Intriguing relationship between chromosome I nuclear aneuploidy and mitochondrial intron deletion in yeast

A curious phenomenon was discovered when examining revertant colonies of the M3041 mutant located in the second intron of mitochondrial cytochrome b gene of Sacchromyces cerevisiae . Several colonies carrying a sector appeared on selective glycerol medium and all correspond to a reversion of M3041 by intron deletion. Colonies of revertants resulting from classical reversion were never sectored. The cells of the sectors, called S, presented a very typical growth profile on glycerol (selective medium for respiration) which differs from that of the rest of the colony called AS for anti-sector. The profiles are identical on glucose-based medium. Molecular analysis, by Pulsed Field Gel Electophoresis, of the S cells showed that they had lost one copy of chromosome 1, suggesting that the original a-haploid mutant had diploidized into mat (a/a). The aneuploid state was validated by crossing with a genetically marked strain followed by mass spore analysis. We then asked whether the aneuploid genetic background would influence the frequency of reversion by intron deletion. This was achieved through the conversion of S and AS cells to rho ⁰ followed by cytoduction of mitochondria carrying M3041 or W91 mutations, knowing that W91 never reverses by intron deletion. Interestingly, there are significantly more revertants (by intron deletion) in the S than AS context for M3041, whereas W91 reversed with the same frequency in both contexts. Nuclear aneuploidy therefore promotes mitochondrial reversion by intron-deletion. We conclude that there is a particular relationship between nuclear aneuploidy and intron deletion during the reversion of mitochondrial intron mutations in yeast.