Plasticity of Mitochondrial DNA Inheritance and its Impact on Nuclear Gene Transcription in Yeast Hybrids

Mitochondrial DNA (mtDNA) in budding yeast is biparentally inherited, but colonies rapidly lose one type of parental mtDNA, becoming homoplasmic. Therefore, hybrids between different yeast species possess two homologous nuclear genomes, but only one type of mitochondrial DNA. We hypothesise that the choice of mtDNA retention is influenced by its contribution to hybrid fitness in different environments, and that the allelic expression of the two nuclear sub-genomes is affected by the presence of different mtDNAs in hybrids. Here, we crossed Saccharomyces cerevisiae with S. uvarum under different environmental conditions and examined the plasticity of the retention of mtDNA in each hybrid. We showed that on fermentable carbon sources at warm temperatures each parental mtDNA was equally likely to be retained, while at colder temperatures, hybrids preferentially retained mtDNA derived from S. uvarum. On a non-fermentable carbon source, hybrids retained S. cerevisiae mtDNA, independent of temperature. By acquiring transcriptome data and co-expression profiles for hybrids harbouring different mtDNA in a selection of environments, we found a clear pattern of concerted allelic transcription of one or the other sub-genome for specific biological pathways, supporting the notion that the hybrid cell works preferentially with one set of parental alleles or the other according to specific cellular functions. We argue that the type of mtDNA retained in hybrids affects the expression of the nuclear genome and the organism fitness in different environments, and therefore may have a role in driving the evolution of the hybrid nuclear genome in terms of gene retention and loss.