Symbiotic bacteria have played a large role in insect diversification. Studies on endosymbionts, specialized bacteria living inside insect tissues and organs, have provided insight into novel ecological adaptations, the origins of plant feeding, and genome evolution. This project will investigate the role of two endosymbionts in the diversification of the native Hawaiian leafhoppers (Cicadellidae: Nesophrosyne). This group is an excellent model system because of the known geologic history of Hawaii, the strict use of particular host plants by these insects, and the restriction of certain species of Nesophrosyne to individual islands. This unique temporal and ecological framework can be used to test hypotheses of insect-endosymbiont coevolution and adaptation to local environments.
This work involves students at multiple educational levels, from elementary and secondary schools to undergraduate and graduate programs. Ongoing work with the NSF GK-12 program (5th and 7th grade) uses project resources to enrich science education of public school students. This project also trains undergraduate students in applied research techniques. Work on the Nesophrosyne-endosymbiont system will provide the groundwork for examining the role of these associations in the evolution and diversification of plant-feeding insects. Taxonomic work on native leafhoppers will provide conservation resource managers and land use planners with more accurate information of the Hawaiian biota.
The relationships between insects and bacteria that live inside them (called endosymbionts) are thought to play an important role in the diversity of insects globally. This project investigated the diversification of the native Hawaiian leafhoppers in the genus Nesophrosyne (Family: Cicadellidae) and their bacterial endosymbionts as a model system to understand these evolutionary interactions. Nesophrosyne are host-plant restricted, sap-feeding insects. This work has identified Nesophrosyne as one of the most diverse (> 200 species) and ecologically abundant plant-feeding insect groups found only in Hawaii. We discovered over 120 new species and 50 previously described species through this project, which are specific to single host-plants and use over 20 plant families in 33 genera. The plant sap (phloem) that Nesophrosyne feed on is nutrient poor, requiring alternative strategies to sustain life. To compensate, Nesophrosyne (like other known sap-feeding insects) have evolved specialized relationships with bacterial endosymbionts, which synthesize essential amino acids and other nutrients. These associations are considered to be a driving force in the adaptation and diversification of plant-feeding insects, allowing them to dominant terrestrial plant life. This includes the success of invasive insect pests. Project results have identified two endosymbionts that facilitate sap feeding in Nesophrosyne, ‘Candidatus Sulcia muelleri’ and a novel betaproteobacterium in the genus ‘Ca. Nasuia’. Both endosymbionts are restricted to specialized organs called bacteriomes, and are transmitted to offspring prior to egg lying. They are unable to be cultured and rely on their host’s basic biological functions. We found that the betaproteobacterium are shared throughout several other leafhopper genera and potentially throughout the leafhopper subfamily, Deltocephalinae. The betaproteobacterium may have been lost or swapped for other bacteria in some groups as a result of diet shifts. The rate of evolution for Nesophrosyne’s endosymbionts reveals the fastest so far recorded for endosymbionts. We hypothesize that this is a result of the rapid evolution of Nesophrosyne (over 200 species in 3.2 million years), and their diversification across many plant families. Association with many plant hosts may increase evolutionary pressure on endosymbionts due to the nutritional needs of a changing diet. Direct outcomes of this work have been in included in public outreach and scientific publications. Work with the NSF GK-12 program (5th and 7th grade) used project resources to enrich science education of public school students in the California Bay Area. Three undergraduate and high school students have participated (and continue to participate) in the long-term (>1 year) development and execution of this project. Work on native leafhoppers has provided conservation resource managers and land use planners with more accurate information of the diversity and distributions of the native Hawaiian insects. Finally, results provide a novel model system for understanding the evolution and diversification of plant-feeding insects, which has not heretofore existed for a circumscribed group of species. A developed understanding to of the evolutionary dynamics between insects, plants, and endosymbionts may offer a profitable route to invasive insect control.
