Ant Trachymyrmex Septentrionalis Research Articles

Trachymyrmex septentrionalis Ant Microbiome Assembly Is Unique to Individual Colonies and Castes

ABSTRACTWithin social insect colonies, microbiomes often differ between castes due to their different functional roles and between colony locations. Trachymyrmex septentrionalis fungus-growing ants form colonies throughout the eastern United States and northern Mexico that include workers, female and male alates (unmated reproductive castes), larvae, and pupae. How T. septentrionalis microbiomes vary across this geographic range and between castes is unknown. Our sampling of individual ants from colonies across the eastern United States revealed a conserved T. septentrionalis worker ant microbiome and revealed that worker ant microbiomes are more conserved within colonies than between them. A deeper sampling of individual ants from two colonies that included all available castes (pupae, larvae, workers, and female and male alates), from both before and after adaptation to controlled laboratory conditions, revealed that ant microbiomes from each colony, caste, and rearing condition were typically conserved within but not between each sampling category. Tenericute bacterial symbionts were especially abundant in these ant microbiomes and varied widely in abundance between sampling categories. This study demonstrates how individual insect colonies primarily drive the composition of their microbiomes and shows that these microbiomes are further modified by developmental differences between insect castes and the different environmental conditions experienced by each colony.IMPORTANCE This study investigates microbiome assembly in the fungus-growing ant Trachymyrmex septentrionalis, showing how colony, caste, and lab adaptation influence the microbiome and revealing unique patterns of mollicute symbiont abundance. We find that ant microbiomes differ strongly between colonies but less so within colonies. Microbiomes of different castes and following lab adaptation also differ in a colony-specific manner. This study advances our understanding of the nature of individuality in social insect microbiomes and cautions against the common practice of only sampling a limited number of populations to understand microbiome diversity and function.

Cold adaptations along a range limit in an obligate symbiosis

Abstract 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, Texas) 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 than 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. Read the free Plain Language Summary for this article on the Journal blog.

Male-biased dispersal in a fungus-gardening ant symbiosis.

For nearly all organisms, dispersal is a fundamental life‐history trait that can shape their ecology and evolution. Variation in dispersal capabilities within a species exists and can influence population genetic structure and ecological interactions. In fungus‐gardening (attine) ants, co‐dispersal of ants and mutualistic fungi is crucial to the success of this obligate symbiosis. Female‐biased dispersal (and gene flow) may be favored in attines because virgin queens carry the responsibility of dispersing the fungi, but a paucity of research has made this conclusion difficult. Here, we investigate dispersal of the fungus‐gardening ant Trachymyrmex septentrionalis using a combination of maternally (mitochondrial DNA) and biparentally inherited (microsatellites) markers. We found three distinct, spatially isolated mitochondrial DNA haplotypes; two were found in the Florida panhandle and the other in the Florida peninsula. In contrast, biparental markers illustrated significant gene flow across this region and minimal spatial structure. The differential patterns uncovered from mitochondrial DNA and microsatellite markers suggest that most long‐distance ant dispersal is male‐biased and that females (and concomitantly the fungus) have more limited dispersal capabilities. Consequently, the limited female dispersal is likely an important bottleneck for the fungal symbiont. This bottleneck could slow fungal genetic diversification, which has significant implications for both ant hosts and fungal symbionts regarding population genetics, species distributions, adaptive responses to environmental change, and coevolutionary patterns.

Thiopeptide Defense by an Ant's Bacterial Symbiont.

Fungus-growing ants and their microbial symbionts have emerged as a model system for understanding antibiotic deployment in an ecological context. Here we establish that bacterial symbionts of the ant Trachymyrmex septentrionalis antagonize their most likely competitors, other strains of ant-associated bacteria, using the thiopeptide antibiotic GE37468. Genomic analysis suggests that these symbionts acquired the GE37468 gene cluster from soil bacteria. This antibiotic, with known activity against human pathogens, was previously identified in a biochemical screen but had no known ecological role. GE37468's host-associated defense role in this insect niche intriguingly parallels the function of similar thiopeptides in the human microbiome.

Evaluation of DESS as a storage medium for microbial community analysis.

