  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,
since 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 S. 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.
