Hannes Schuler

Department of Biological Sciences

University of Notre Dame

My research addresses the ecological and evolutionary role of Wolbachia in different insects. Wolbachia is an endosymbiotic bacterium that is present in a high number of insects and other arthropods. The endosymbiont can influence the reproduction of its host to enhance its own fitness and has therefore a crucial impact on the ecology and evolution of its host.


The goal of my PhD research, as well as the research I’ve conducted as a postdoc, is to genetically characterize Wolbachia in different fruit flies of the genus Rhagoletis. During my PhD, I studied  1) the endosymbiont Wolbachia in different host races of the apple maggot fly, R. pomonella, 2) horizontal transmission of Wolbachia between the native cherry fruit fly R. cerasi and the invasive R. cingulata and 3) the presence of low-titre Wolbachia in the walnut husk fly, R. completa, in Europe.


My first postdoc aimed at characterizing Wolbachia and the genotype of its host, R. cerasi, to reconstruct their (co)evolutionary history. Additionally, I was involved in the characterization of Wolbachia in several other insects, such as in parasitoids of R. cerasi, in the oriental fruit fly Bactocera dorsalis and in haplodiploid and diploid bark and ambrosia beetles. After a short intermezzo at the Free University of Bolzano where I studied the interaction of Varroa and the honeybee I returned to my passion for Wolbachia and am currently working on the following research projects:


Wolbachia in Rhagoletis sp.

Rhagoletis pomonella is a famous textbook example of sympatric speciation. The recent shift of one population from the native hawthorn to the introduced domestic apples in the last ~160 years has resulted in the formation of an ecological and genetically different host race. Similarly, the closely related sister species R. mendax (blueberry maggot), R. zephyria (snowberry maggot) and the undescribed flowering dogwood fly speciated through adaptation to different hosts.


In contrast, species of the Walnut-infesting R. suavis species group speciated due to allopatric divergence. The six species are largely allopatric, but do display parapatry in parts of their respective ranges. Finally, cherry fruit flies in the R. cingulata group appear to have speciated through a variety of modes. While R. cingulata and R. indifferens speciated due to allopatric separation, R. chionanthi and R. osmanthi speciated due to adaptation to different hosts.


Currently we are characterizing the Wolbachia community in the different Rhagoletis species. This will show if the bacterium has a coevolutionary history with its host or if different species harbor different strains with potential origin of independent horizontal transmission events.


Wolbachia as incompatibility factor in different Rhagoletis sp.

Crossing studies by Juan Rull and colleagues (2010) showed that certain crosses of different hawthorn- infesting R. pomonella populations from Mexico and the US produced no offspring. Those authors hypothesized intrinsic nuclear reproductive isolation as the cause of the incompatibilities. We are currently screening the different populations for Wolbachia to see if the endosymbiont played a role in postzygotic reproductive isolation.


Similar to R. pomonella, recent crossings of R. cingulata populations from Mexico and the US showed incompatibilities between different populations. We are currently genotyping different R. cingulata populations by microsatellites and mitochondrial DNA and compare those data with the genotype of their Wolbachia. This comparative approach will help us understand the evolutionary history of Wolbachia in R. cingulata and help us determine if reproductive isolation is caused by nuclear incompatibilities or by the endosymbiont.


Wolbachia in different parasitoids of Rhagoletis sp.

Rhagoletis sp. are hosts of a broad range of different parasitoids. The community of parasitoid wasps that specializes in attacking Rhagoletis flies (Forbes et al. 2010) is a potential vector for horizontal transfer. These parasitoids directly oviposit their eggs into fly eggs or larvae, possibly acting as an infective hypodermic needle.


We are currently characterizing the Wolbachia infection in different Rhagoletis-infesting parasitoids and compare it with those of their hosts. Resolving the distribution of Wolbachia in braconid wasps will contribute to an explanation for the current distribution of Wolbachia in Rhagoletis and help reveal whether  different strains may serve as an incompatibility barrier between parasitoids attacking different hosts or between multiple geographic populations.



Hannes Schuler

Department of Biological Sciences, University of Notre Dame,

291 Galvin Life Sciences Building , Notre Dame, IN 46556