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Research: Recent Projects

[2009]
Antagonistic interactions within natural populations of bacteria
Hadas Hawlena (postdoc, IU Dept of Biology), Farrah Bashey-Visser (postdoc, IU Dept of Biology), and Curt Lively (professor, IU Dept of Biology)

caterpillars in tray of soil
Galleria mellonella caterpillars being exposed to soil from Moores Creek (photo by H. Hawlena)
Social interactions among bacteria are thought to be a major mechanism affecting population and community dynamics of microbes. Nevertheless, the actual distribution and variability of these interactions among bacteria in nature is largely unknown. We are studying the antagonistic interactions among nematode-associated, insect-pathogenic bacteria. In a first study, we focused on 32 isolates of Xenorhabdus bovienii collected from 4 insects, which had been exposed to four different soils samples from the Moores Creek site. The isolates were cross-tested to see whether one isolate could inhibit the growth of another via the production of bacteriocins (narrow-spectrum antibiotics). We found an incredible diversity, with up to eight phenotypes recovered from a single insect host. Moreover, this small sample indicates the presence of at least seven different bacteriocin toxins. Most excitingly, the relationships among the phenotypes appear to be non-hierarchical or non-transitive (such that A>B, B>C, and C>A). This result suggests that diversity may be maintained in this system endogenously, that is, in the absence environmental heterogeneity. We plan to use these isolates to explore the potential for non-transitive interactions for maintaining genetic diversity in this species. Additionally, we are examining the effect of bacterial interactions on their insect hosts and nematode mutualists.
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Xenorhabdus bacteria grown on NBTA agar (photo F. Bashey)
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Growth inhibition of one bacterial isolate caused by a bacteriocin produced by a second (photo F. Bashey)


[2009]
Life-history variation and competitive interactions among entomopathogenic nematodes
Farrah Bashey-Visser (postdoc, IU Dept of Biology), Courtney Reynolds (undergraduate, IU Dept of Biology), Stephanie Rangel (undergraduate, IU Dept of Biology), Sara Young (undergraduate, IU Dept of Biology), Tara Sarin (undergraduate, IU Dept of Biology), Leslie Schenck (undergraduate, IU Dept of Biology)

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Soil core being placed in a plastic bag for transport to the lab (photo C. Sears)
We have been able to repeatedly collect three species of entomopathogenic nematode from the Moores Creek site. Two of our nematode species, Steinernerma affine and Steinernema kraussi are associated with the bacteria Xenorhabdus bovienii while the third, as yet undescribed, Steinernema species is associated with Xenorhabdus koppenhoefferi. While the three species appear to be elevationally distinct in distribution, regions of sympatry occur, where two of the three species have been recovered from soil samples only meters apart. As Steinenematid nematodes have broad host ranges (Peters, 1996), co-infection of a single host by more than one species is possible.

We have begun to characterize the nematode life-history patterns, interspecific competitive abilities, and virulence in these three species. Thus far we have found that the trade-off between the size and number of nematodes emerging from a host varies across species. Additionally, high virulence is correlated with competitive dominance across species.


Steinernema affine (photo L.Schenck)

Steinernema kraussi (photo L.Schenck)

Undescribed species of Steinernema from Moores Creek (photo L.Schenck)


Predicting patterns of spread and impacts of the invasive woodland species, Japanese stiltgrass
Angie Shelton (postdoc, IU Dept. of Biology and IURTP), Dan Johnson (grad student, IU Dept. of Biology), S. Luke Flory (postdoc, IU Dept. of Biology), Keith Clay (professor, IU Dept. of Biology), Burney Fischer (professor, IU School of Public and Environmental Affairs), Cindy Huebner (research scientist, US Forest Service Northern Research Station, Morgantown, WV)

Japanese stiltgrass
Japanese stiltgrass
Invasive species can significantly alter natural communities and are considered one of the greatest threats to the integrity of natural ecosystems. Japanese stiltgrass (Microstegium vimineum) is an invasive species new to southern Indiana. It was first documented in Tennessee in 1919 but has only recently been recognized as invasive. It arrived in Monroe County, Indiana, around 2001 and has rapidly spread through the county.

As part of a Joint Research Venture with the Northern Research Station of the U.S. Forest Service, we are monitoring invasions in the field to predict patterns and rates of future invasions and investigating the impact of Japanese stiltgrass on tree seedlings. The project has three primary components:

  1. Characterize invaded sites and monitor the process of natural and experimental invasions
  2. Determine the impacts of Microstegium invasion on tree regeneration and growth in natural systems
  3. Develop spatially-explicit predictive models for Microstegium invasions and forest impacts

Read more about stiltgrass invasion research. Also see "Spread of Invasive Species"


Colonization: asexual vs. sexual mites
Jen Patterson (graduate student, IU Dept. of Biology)

Allosuctobelbella mite under microscope
View more mites
Asexual organisms are thought to be evolutionary dead ends due to an inability to respond to selection pressures and to eliminate accumulating deleterious mutations. Many species of oribatid mites appear to have been asexual for many millions of years and do not have close sexual relatives; these are so-called ancient asexuals. Other asexual oribatid mites have close sexual relatives and are likely younger. Together these make up about 10 percent of the suborder, and the rest are sexual.

