Cold tolerance of Trachymyrmex fungus gardening ants and Leucocoprinus symbionts

Symbionts can have profound effects on host fitness, adaptations, and
range distributions.  Stress-induced evolution is difficult to
show in obligate symbioses, however, adaptive evolution within an obligate
symbiosis can be investigated experimentally or by correlating trait
variation with stress along an ecological cline (i.e., temperature-stress
gradient). We investigated the cold-tolerance of the fungus-growing ant
Trachymyrmex septentrionalis by performing cold-tolerance assays comparing
two populations collected from either the southernmost range of their
distribution (Bastrop, TX) or from a site that is approximately 600 km
further north (Norman, Oklahoma). We first compared isolated fungal
symbionts grown on artificial media to determine cold-tolerance of fungus
alone. Subsequently, we conducted cross-fostering experiments between
northern and southern host and symbionts to test for synergisms between
the partners in generating adaptations of cold tolerance.   Ants
of the northern fungal populations were more cold-adapted then southern
fungal populations. Northern nests were deeper and northern colonies
initially rejected fungi from the southern population. The cross-fostering
experiments demonstrated that only one partner must be cold tolerant to
confer maximum cold-tolerance to the ant-fungus symbiosis, because
northern ants growing southern fungus under cold stress performed just as
well as northern ants growing northern fungi. Our results suggest that
cold stress has been an important selective factor during the migration of
this ant-fungus symbiosis into northern latitudes during the last 10,000
years, and that cold tolerance likely is an energetically demanding trait
that may be traded off with other aspects of the symbiosis’ life history.
The symbiosis also appears to have evolved several additional adaptations
that increase survival in cold environments, such as building deeper nests
that insulate the fungi from cold surface