Adelgids are sap-feeding insects that feed upon conifers and are related to aphids, phylloxera, and other hemipteran species. Plant sap is a very poor source of nutrients, particularly the Nitrogen-based essential amino acids that all animals must consume in their diet. An innovative strategy to overcome this deficiency is the acquisition of bacterial symbionts that reside intracellularly in specialized host cells and convert nutrients in plant sap into essential amino acids. Bacterial symbionts are passed to the next generation vertically, which creates population bottlenecks and prevents genetic exchange with other symbiont lineages. Over the long term, this results in the slow degradation of symbiont genomes; increased genetic drift and reduced efficacy of selection cause symbiont genomes to become massively streamlined to the essential content.
To date, much of our understanding of symbiont genome evolution and regulatory control comes from only a few insect species with simple life cycles. However, plant-sap-feeding insects embrace a broad diversity of life cycles that may include unrelated host plants, large nutritional disparities, and dormancies; all are factors that govern nutrient availability and demands on symbiont function. What therefore remains virtually unexplored is the evolutionary interplay between insect life cycles and symbiosis. Lack of such knowledge is an important problem because it thwarts our comprehension of how symbionts, while initially opening up new niches for their hosts, might ultimately limit future host evolution and diversification.
In collaboration with Dr. Carol von Dohlen at Utah State University, Dr. Burke’s long-term goal for this project is to understand how the particular biology of an insect host affects the stability, longevity, and dynamism of evolutionary transitions arising from symbiosis. The overall objective for this project is to quantify the interplay between selection, symbiont genome decay, symbiont acquisition, and life cycle evolution in adelgids. While surveying and sequencing the genomes of several different lineages of adelgid symbionts, we found a pattern of each species possessing dual symbionts that have been gained and lost over evolutionary time in a pattern that is much more dynamic than other sap-sucking insects. Our research aims to provide an explanation for the novel situation in adelgids that will be consistent with their biology and with accepted processes of symbiont genome evolution. This in turn will form the justification for future exploration of symbiont contributions to the evolution of life cycle complexity in other herbivorous insects. As both native and non-native adelgid species can be devastating forestry pests, greater understanding of adelgid-plant-microbe interactions could help to inform biocontrol measures and development of host-plant resistance.
We would like to thank Garden Delights for generously donating hemlock trees for our research.
Selected publications:
Dial DT, Weglarz KM, Brunet B, Havill NP, von Dohlen CD, Burke GR (2023) Whole genome sequence of the Cooley spruce gall adelgid, Adelges cooleyi (Hemiptera: Sternorrhyncha: Adelgidae). G3. jkad224.
Dial DT+, Weglarz KM+, UAremu AO, Havill NP, UPearson TA, Burke GR, von Dohlen CD(2021) Transitional genomes and nutritional role reversals identified for dual symbionts of adelgids (Aphidoidea: Adelgidae). ISME J. https://doi.org/10.1038/s41396-021-01102-w. +Co-first authors
Weglarz KM, Havill NP, Burke GR, von Dohlen CD (2018) Partnering with a pest: genomes of hemlock woolly adelgid symbionts reveal atypical nutritional provisioning patterns in dual-obligate bacteria. Genome Biology and Evolution. evy114, https://doi.org/10.1093/gbe/evy114
Mech AM, Harper SJ, Havill NP, von Dohlen CD, Burke GR (2017) Ecological factors influencing the beneficial endosymbionts of the Hemlock Woolly Adelgid (Hemiptera: Adelgidae). Insect Science. Epub ahead of print.
von Dohlen CD, Spaulding U, Patch KB, Weglarz K, Foottit R, Havill NP, Burke GR (2017) Dynamic acquisition and loss of dual-obligate symbionts in the plant-sap-feeding Adelgidae (Hemiptera: Sternorrhyncha: Aphidoidea). Frontiers in Microbiology. 8:1037.