in decreased fecundity, increased lipid content, and increased mitochondrial reactive oxygen species levels. Many of these effects were asymmetric with respect to cross direction, as expected if cytoplasmic-nuclear Dobzhansky-Muller incompatibilities exist. One such effect is consistent with the interpretation that disrupting coevolved mitochondrial and nuclear loci impacts mitochondrial function and organismal fitness. These findings enhance efforts to study the genesis, identity, and maintenance of genetic incompatibilities that precipitate the speciation process.
Preview · Article · Nov 2015 · G3-Genes Genomes Genetics
"Backcross studies on this species have revealed that hybrid breakdown can be attributed to the nuclear-mitochondrial interactions, raising the possibility that mitonuclear incompatibilities may represent an important mechanism of reproductive isolation and speciation (Burton and Barreto 2012). Mitonuclear mismatch also produced sterile hybrids between the yeast species Saccharomyces cerevisiae and S. bayanus (Lee et al. 2008), and hybrid breakdown between species of Nasonia wasps (Ellison et al. 2008), Drosophila (Sackton et al. 2003), centrarchid fishes (Bolnick et al. 2008) and several plant species (Bomblies 2010 and references therein). Studies on Drosophila that have manipulated nuclear and mitochondrial genetic variation have found that mitonuclear epistasis is an important force in shaping longevity variation (James and Ballard 2003; Rand et al. 2006; Maklakov et al. 2006; Ballard et al. 2007; Clancy 2008; Dowling et al. 2009). "
[Show abstract][Hide abstract] ABSTRACT: Products and regulatory motifs of the mitochondrial and nuclear genomes interact closely to enable efficient cellular energy production within mitochondria. Although recent evidences support the prediction that during evolutionary time combinations of these interactions are optimized by selection acting on important life history traits, relatively few studies have directly tested it. The goal of this study was to test the role of mitonuclear interactions in shaping preadult and adult life history traits under age-specific selection in the seed beetle (Acanthoscelides obtectus). In order to disentangle the effects of mitochondria, nuclei and their interaction in the evolutionary response to the long-term laboratory selection for early (E) and late (L) reproduction, we used mitonuclear introgression lines in which E and L mitochondrial genomes were expressed in both E and L nuclear background. We found that mitonuclear genotypes carrying disrupted pair of nuclear and mitochondrial genomes mainly affected preadult life history traits—egg-to-adult viability and developmental time. Neither mitochondria nor their interaction with nuclear genomes had effects on realized fecundity of mated females and longevity of virgin beetles. However, when involved in reproductive activities females and males with disrupted genotypes mostly exhibited reduced longevity. Furthermore, since reproduced males exhibited greater longevity cost than females, our results are in accordance with the mother’s curse hypothesis. Being that for the most life history traits we detected smaller additive mitochondrial genetic effects compared with epistatic mitonuclear effects, we concluded that mitonuclear interactions might be the target of age-specific selection.
"The effects on F1 hybrids however are often unpredictable ; hybrids can display hybrid vigor — increased fitness in certain traits relative to the parental species (Livesay, 1930; Manwell et al., 1963; McDaniel and Grimwood, 1971) — but more commonly display defects. In many cases, defects in hybrids are linked to incompatibilities between nuclear and mitochondrial genomes (Liepins and Hennen, 1977; Edmands and Burton, 1999; Sackton et al., 2003; Ellison and Burton, 2006, 2008a; Ellison et al., 2008; Lee et al., 2008; Niehuis et al., 2008). While a hybrid's nuclear DNA is inherited from both parents (and therefore both species), its mitochondrial DNA is inherited from only one parent (in most cases, the mother). "
[Show abstract][Hide abstract] ABSTRACT: Previous studies have shown evidence of genomic incompatibility and mitochondrial enzyme dysfunction in hybrids of bluegill (Lepomis macrochirus Rafinesque) and pumpkinseed (L. gibbosus Linnaeus) sunfish (Davies et al., 2013 Physiol. Biochem. Zool. 85, 321–331). We assessed if these differences in mitochondria had an impact on metabolic processes that depend on mitochondrial function, specifically hypoxia tolerance and recovery from burst exercise. Bluegill, pumpkinseed, and their hybrids showed no difference in the critical oxygen tension (Pcrit) and no differences in tissue metabolites measured after exposure to 10% O2 for 30 min. In contrast, loss of equilibrium (LOE) measurements showed that hybrids had reduced hypoxia tolerance and lacked the size-dependence in hypoxia tolerance seen in the parental species. However, we found no evidence of systematic differences in metabolite levels in fish after LOE. Furthermore, there were abundant glycogen reserves at the point of loss of equilibrium. The three genotypes did not differ in metabolite status at rest, showed an equal disruption at exhaustion, and similar metabolic profiles throughout recovery. Thus, we found no evidence of a mitochondria dysfunction in hybrids, and mitochondrial differences and oxidative metabolism did not explain the variation in hypoxia tolerance seen in the hybrid and two parental species.
Full-text · Article · Dec 2014 · Comparative Biochemistry and Physiology - Part A Molecular & Integrative Physiology