Determining abundance and diversity of fungi in the Harvester Ants (Genus: Pogonomyrmex)
Hosts and pathogens are constantly evolving in response to one another. This coevolutionary process can lead to strong differences in pathogen impact among individuals and populations, as pathogens adapted to infect one genotype may not be able to infect other genotypes. One possible mechanism to increase resistance to pathogens is interspecific hybridization, which creates novel genotypes unfamiliar to pathogens. Hybridization between two species of harvester ants, Pogonomyrmex barbatus and Pogonomyrmex rugosus, has given rise to two hybrid lineage pairs (“H” and “J”) that are highly ecologically successful, occurring between the desert range of P. rugosus in California, Arizona and northwestern Mexico, and the mesic range of P. barbatus in Texas and eastern Mexico. As fungal growth is favored by high humidity and moisture, these ant species may differ in the diversity and abundance of fungal pathogens, and thus in their anti-fungal adaptations. To characterize differences in abundance and diversity of fungi among habitats, we isolated fungi from three ant and soil samples from colonies of three sites collected for each parental species and hybrid lineages. We then exposed pupa from each taxa to soils from across the range to test for local adaptation or hybrid advantage.
As predicted, ant samples from higher precipitation sites had significantly more fungal spores than ants from lower precipitation sites. Soil samples from low-precipitation P. rugosus sites also had the lowest diversity, but hybrid soils, especially H, tended to be highest diversity despite being intermediate in precipitation level. From the fungal spores found we identified 16 genera, of which five were entomopathogenic. These ranged in geographic distribution from widespread to highly species-specific; Paecilomyces spp were found in both parental and hybrid ranges, while Metarhizium was only associated with one parental species (P. barbatus). When pupae were exposed to soils collected from across the geographical range, we found that pupae in soils from higher precipitation areas were associated with higher infection frequency. We also found that the frequency of infection was significantly lower in P. rugosus than P. barbatus, J and H lineage, suggesting P. rugosus species may to be less susceptible to pathogens compared to P. barbatus, as well as the J and H lineages. These results also suggest hybrid lineages did not seem to have any advantage when exposed to pathogens in both hybrid and parental soil.