title: Different selective pressures lead to different genomic outcomes as newly-formed hybrid yeasts evolve.
abstract: BACKGROUND
  Interspecific hybridization occurs in every eukaryotic kingdom. While hybrid progeny are frequently at a selective disadvantage, in some instances their increased genome size and complexity may result in greater stress resistance than their ancestors, which can be adaptively advantageous at the edges of their ancestors' ranges. While this phenomenon has been repeatedly documented in the field, the response of hybrid populations to long-term selection has not often been explored in the lab. To fill this knowledge gap we crossed the two most distantly related members of the Saccharomyces sensu stricto group, S. cerevisiae and S. uvarum, and established a mixed population of homoploid and aneuploid hybrids to study how different types of selection impact hybrid genome structure.
  
  
  RESULTS
  As temperature was raised incrementally from 31°C to 46.5°C over 500 generations of continuous culture, selection favored loss of the S. uvarum genome, although the kinetics of genome loss differed among independent replicates. Temperature-selected isolates exhibited greater inherent and induced thermal tolerance than parental species and founding hybrids, and also exhibited ethanol resistance. In contrast, as exogenous ethanol was increased from 0% to 14% over 500 generations of continuous culture, selection favored euploid S. cerevisiae x S. uvarum hybrids. Ethanol-selected isolates were more ethanol tolerant than S. uvarum and one of the founding hybrids, but did not exhibit resistance to temperature stress. Relative to parental and founding hybrids, temperature-selected strains showed heritable differences in cell wall structure in the forms of increased resistance to zymolyase digestion and Micafungin, which targets cell wall biosynthesis.
  
  
  CONCLUSIONS
  This is the first study to show experimentally that the genomic fate of newly-formed interspecific hybrids depends on the type of selection they encounter during the course of evolution, underscoring the importance of the ecological theatre in determining the outcome of the evolutionary play.
authors.affiliation: Chemical Genomics Research Group, RIKEN Advance Science Institute, Wako, Wako, Japan.
authors.forename: Jeff S
authors.initials: JS
authors.lastname: Piotrowski
authors.forename: Saisubramanian
authors.initials: S
authors.lastname: Nagarajan
authors.forename: Evgueny
authors.initials: E
authors.lastname: Kroll
authors.forename: Alison
authors.initials: A
authors.lastname: Stanbery
authors.forename: Kami E
authors.initials: KE
authors.lastname: Chiotti
authors.forename: Arthur L
authors.initials: AL
authors.lastname: Kruckeberg
authors.forename: Barbara
authors.initials: B
authors.lastname: Dunn
authors.forename: Gavin
authors.initials: G
authors.lastname: Sherlock
authors.forename: Frank
authors.initials: F
authors.lastname: Rosenzweig
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics*; D055028:Comparative Genomic Hybridization; D000431:Ethanol; D016681:Genome, Fungal / Q000235:genetics*; D006824:Hybridization, Genetic / Q000235:genetics*; D059785:Karyotype; D011156:Population Density; D012440:Saccharomyces / Q000235:genetics*; D012641:Selection, Genetic / Q000235:genetics*; D013045:Species Specificity; D013696:Temperature
chemical_list: D000431:Ethanol