Ancient balancing selection maintains incompatible versions of the galactose pathway in yeast (Figure creation)

Metabolic pathways differ between species, but are expected to be similar
within a species. We discovered two functional, incompatible versions of
the galactose pathway in S. cerevisiae. We identified a 3-locus genetic
interaction for growth in galactose, and used precisely engineered alleles
to show that is arises from variation in the metabolic genes GAL2,
GAL1/10/7, and PGM1, and that the reference allele of PGM1 is incompatible
with the alternative alleles of the other genes. Multi-locus balancing
selection has maintained the two incompatible versions of the pathway for
millions of years. Strains with alternative alleles are found primarily in
galactose-rich dairy environments, and they grow faster in galactose, but
slower in glucose, revealing a tradeoff on which balancing selection may
have acted.