Environmental change is occurring across the globe at an unprecedented rate. With atmospheric CO 2 concentrations now exceeding 400 ppm (Blunden, Arndt, & Hartfield, 2018), global mean surface temperatures are rising (Stocker et al., 2013) and the world ocean is becoming more acidic (Gattuso et al., 2015). Habitat is being degraded and homogenized via land-use change while nutrient runoff from industrial-scale agriculture is expanding anoxic dead zones in coastal ecosystems (Foley, 2005; Stocker et al., 2013). Meanwhile, altered precipitation regimes are creating more intense and prolonged periods of drought and flooding that threaten our ability to reliably feed the growing human population (Trenberth, 2011). These and other global changes pose severe threats to the biodiversity of virtually all ecosystems on Earth. For species to persist in the face of widespread and rapid environmental change, it is important to identify biological mechanisms that can reverse trends of population decline. Some populations can achieve this by moving into more favourable habitats, for example, through the migration of heat-stressed individuals to sites at higher latitudes with cooler temperatures (Chen,

Environmental opportunistic pathogens can exploit vulnerable hosts through expression of traits selected for in their natural environments. Pathogenicity is itself a complicated trait underpinned by multiple complex traits, such as thermotolerance, morphology, and stress response. The baker’s yeast, Saccharomyces cerevisiae, is a species with broad environmental tolerance that has been increasingly reported as an opportunistic pathogen of humans. Here we leveraged the genetic resources available in yeast and a model insect species, the greater waxmoth Galleria mellonella, to provide a genome-wide analysis of pathogenicity factors. Using serial passaging experiments of genetically marked wild-type strains, a hybrid strain was identified as the most fit genotype across all replicates. To dissect the genetic basis for pathogenicity in the hybrid isolate, bulk segregant analysis was performed which revealed eight quantitative trait loci significantly differing between the two bulks with alleles from both parents contributing to pathogenicity. A second passaging experiment with a library of deletion mutants for most yeast genes identified a large number of mutations whose relative fitness differed in vivo vs. in vitro, including mutations in genes controlling cell wall integrity, mitochondrial function, and tyrosine metabolism. Yeast is presumably subjected to a massive assault by the innate insect immune system that leads to melanization of the host and to a large bottleneck in yeast population size. Our data support that resistance to the innate immune response of the insect is key to survival in the host and identifies shared genetic mechanisms between S. cerevisiae and other opportunistic fungal pathogens.

1.Rescue effects arise when ecological and evolutionary processes restore intrinsic positive growth rates in populations that are at risk of going extinct. Rescue effects have traditionally focused on the roles of immigration, phenotypic plasticity, gene flow, and adaptation. However, species interactions are also critical for understanding how populations respond to environmental change.2.In particular, the fitness of plant and animal hosts is strongly influenced by symbiotic associations with the bacteria, archaea, microeukaryotes, and viruses that collectively make up a host’s microbiome. While some are pathogenic, many microorganisms confer nutritional, immunological, and developmental benefits that can protect hosts against the effects of rapid environmental change. 3.Microbial rescue occurs when changes in microbiome abundance, composition, or activity influence host physiology or behavior in ways that improve host fitness. If these microbial attributes and their beneficial effects are transmitted through a population, it may stabilize growth rates and reduce the probability of extinction.4.In addition to providing a framework to guide theoretical and empirical efforts in host-microbiome research, the principles of microbial rescue may also be useful for adaptively managing at-risk species. We discuss the risks and rewards of incorporating microbial rescue into conservation strategies such as probiotics, assisted migration, and captive breeding.