Microbial ecology research requires sampling strategies that accurately represent the microbial community under study. These communities must typically be transported from the collection location to the laboratory and then stored until they can be processed. However, there is a lack of consensus on how best to preserve microbial communities during transport and storage. Here, we evaluated dimethyl sulfoxide, ethylenediamine tetraacetic acid, saturated salt (DESS) solution as a broadly applicable preservative for microbial ecology experiments. We stored fungus gardens grown by the ant Trachymyrmex septentrionalis in DESS, 15% glycerol, and phosphate buffered saline (PBS) to test their impact on the fungus garden microbial community. Variation in microbial community structure due to differences in preservative type was minimal when compared to variation between ant colonies. Additionally, DESS preserved the structure of a defined mock community more faithfully than either 15% glycerol or PBS. DESS is inexpensive, easy to transport, and effective in preserving microbial community structure. We therefore conclude that DESS is a valuable preservative for use in microbial ecology research.

Instability of novel ant-fungal associations constrains horizontal exchange of fungal symbionts

One of the more fascinating features of fungus-gardening ants (Attini: Formicidae) is their fidelity to their lineage-specific fungal symbionts. Among the derived higher-attine ants (leafcutter ants and close relatives), it is thought that most leaf-cutting ants grow Attamyces fungus whereas most Trachymyrmex ants grow ‘Trachymyces’ fungus, but there exist exceptions to this clade-to-clade correspondence between ants and fungi. The exceptions are inconsistent with strict one-to-one coevolution, which suggests that ants sometimes are able to switch to novel fungi. Such switches appear to be largely constrained and ants are generally faithful to their species-specific fungi. Prior experiments demonstrated no clear fitness consequences of growing novel fungi over the short-term when the ant Trachymyrmex septentrionalis was symbiont-switched by forcing it to grow Attamyces leaf-cutter fungus. We hypothesized that long-term ant-fungal fidelity is constrained either by physiological differences among fungal species or by garden diseases that symbiont-switched ants cannot control. Repeat experiments in a different location show that T. septentrionalis colonies switched to grow Attamyces exhibit sudden declines in garden biomass and consequent fitness reductions due to garden destruction by pathogens, whereas control colonies (Trachymyrmex ants cultivating Trachymyces fungus) do not show parallel garden declines. These patterns are mirrored in symbiont-switch experiments conducted on colonies in Trachymyrmex turrifex. Disease microbes selecting on ant-cultivar combinations therefore can constrain switches to novel cultivars and maintain combinations that are more resistant to disease.

Fungus-gardening ants prefer native fungal species: do ants control their crops?

Fundamental features of mutualisms are solutions to conflict. The mutualism between fungus-gardening ants and their fungi is characterized by pronounced evolutionary ant-fungus fidelity. Ant-fungus fidelity is thought to be controlled by the fungus, which secretes somatic incompatibility compounds that suppress incompatible fungal strains that may invade gardens. Accordingly, the ants are thought to perceive incompatibilities and then reject invading cultivars. During cultivar-switch experiments, we discovered that if switched colonies of the ant Trachymyrmex septentrionalis are successful in retaining a minute fragment of their original garden, the ants will gradually co-cultivate the novel and the original fungus strains in a chimeric (polycultural) garden; this chimeric garden will revert after 1–2 weeks completely to a monoculture of the original fungal strain. Experimental examination of worker preference suggests that symbiont rejection behavior seems to be an innate response toward foreign fungi rather than mediated solely by incompatibility compounds secreted by the fungus. These observations suggest that the fungal strain cultivated by a colony imprints the attending ants, and this imprinting modifies ant behavior so that the ants prefer their original cultivar even when experimentally forced to grow a foreign cultivar for several weeks. Fungal monocultures and ant-fungus fidelities in this symbiosis therefore seem to be reinforced by factors intrinsic to both the ants and the fungi. Key words: Attini, chimera, cultivar switch, somatic compatibility, symbiosis. [Behav Ecol]

Microbiomes of ant castes implicate new microbial roles in the fungus-growing ant Trachymyrmex septentrionalis

Fungus-growing ants employ several defenses against diseases, including disease-suppressing microbial biofilms on their integument and in fungal gardens. Here, we compare the phenology of microbiomes in natural nests of the temperate fungus-growing ant Trachymyrmex septentrionalis using culture-dependent isolations and culture-independent 16S-amplicon 454-sequencing. 454-sequencing revealed diverse actinobacteria associated with ants, including most prominently Solirubrobacter (12.2–30.9% of sequence reads), Pseudonocardia (3.5–42.0%), and Microlunatus (0.4–10.8%). Bacterial abundances remained relatively constant in monthly surveys throughout the annual active period (late winter to late summer), except Pseudonocardia abundance declined in females during the reproductive phase. Pseudonocardia species found on ants are phylogenetically different from those in gardens and soil, indicating ecological separation of these Pseudonocardia types. Because the pathogen Escovopsis is not known to infect gardens of T. septentrionalis, the ant-associated microbes do not seem to function in Escovopsis suppression, but could protect against ant diseases, help in nest sanitation, or serve unknown functions.

