Accidental Allies: The Evolutionary Consequences of Symbiont Switching and Host-Range Expansion in Leaf-Footed Bugs (Hemiptera: Coreoidea)

Leaf footed Bugs on dandelion

Supervisor: Christine Miller 

Project Summary (PhD):

Unlike many insects that pass vital bacterial partners directly to their offspring, true bugs within the superfamily Coreoidea (leaf-footed bugs and their allies) must re-acquire their primary gut symbionts—predominantly from the genus Caballeronia (formerly Burkholderia)—anew from the environment every single generation. These bacteria reside in specialized midgut outgrowths (crypts), providing crucial nutritional supplementation and pesticide resistance that strongly influence host fitness. 

Because these bugs frequently share host plants and soil microhabitats, environmental acquisition creates a fascinating evolutionary vulnerability: accidental horizontal exchange. When different species of Coreoidea feed on the same plant tissue or aggregate in the same soil patch, they are exposed to non-native, allopatric symbiont strains. This project will investigate what happens when these accidental "partner swaps" occur.

Potential questions addressed (based on student interest) include the following:

1.The Phenotypic Payoff (or Penalty): How does acquiring a non-native symbiont strain alter host fitness, development rate, and survival? Does a mismatched symbiont provide novel metabolic capabilities, such as allowing the bug to detoxify or feed on previously unexploitable plant species?

2. Interspecies Competition & Selection: How strict is the host’s internal gating? The student will use competitive co-inoculation assays to determine if native symbiont strains consistently outcompete non-native strains, or if priority effects (who gets there first) allow accidental strains to persist.

3. The Engine of Speciation: Can an accidental symbiont shift trigger reproductive isolation? If a non-native bacterium alters host pheromone profiles, mate choice, or seasonal timing (phenology), a simple ecological accident could ultimately spark the emergence of a new insect species.

Type of Work: 

  • Experimental Entomology & Gnotobiotic Rearing: Rearing germ-free (aposymbiotic) bugs and inoculating them with targeted bacterial strains to isolate phenotypic effects
     
  • High-Throughput Sequencing & Genomics: Using 16S rRNA amplicon sequencing and comparative genomics to track symbiont communities and identify genomic islands associated with host-switching.
     
  • Behavioral & Ecological Assays: Conducting mate choice experiments, host-plant preference trials, and feeding efficiency metrics.

Importance of the area of research concerned:

Horizontally transmitted symbionts challenge the traditional view that stable mutualisms require strict parent-to-offspring inheritance. Because insects acquire these microbes from the environment, symbiotic partnerships can form, dissolve, and change rapidly, providing a powerful system for studying the evolution of cooperation, partner choice, and coevolution. Importantly, novel microbial partners may enable insects to exploit new host plants or habitats, facilitating host range expansion and potentially driving ecological divergence and speciation.

Understanding these processes is also crucial for conservation and invasion biology, as environmental microbes can influence species persistence, conservation success, and the capacity of invasive insects to establish and spread in new environments.

References: 

Kikuchi, Y., Hosokawa, T. & Fukatsu, T. (2007). Insect–Microbe Mutualism without Vertical Transmission: A Stinkbug Acquires a Beneficial Gut Symbiont from the Environment Every Generation. Applied and Environmental Microbiology, 73, 4308–4316;    Kikuchi, Y., Hosokawa, T. & Fukatsu, T. (2011). Specific Developmental Window for Establishment of an Insect–Microbe Gut Symbiosis. Applied and Environmental Microbiology, 77, 4075–4081;  Oliver, K. M., Degnan, P. H., Hunter, M. S. & Moran, N. A. (2009). Bacteriophages Encode Factors Required for Protection in a Symbiotic Mutualism. Science, 325, 992–994.