Signatures of Innovation and Selection in the Extremotolerant Yeast Kluyveromyces marxianus

Organisms specialized to extreme environments can be the product of millions of years of evolutionary engineering and refinement. The underlying genetics can be quite distinct from those operating at earlier stages of trait innovation. In this work, we have developed the multistress-resistant yeast Kluyveromyces marxianus, which diverged from its closest relative >20 million years ago, as a model for interspecies comparative biology and genomics. In growth assays of the Kluyveromyces genus, we found that K. marxianus exhibited unique tolerance of high heat and a subset of chemical stress conditions. We then generated and analyzed omic profiles from across the genus to find molecular features associated with-and potentially causal for-K. marxianus traits. Expression profiling revealed divergent lipid processing and membrane transport programs in K. marxianus, borne out in changes in lipid utilization in experimental assays. Sequence analyses found robust evidence for expansions in gene families in the K. marxianus genome, most notably among transmembrane transporters and in metabolic enzymes. In molecular-evolution tests, we identified adaptive protein variants throughout the K. marxianus genome, among which plasma membrane transporters were over-represented. These data enable a model of the molecular mechanisms and evolutionary pressures underlying K. marxianus traits, including adaptive changes to transporters, lipid processing, and membrane functions mediating stress resistance.