My research program has branched out to cover various basic and applied aspects of the life history of EPN and their bacterial symbionts.
Evolutionary relationships among Steinernema spp. and their bacterial symbionts, Xenorhabdus spp.
Infective juvenile of Steinernema riobrave (left)
and its bacterial symbiont, Xenorhabdus cabanillasii (right)
We have recently proposed a multigene-based scenario to assess the evolutionary histories of both symbiotic bacteria and their nematode hosts. Fundamentally this project produced, for the first time, a testable hypothesis of phylogenetic affinities among Xenorhabdus and Steinernema spp. Furthermore, this study also provides a framework for documenting and interpreting ecological (i.e., insect host specificity, pathogenesis), physiological (i.e., symbiont and host metabolic contributions), and morphological (i.e., nematode bacterial receptacle morphology, bacteria colony morphology, color, etc) aspects of this nematode-bacterium partnership.
Establishment and maintenance of the mutualistic association between steinernematids and their bacterial symbionts
Steinernema carpocapsae showing uncolonized extruded intestine (left)
and ‘in situ’ colonized bacterial receptacle (right)
Over the past six years my lab in collaboration with H. Goodrich-Blair and S. Fort’s labs have been investigating the bacterial receptacle in Steinernema nematodes. This structure is dedicated to the establishment and maintenance of a symbiotic relationship.
Key findings include:
- The bacterial receptacle size is dependent on nematode age and bacterial load. (Forst and Stock)
- Bacterial load varies among symbiotic pairs. (Forst, Stock, Goodrich-Blair)
- The bacterial receptacle is only present in the infective juvenile stage and it is a modification of the two most anterior cells of the intestine. (Stock)
- Hemolymph stimulates of Xenorhabdus symbionts in which bacteria move through the nematode intestine and exit through the anus. (Forst, Stock, Goodrich-Blair)
- The receptacle contains a nematode-derived subcellular structure (IVS) to which bacterial symbionts adhere. (Goodrich-Blair)
- In several species (remember please that this is a formal document) the IVS is wrapped by a membrane-like structure. (Goodrich-Blair and Stock)
- The receptacle contains sufficient levels of several amino acids, vitamins, and nucleosides, but insufficient levels of methionine and threonine to support bacterial division. (Goodrich-Blair).
Transmission electronon micrograph of Steinernema
carpocapsae showing bacterial symbionts inside.
Together with my collaborators, we are now focused on the study of symbiont specificities and dependencies to investigate physiological and molecular underpinnings of symbiont contributions to nematode fitness.
Major focus areas are: (a) determine the extent of symbiont specialization among Steinernema nematodes, (b) investigate bacterial traits contributing to nematode fitness and competitiveness and (c) study the molecular basis of symbiont selection during transmission with focus on bacterial genes necessary for colonization. National Science Foundation and hope to initiate this research in 2011.
Rainforest, La Selva, Costa Rica (left), Semidesert habitats: Jordan (middle), Sabino Canyon, Arizona (right)
My lab continues to be actively engaged in survey and inventory projects in different geographic regions of the world (including USA, Canada, Mexico, Costa Rica, Colombia, France, South Africa, Jordan and Egypt). These projects require the improving of molecular techniques by which nematode species are recovered, as well as refining philosophical and operational methods for species delimitation.
Some recent examples:
La Selva Preserve in Costa Rica
Through this project, we found that nematode diversity in this region exceeds that of temperate regions (i.e., Konza Prairie, NE) which were previously recognized as the ecosystem with the greatest reported terrestrial species richness. We conducted molecular analysis of more than 300 nematodes, yielding 167 distinct MOTUs (Molecular Operational Taxonomic Units).
This research was a collaborative effort with D. Neher (University of Vermont), R. Giblin-y of Florida) and T. Powers (University Nebraska).
EPN diversity in oak woodlands of southeastern Arizona sky-islands
In Arizona we are currently investigating seasonal fluctuations of EPN populations in relation to abiotic (i.e. precipitation, temperature, etc) and biotic parameters (i.e. presence of predators, potential insect hosts and other nematode trophic groups).
We have conducted and are currently conducting various projects on this area. Some of the ongoing research projects are listed below:
We focused on the fitness cost of resistance to Cry1Ac by the pink bollworm P. gossypiella, when challenged with EPN.
Insect- pathogen-plant interactions
We are studying the EPN-bacterium-insect model system to explore how physiological conditions of one of the three partners – the host insect – affects the system as a whole.
EPN- plant- pathogen interactions
We are investigating direct and indirect interactions between plant pathogenic fungi and EPN and their bacterial symbionts. This study considers a multidisciplinary approach including ecology, insect pathology, plant pathology and biochemistry.