title: Crowded growth leads to the spontaneous evolution of semistable coexistence in laboratory yeast populations.
abstract: Identifying the mechanisms that create and maintain biodiversity is a central challenge in biology. Stable diversification of microbial populations often requires the evolution of differences in resource utilization. Alternatively, coexistence can be maintained by specialization to exploit spatial heterogeneity in the environment. Here, we report spontaneous diversification maintained by a related but distinct mechanism: crowding avoidance. During experimental evolution of laboratory Saccharomyces cerevisiae populations, we observed the repeated appearance of "adherent" (A) lineages able to grow as a dispersed film, in contrast to their crowded "bottom-dweller" (B) ancestors. These two types stably coexist because dispersal reduces interference competition for nutrients among kin, at the cost of a slower maximum growth rate. This tradeoff causes the frequencies of the two types to oscillate around equilibrium over the course of repeated cycles of growth, crowding, and dispersal. However, further coevolution of the A and B types can perturb and eventually destroy their coexistence over longer time scales. We introduce a simple mathematical model of this "semistable" coexistence, which explains the interplay between ecological and evolutionary dynamics. Because crowded growth generally limits nutrient access in biofilms, the mechanism we report here may be broadly important in maintaining diversity in these natural environments.
authors.affiliation: Department of Organismic and Evolutionary Biology, Department of Physics, and Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, MA 02138; Program in Biophysics, Harvard University, Boston, MA 02115;
authors.forename: Evgeni M
authors.initials: EM
authors.lastname: Frenkel
authors.affiliation: Department of Organismic and Evolutionary Biology, Department of Physics, and Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, MA 02138;
authors.forename: Michael J
authors.initials: MJ
authors.lastname: McDonald
authors.affiliation: Department of Biology, University of Miami, Coral Gables, FL 33146;
authors.forename: J David
authors.initials: JD
authors.lastname: Van Dyken
authors.affiliation: Department of Organismic and Evolutionary Biology, Department of Physics, and Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, MA 02138;
authors.forename: Katya
authors.initials: K
authors.lastname: Kosheleva
authors.affiliation: Department of Biological Sciences, Lehigh University, Bethlehem, PA 18015.
authors.forename: Gregory I
authors.initials: GI
authors.lastname: Lang
authors.affiliation: Department of Organismic and Evolutionary Biology, Department of Physics, and Faculty of Arts and Sciences Center for Systems Biology, Harvard University, Cambridge, MA 02138; mdesai@oeb.harvard.edu.
authors.forename: Michael M
authors.initials: MM
authors.lastname: Desai
mesh_terms: D000465:Algorithms; D000935:Antifungal Agents / Q000494:pharmacology; D044822:Biodiversity; D005075:Biological Evolution; D017753:Ecosystem; D004777:Environment; D015725:Fluconazole / Q000494:pharmacology; D008825:Miconazole / Q000494:pharmacology; D008954:Models, Biological; D011156:Population Density; D011157:Population Dynamics; D012441:Saccharomyces cerevisiae / Q000145:classification / Q000187:drug effects / Q000254:growth & development*; D059008:Time-Lapse Imaging
chemical_list: D000935:Antifungal Agents; D008825:Miconazole; D015725:Fluconazole
keywords: coexistence; crowding; experimental evolution; fungal adherence