Phylogeography of post‐Pleistocene population expansion in a fungus‐gardening ant and its microbial mutualists

Although historical biogeographical forces, such as climate-driven range shifts, greatly influence the present-day population genetic structure of animals and plants, the extent to which they affect microbial communities remains largely unknown. We examined the effect of postglacial expansion on the population structure of the northern fungus-gardening ant Trachymyrmex septentrionalis and compared it with that of its two microbial mutualists: a community of lepiotaceous fungal cultivars and associated antibiotic-producing Pseudonocardia bacteria. The ant population genetic structure showed signs of population expansion and subdivision into eastern and western phylogroups that likely originated in the Pleistocene - a pattern shared by many other North American taxa found in the same region. Although dispersal limitation was present in all three symbionts, as suggested by genetic isolation increasing with distance, the host's east-west subdivision of population genetic structure was absent from the microbial mutualist populations. While neither the cultivar nor the Pseudonocardia genetic structure was correlated with that of the ants, they were significantly correlated with each other. These results show that biogeographical forces may act differently on macro- and microscopic organisms, even in the extreme case where microbial mutualists are vertically transmitted from generation to generation and share the same joint ecological niche. It may be that historical climate change played a larger role in determining the population structure of the ant hosts, whereas present-day environmental forces, such as pathogen pressure, determine the structure of associated microbial populations.

Colony Productivity of the Fungus-Gardening Ant <I>Trachymyrmex septentrionalis</I> (Hymenoptera: Formicidae) in a Florida Pine Forest

The ecology of the fungus-gardening ant Trachymyrmex septentrionalis McCook (Hymenoptera: Formicidae) was investigated in a northern Florida longleaf pine, Pinus palustris Mill., forest. This ant is extremely abundant in pine sandhill in the Apalachicola National Forest, in north Florida; a hectare contains on average >1,000 nests, 235,000 T. septentrionalis workers, and 3.5 kg of fungus garden. When colony size and performance were estimated from excavations and the weight of sand in the tumulus, nests were larger and produced the most offspring in open, treeless habitats, whereas the smallest and least productive nests occurred in wooded areas. Our data suggest that the warm soils of open sites stimulate worker activity and colony growth, and cool soils of shaded, wooded sites depress performance. Moreover soils with extremely warm temperatures may have an inhibitory effect because the ants and fungus are susceptible to desiccation and excessive warmth, respectively. This suggests that T. septentrionalis is an indicator ant species of habitat quality in longleaf pine sandhills because its performance seems to be positively related to the natural disturbance regime—frequent fires occurring in the summer. Soil displacement is possibly a major effect that this ant has on the forest ecosystems: colonies may excavate over 1 metric ton of soil each year in a typical hectare of pine forest. The effect of this soil turnover is probably considerable in these nutrient-poor, sandy soils. Experiments are needed to determine the role of this highly abundant ant in longleaf pine forests.

Polyethism and the adaptiveness of worker size variation in the attine antTrachymyrmex septentrionalis

Division of labor was studied in colonies from Long Island and Florida populations of the fungus-gardening antTrachymyrmex septentrionalis. Workers showed age polyethism and a weak size polyethism, and these patterns of division of labor were not different in colonies from the two populations. Individual workers had repertoires comprised of three to five roles but tended to concentrate their labor within a single role. The data are consistent with the hypothesis that worker polymorphism and the elaboration of size-related behavior in the attine ants evolved along with the use of fresh vegetation as the fungal substrate. The data do not support the hypothesis that size variation in colonies ofT. septentrionalis evolved to promote efficient division of labor. Division of labor within the worker caste is based mainly on age and appears to be an attribute of the species rather than an adaptation to a particular habitat.