Nucleo-cytoplasmic compatibility in interspecies Saccharomyces hybrids and the destabilisation of the mitogenome by allospecific recombination

The postzygotic reproductive isolation of species in the Saccharomyces genus is maintained by failure of the homeologous chromosomes to pair in meiosis and by repression of the mating programme by MATa/MATalpha heterozygosity, but nucleo-mitochondrial incompatibilities causing respiration deficiency were also proposed to be involved. In this study, we investigated the compatibility of parental and recombinant mitogenomes with alloploid nuclear genomes in synthetic S. cerevisiae x S. uvarum hybrids. All hybrids were homoplasmic but their mitogenomes showed high structural diversity due to frequent allospecific recombination. 34 recombination sites were identified, all in coding regions. From the location of the sites, it could be inferred that both intron-homing and crossing-over were involved. The recombinant mitogenomes had diverse combinations of parental and chimeric genes. The respiration proficiency of the hybrids indicates compatibility between their (parental or recombinant) mitogenomes and alloploid nuclear genomes. However, the recombinant mitogenomes were unstable, resulting in respiration-deficient segregants in the hybrid cultures. Since lack of respiration impairs sporulation efficiency and also competitiveness in mixed populations with respiration-proficient (e.g. parental) cells, destabilisation of the mitogenome by allospecific recombination may (indirectly) contribute to the biological isolation of Saccharomyces species.