I tested the hypothesis that asexuals have a colonization advantage, in this case, specifically that this advantage works on a local scale to allow long-term persistence of asexual species. Such an advantage could free asexuals from competition from sexuals and permit the co-existence of both. In order to assess colonization ability I compared the presence of individuals in forest habitat to their presence in constructed litter bags at Moores Creek in Monroe County, Indiana. In order to construct the treatments, soil cores were collected, from which mites were extracted and discarded; the mite-free habitat was returned to the forest in containers made of fiberglass window screen from which mites could enter and leave at will, called soil-core litter bags (LBC). Leaves from fallen tree branches were also collected and used to fill litter bags (LBN). This treatment represents mite habitat from its natural initiation point and is also mite-free until the leaves start to decay. Pairs of one LBC and one LBN were placed in the field together at their places of origin. Seven pairs of litter bags and an adjacent soil core were collected bimonthly for two years in order to compare the presence of mites in the litter bags to mites in the surrounding habitat.

Contrary to expectation, sexuals tended to colonize litter bags more than either recent or ancient asexuals, although ancient asexuals colonized more than recent asexuals. Although this indicates that asexual oribatid mites do not have a colonization advantage, it suggests that colonization ability may be a component of the long-term success strategy of ancient asexual oribatid mites.


Chlorophyll composition in Griffy Lake sediment cores: has land use in the watershed shifted organic matter content over time?
Ryan Queen (lab technician, IU Dept. of Geological Sciences; undergraduate student, IU Dept. of Chemistry)

We are examining sediment cores of Griffy Lake and measuring the concentration of the different types of chlorophylls a, b, and c throughout sediment layers in the cores. The sediment cores will provide chlorophyll concentrations dating back to the construction of the lake in 1924 up to present day [2008]. By looking at the chlorophyll concentrations, we can track the changes of organic carbon types in the lake over time. Chlorophylls a, b, and c are our primary focus, which correlate to aquatic algal and macrophyte communities within the ecosystem. The concentrations of total nitrogen and total phosphorous in the cores will also be analyzed through the levels of the core. Previous experimentation determining nitrogen and phosphorous content has been performed on past cores extracted from the lake, which will function as a baseline to tie into data of previous cores taken in 2002; thus, comparing the new core to previous data, a timeline from lake cores can be established.

By examining the cores, organic carbon in the sediment layers will reflect how land use in the watershed has affected Griffy Lake over time. Sedimentation at the bottom of the core has been dated back to the construction period of the lake and its early history, established by Pb-210 analysis (M. Hill, BSES 2002). The sedimentation near the top of the core will reflect the most recent activity Griffy has experienced, if any changes have occurred at all over time. Griffy Lake may be used as a good case study to be analyzed in a broader perspective. It functions similarly to numerous manmade lakes in the midwestern United States. By researching how land use around Griffy Lake has impacted this aquatic ecosystem over time may give an analogue of the stresses other aquatic ecosystems in our region experience. By understanding the biological and historical processes that have occurred at Griffy Lake also furthers our understanding of the history of the Griffy Preserve.


Effects of the exotic annual grass Microstegium vimineum (Japanese stiltgrass) on native communities
S. Luke Flory (postdoc, IU Dept. of Biology)

experimental plots
Experimental setup at Bayles Road property (Fall 2006)
Observational studies of exotic species commonly suggest that invasions have negative effects on native communities, but in fact we have surprisingly little experimental evidence regarding the impacts of invasions on native systems. Quantitative studies of the consequences of exotic plant invasions are needed to understand the ecological implications of plant invasions, prioritize management efforts, and leverage resources for invasive plant control.

Microstegium vimineum (Japanese stiltgrass) is a highly shade tolerant C4 annual grass that is rapidly invading deciduous forests in the eastern U.S. I am conducting a large-scale, long-term experiment to quantify the effects of Microstegium invasion on forest communities. I established 32 replicated early-successional forest communities containing nine species of tree seeds or seedlings plus twelve herbaceous species. I then added Microstegium seed to half of the plots to experimentally manipulate invasion. Within this experimental setup we are determining the effects of invasion on:

  1. Native trees at the seed and seedling life-history stages
  2. The native herbaceous community
  3. Arthropod abundance and diversity (with undergraduate Ben Eddy, Dr. Jennifer Rudgers at Rice University, and Dr. Al Cady at Miami of Ohio)
  4. Plant resource availability including light, water, and nutrients (with fellow graduate student Brett Mattingly)
  5. Survival of two species of ticks (with fellow graduate student Dave Civitello)

Thus far, I have found that Microstegium invasion reduces the survival and alters the growth of some, but not all tree species and dramatically decreases the productivity and diversity of herbaceous communities. Microstegium also indirectly impacts tree regeneration by encouraging vole activity and may decrease the diversity and abundance of ground and canopy-dwelling arthropods. Finally, Microstegium invasion may reduce human disease risk by decreasing the survival of two tick species that are important disease vectors.

These results suggest that Microstegium invasions will significantly alter forest communities in the eastern U.S. by changing tree species composition and herbaceous community structure. Furthermore, these changes in plant community composition and structure may result in cascading effects on other trophic levels. Studies of other plant invaders should adopt this experimental approach for quantifying the impacts of exotic plant invasions.

Read more about Luke Flory's research at http://lukeflory.com/.




Revised: January 17, 2009


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