title: Genetic drift opposes mutualism during spatial population expansion.
abstract: Mutualistic interactions benefit both partners, promoting coexistence and genetic diversity. Spatial structure can promote cooperation, but spatial expansions may also make it hard for mutualistic partners to stay together, because genetic drift at the expansion front creates regions of low genetic and species diversity. To explore the antagonism between mutualism and genetic drift, we grew cross-feeding strains of the budding yeast Saccharomyces cerevisiae on agar surfaces as a model for mutualists undergoing spatial expansions. By supplying varying amounts of the exchanged nutrients, we tuned strength and symmetry of the mutualistic interaction. Strong mutualism suppresses genetic demixing during spatial expansions and thereby maintains diversity, but weak or asymmetric mutualism is overwhelmed by genetic drift even when mutualism is still beneficial, slowing growth and reducing diversity. Theoretical modeling using experimentally measured parameters predicts the size of demixed regions and how strong mutualism must be to survive a spatial expansion.
authors.affiliation: Faculty of Arts and Sciences Center for Systems Biology, Department of Physics, and Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138.
authors.forename: Melanie J I
authors.initials: MJ
authors.lastname: Müller
authors.forename: Beverly I
authors.initials: BI
authors.lastname: Neugeboren
authors.forename: David R
authors.initials: DR
authors.lastname: Nelson
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Murray
mesh_terms: D000362:Agar / Q000737:chemistry; D000596:Amino Acids / Q000378:metabolism; D003470:Culture Media / Q000378:metabolism; D017753:Ecosystem; D019143:Evolution, Molecular; D040961:Genetic Drift; D014644:Genetic Variation; D008856:Microscopy, Fluorescence; D008962:Models, Theoretical; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000502:physiology*; D013559:Symbiosis
chemical_list: D000596:Amino Acids; D003470:Culture Media; D000362:Agar
keywords: auxotroph; evolution; invasion; microbes; snow-drift game