NSF-BSF: Ecological Implications and Evolutionary Dynamics of a Nascent Sodalis-allied symbiont in a Parasitoid Wasp.

This NSF-BSF project is a collaboration between scientists at the University of Utah, USA and the University of Haifa, Israel. It focuses on analyzing the connection between parasitoid wasps and a symbiotic bacterium named Sodalis. Many insects on our planet have evolved associations with Sodalis bacteria which often produce nutrients or other benefits for their insect hosts, allowing them to persist under conditions in which nutrient availability is sub-optimal.

The parasitoid wasp under study in this project has the most-recently derived Sodalis symbiosis yet discovered in nature. It provides an unprecedented opportunity to study the mechanistic and evolutionary bases of emerging symbiotic relationships using techniques including laboratory insect manipulation, bacterial genetic modification and genome sequencing.

It will yield important insight into a phenomenon known as degenerative evolution, in which symbiotic bacteria (including many pathogens) undergo drastic gene loss in response to confinement in a host. Notably, the genome of the Sodalis symbiont in this project is undergoing proliferation of parasitic genetic elements called insertion sequence elements, that replicate and insert themselves into bacterial genes, causing extensive disruption and loss of gene functions.

This proliferation will be studied using experimental evolution and genome sequencing. In addition, it will provide a mechanistic understanding of the process of maternal symbiont transmission. Broader impacts will focus on the education of middle and high school students in Utah and Haifa, who will learn about the biology and bio-control potential of parasitoids and will engage in the collection of parasitoids from their local environment.

Insects from many different families, encompassing myriad lifestyles and habitats, maintain long-term associations with symbiotic bacteria of the genus Sodalis. In some cases, these associations are ancient in origin and the resulting Sodalis symbionts demonstrate extensive genome degeneration as a consequence of the isolated and uniform symbiotic lifestyle.

Genome degeneration involves accumulation of mutations that inactivate and erode genes evolving under relaxed selection. Inspection of the genomes of nascent symbionts reveal that selfish insertion sequence (IS)-elements play an important role in the process of degeneration. These IS-elements undergo proliferation in the early-stages of host-association, presenting a potent mechanism for inactivation of bacterial genes and yielding repetitive templates for deletogenic recombination events.

This project focuses on a nascent Sodalis symbiont that lives in association with a parasitoid wasp, has been cultured in the laboratory and is amenable to genetic manipulation. The mechanistic basis and evolutionary consequences of IS-element proliferation will be studied in vitro and in vivo using experimental evolution. Notably, some IS-elements in this Sodalis symbiont have acquired bacterial genes that likely enable them to silence the transposition of competing elements, facilitating IS-element “war games”.

The role of these genes in the proliferation process will be examined via genetic manipulation of the symbionts. Genetic approaches will also be utilized to identify the bacterial determinants of vertical symbiont transmission in this system. Together, these activities will provide important new insight into the early, deterministic stages of genome degeneration, enhancing our understanding of the evolution of mutualistic and pathogenic associations.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.