text: Aerobic fermentation  evolved independently in at least three yeast lineages (Saccharomyces, Dekkera, Schizosaccharomyces).  It has also been observed in plant pollen, trypanosomatids, mutated E. coli, and tumor cells.  Crabtree-positive yeasts will respire when grown with very low concentrations of glucose or when grown on most other carbohydrate sources.  The Crabtree effect is a regulatory system whereby respiration is repressed by fermentation, except in low sugar conditions.  When Saccharomyces cerevisiae is grown below the sugar threshold and undergoes a respiration metabolism, the fermentation pathway is still fully expressed, while the respiration pathway is only expressed relative to the sugar availability.  This contrasts with the Pasteur effect, which is the inhibition of fermentation in the presence of oxygen and observed in most organisms.
  The evolution of aerobic fermentation likely involved multiple successive molecular steps, which included the expansion of hexose transporter genes, copy number variation (CNV) and differential expression in metabolic genes, and regulatory reprogramming.  Research is still needed to fully understand the genomic basis of this complex phenomenon.  Many Crabtree-positive yeast species are used for their fermentation ability in industrial processes in the production of wine, beer, sake, bread, and bioethanol.  Through domestication, these yeast species have evolved, often through artificial selection, to better fit their environment.  Strains evolved through mechanisms that include interspecific hybridization, horizontal gene transfer (HGT), gene duplication, pseudogenization, and gene loss.