Data from: No evidence for extrinsic postzygotic isolation in a wild Saccharomyces yeast system

Although microorganisms account for the largest portion of Earth’s biodiversity, we know little about the reproductive barriers that lead to species formation. In young lineages with no or weak pre-zygotic isolation, the environment could contribute to prevent gene flow via selection against hybrid phenotypes. Sexual species such as Saccharomyces yeasts have remained isolated enough to diverge and develop strong intrinsic post-zygotic isolation, which prevents gene flow despite overlapping distributions and co-occurrence on the same substrates. We measured growth of F1 hybrids between two incipient species of Saccharomyces paradoxus to assess the inheritance of the parental phenotypes across 32 environments. More than 80% of hybrids showed either partial dominance of the best parent or over-dominance for growth, revealing no or relatively weak selection against F1 hybrids. Extrinsic reproductive isolation based on growth performance therefore plays little role in limiting gene flow between incipient yeast species and is not a requirement for speciation.