Symbiosis is a defining feature of the eukaryotes that has contributed major evolutionary innovations including aerobic respiration, autotrophy, nitrogen fixation, and the capacities to feed upon low quality diets. Many symbioses involve larger eukaryotes harboring microscopic microbial partners, which execute beneficial functions. Many of these associations have become such an integral part of eukaryotic biology that a vast number of plants, animals, fungi, and protists would cease to exist without their associated microbial symbionts.
FISH micrograph illustrating the localization of bacteria in the ileum (hindgut) of an army ant. (Photo credit: Piotr Lukasik.)
In our research, we aim to uncover the costs and benefits of symbioses with microbes along with their underlying mechanisms. We also aim to uncover how these associations have evolved and how they function in the real world. We measure how symbioses vary across the host phylogeny and in relation to hosts' ecological attributes. To achieve these goals we combine a mixture of computational-, bench-, and field-based research across different research systems
Currently, the two primary research systems for our explorations involve ant-gut bacteria associations and interactions between aphids and maternally transmitted microbes. But we have also explored symbioses in additional eukaryotic groups, including fish, moths and butterflies, and ornamental plants from living biofilters. In addition, we are fascinated by interactions between insects and their prevalent Wolbachia symbionts, along with the factors that govern Wolbachia distributions.
Many ants are symbiotic creatures, engaging in associations with visible partners such as sap-feeding insects or mutualistic fungi that they cultivate for food. Bacteria are also important associates of ants, though our knowledge of such partnerships has been slow to develop. A major goal of our work is to catalog the diversity of symbiotic bacteria across the ants. We have placed some emphasis on heritable ant-associated symbionts such as Wolbachia. But our main interests in this realm are centered on the communities of bacteria that colonize ant guts.
Cephalotes ant tends a membracid. (Photo credit: Jon Sanders)
Ants tending scale insects. (Photo credit: Piotr Lukasik.)
Stages of Cephalotes varians development (Photo credit: Yi Hu.)
Gut bacteria from ant-specific lineages are prevalent, if not ubiquitous, within groups such as Cephalotes turtle ants and army ants. We aim to understand the functional roles of these bacteria and the forces that have shaped their maintenance as symbionts for tens of millions of years.
Highly specialized herbivores and carnivores show convergence in their symbioses with bacteria. The link between symbiosis and diet level is intriguing, suggesting potential involvement of bacteria in the maintenance of extreme trophic specialization.
In the pea aphid (Acyrthosiphon pisum) system, we have taken lab-derived predictions to the field, exploring diversity and dynamics of protective symbionts across multiple scales in nature. Frequencies of these bacteria, which protect their hosts from natural enemies or high temperatures, vary between aphid host races and populations from different locations. Yet within populations, symbiont frequencies can fluctuate drastically across a single season. The heritable nature of these microbes equates them to cytoplasmically inherited genetic elements and, thus, such changes themselves equate to rapid, symbiont-driven evolution at the host level. So what are the consequences of such evolution? And what drives these temporal dynamics?
Pea aphid on a senescent leaf. (Photo credit: Patrick McLaughlin.)
Pea aphid mummy harboring the wasp parasitoid Aphidius ervi. (Photo credit: Pat McLaughlin.)
Pea aphid cadaver with infection by the fungal pathogen Pandora neoaphidis. (Photo credit: Pat McLaughlin.)
Our data suggest that some symbiont frequency fluctuations correlate with pressures from parasitoid wasps in the field, which are often a major source of pea aphid mortality. Yet correlations with wasps and fungal pathogen enemies predicted to shape the benefits of these symbionts in nature are not ubiquitous, suggesting alternative forces to be at play. Future field work aims to elucidate these forces, and to more directly assess whether symbionts are driving seasonal (or longer-term) aphid adaptation in response to natural enemies in the field.
Lastly, it is important to note that many aphids harbor more than one facultative symbiont, meaning that these bacteria are members of low-diversity microbial communities. Our abilities to manipulate these fairly simple communities make this system ideal for understanding processes that govern assembly and dynamics of symbiont communities in other systems. Intriguingly, in nature particular symbiont pairings or communities are either highly enriched or quite rare. Such associations appear stable over time and may even extend across geographic regions. We are assessing whether these might, thus, implicate symbiont-symbiont mutualisms or synergistic benefits, enabling the spread of highly stable or beneficial communities.
Our research tools involve a mixture of computational-, bench-, and field-based approaches across different research systems. These approaches are heavily steeped in the realm of molecular ecology, as we use tools such as PCR and DNA sequencing to elucidate the distributions of symbionts across hosts. In addition, phylogenetic analyses enable us to understand how symbionts have evolved and to identify the factors that govern their distributions.
FISH micrograph showing bacteria in gut of an army ant. (Photo credit: Piotr Lukasik).
Genomic similarity among cultured symbiont isolates from Cephalotes varians. (Photo credit: Jon Sanders)
FISH microscopy (performed at Drexel's CIC facility: website) is a key component of localizing particular symbionts within hosts. This can shed light on the relationships among microbes. It can also provide insight into the interplay between host anatomy/physiology and microbial function.
Amplicon sequencing of 16S rRNA genes gives us deep insight into the composition of symbiotic communities at various taxonomic levels. More recently, metagenomic analyses (using Drexel's Proteus cluster: website) have given insight into both function and mechanism for microbial symbionts.
Red mangrove trees in the Florida Keys are home to a variety of ants. (Photo credit: Piotr Lukasik).
Manipulative field cage experiments performed in Montgomery County, Pennsylvania in the summer of 2013.
Manipulative experiments are facilitated in the pea aphid system by the general ease of symbiont manipulation via antibiotic curing or microinjection. Clonal reproduction of aphids in the laboratory is also a major factor in enabling us to distinguish the effects of symbionts from those of host genotype. The capacity to rear parasitoids and pathogens of these insects in the lab is further key to our pea aphid research.
In the ants, we can similarly remove symbionts via antibiotic treatment, determining how this impacts nutrient provisioning through the study of heavy isotopes.