Facultative symbiont virulence determines horizontal transmission rate without host specificity in Dictyostelium discoideum social amoebas.

In facultative symbioses, only a fraction of hosts are associated with symbionts. Specific host and symbiont pairings may be the result of host-symbiont coevolution driven by reciprocal selection or priority effects pertaining to which potential symbiont is associated with a host first. Distinguishing between these possibilities is important for understanding the evolutionary forces that affect facultative symbioses. We used the social amoeba, Dictyostelium discoideum, and its symbiont, Paraburkholderia bonniea, to determine whether ongoing coevolution affects which host-symbiont strain pairs naturally cooccur within a facultative symbiosis. Relative to other Paraburkholderia, including another symbiont of D. discoideum, P. bonniea features a reduced genome size that indicates a significant history of coevolution with its host. We hypothesized that ongoing host-symbiont coevolution would lead to higher fitness for naturally cooccurring (native) host and symbiont pairings compared to novel pairings. We show for the first time that P. bonniea symbionts can horizontally transmit to new amoeba hosts when hosts aggregate together during the social stage of their life cycle. Here we find evidence for a virulence-transmission trade-off without host specificity. Although symbiont strains were significantly variable in virulence and horizontal transmission rate, hosts and symbionts responded similarly to associations in native and novel pairings. We go on to identify candidate virulence factors in the genomes of P. bonniea strains that may contribute to variation in virulence. We conclude that ongoing coevolution is unlikely for D. discoideum and P. bonniea. The system instead appears to represent a stable facultative symbiosis in which naturally cooccurring P. bonniea host and symbiont pairings are the result of priority effects.

Lessons Learned from the Design, Construction, and Commissioning of a Retrofitted Arthropod Containment Level 3 Insectary.

Arthropods are vectors for many pathogens that significantly harm human and animal health globally, and research into vector-borne diseases is of critical public health importance. Arthropods present unique risks for containment, and therefore insectary facilities are essential to the safe handling of arthropod-borne hazards. In 2018, the School of Life Sciences at Arizona State University (ASU) began the process to build a level 3 arthropod containment (ACL-3) facility. Even with the COVID-19 pandemic, it took more than 4 years for the insectary to be granted a Certificate of Occupancy. At the request of the ASU Environmental Health and Safety team, Gryphon Scientific, an independent team with biosafety and biological research expertise, studied the project lifecycle through the design, construction, and commissioning of the ACL-3 facility with the goal of identifying lessons learned from the delayed timeline. These lessons learned convey insight into best practices for assessing potential facility sites, anticipating challenges with retrofitted construction, preparing for commissioning, equipping the project team with necessary expertise and expectations, and supplementing the gaps in available containment guidance. Several unique mitigations designed by the ASU team to address research risks not specifically addressed in the American Committee of Medical Entomology Arthropod Containment Guidelines are also described. Completion of the ACL-3 insectary at ASU was delayed, but the team thoroughly assessed potential risks and enabled appropriate practices for the safe handling of arthropod vectors. These efforts will enhance future ACL-3 construction by helping to avoid similar setbacks and streamlining progress from concept to operation.

Facultative symbiont virulence determines horizontal transmission rate without host strain specificity.

In facultative symbioses, only a fraction of hosts are associated with a symbiont. Understanding why specific host and symbiont strains are associated can inform us of the evolutionary forces affecting facultative symbioses. Possibilities include ongoing host-symbiont coevolution driven by reciprocal selection, or priority effects that are neutral in respect to the host-symbiont interaction. We hypothesized that ongoing host-symbiont coevolution would lead to higher fitness estimates for naturally co-occurring (native) host and symbiont combinations compared to nonnative combinations. We used the Dictyostelium discoideum - Paraburkholderia bonniea system to test this hypothesis. P. bonniea features a reduced genome size relative to another Paraburkholderia symbiont of D. discoideum, indicating a significant history of coevolution with its host. Facultative symbionts may experience continued genome reduction if coevolution is ongoing, or their genome size may have reached a stable state if the symbiosis has also stabilized. Our work demonstrates that ongoing coevolution is unlikely for D. discoideum and P. bonniea. The system instead represents a stable facultative symbiosis. Specifically associated host and symbiont strains in this system are the result of priority effects, and presently unassociated hosts are simply uncolonized. We find evidence for a virulence-transmission trade-off without host strain specificity, and identify candidate virulence factors in the genomes of P. bonniea strains that may contribute to variation in benevolence.