Trachymyrmex ant and Leucocoprinus fungus genotypes

Over the past few decades, large-scale phylogenetic analyses of
fungus-gardening ants and their symbiotic fungi have depicted strong
concordance among major clades of ants and their symbiotic fungi, yet
within clades, fungus sharing is widespread among unrelated ant lineages.
Sharing has been explained using a diffuse coevolution model within major
clades. Understanding horizontal exchange within clades has been limited
by conventional genetic markers that lack both interspecific and
geographic variation. To examine whether reports of horizontal exchange
were indeed due to symbiont sharing or the result of employing relatively
uninformative molecular markers, samples of Trachymyrmex
arizonensis and Trachymyrmex pomonae and their
fungi were collected from native populations in Arizona and genotyped
using conventional marker genes and genome-wide single nucleotide
polymorphisms (SNPs). Conventional markers of the fungal symbionts
generally exhibited cophylogenetic patterns that were consistent with some
symbiont sharing, but most fungal clades had low support. SNP analysis, in
contrast, indicated that each ant species exhibited fidelity to its own
fungal subclade with only one instance of a colony growing a fungus that
was otherwise associated with a different ant species. This evidence
supports a pattern of codivergence
between Trachymyrmex species and their fungi, and thus a
diffuse coevolutionary model may not accurately predict symbiont exchange.
These results suggest that fungal sharing across host species in these
symbioses may be less extensive than previously thought.