Progress 06/15/19 to 12/31/21
Outputs
Target Audience:Target audiences for this project included researchers, land managers, and conservation practitioners working with bumblebees. This audience was reached through research presentation at the "BOMBUSS" meeting. Theproject also reached general entomologists, ecologists, and evolutionary biologists through departmental and conference seminars. Finally, teaching and outreach efforts targeted members of the public in Austin and San Antonio, TX, and undergraduate students at UT Austin and the University of the Andes in Colombia. Changes/Problems:The pandemic disrupted experiments that were planned to test the effects and molecular mechanisms of the full range of stressors described in the proposal. Hence, data on thermal ecology of bumblebee microbiota that had been collected by March 2020 were analyzed and written for publication (Hammer et al. Proc R Soc B 2021). As laboratory work was not allowed for several months, Hammer led the preparation of a review paper on bumblebee gut microbiota (Hammer et al. InsectSoc 2021). When laboratory work was permitted again, only limited days and times were available, and undergraduate involvement was not allowed; hence,a smaller-scale project was designed and conducted. This project focused on changes in microbiota and host gene expression over the bumblebee lifespan. A no-cost extension was granted to allow Hammer time to obtain and analyze sequence data from this last project. Data have now been analyzed and amanuscript is in preparation for submissionin May 2022. What opportunities for training and professional development has the project provided?Hammer received training in new research techniques (e.g. RNAseq, shotgun metagenomics) and sub-fields (e.g., thermal ecology, evolution of aging, bacterial pathogenesis, social insect biology) and opportunities to present research findings at numerous conferences and other venues. Hammer also gained experience in mentorship bysupervising multiple undergraduate researchers. The training and professional development provided by the project were instrumental in enabling Hammer to obtain a tenure-track faculty position at UC Irvine. The project also providedopportunities for training and professional developmentfor undergraduate student researchers. Three students from UT Austin were involved in experiments testing thermal tolerances of gut bacteria and host-Serratia-symbiont interactions. These students received training in a variety of technical laboratory skills. Onestudent was alsoinvolved in a review paper (Hammer et al. Insectes Sociaux 2021), gaining experience in literature synthesis and coauthorship on the paper. A fourth student from the Universidad de los Andes was also mentored by Hammer. This student received a fellowship to conduct summer research at UT Austin and gained molecular and bioinformatic skills and professional communication skills; she is currently lead author on a manuscript that is being prepared for publication. How have the results been disseminated to communities of interest?Results have been disseminated to researchers studying host-microbe interactions, bumblebee biology, and microbial ecology through peer-reviewed publications and presentations at several conferences. Presentation at the BOMBUSS meeting specifically targeted bumblebee researchers and conservation practitioners. Further, Hammer led or wasinvolved in five outreach events focused on the importance of pollinators for agriculture and society, and threats to pollinator health. Hammer gave talks on: bee biology, to an undergraduate student group in Colombia; social insects,to the public in AustinTX through a webinar organized by the Austin Nature & Science Center; bumblebees, to the Alamo Area Beekeeper's Association meeting. Hammer also participated in a pollination exhibit at the San Antonio Rodeo Wildlife Expo, and in an 'Explore UT' campus exhibition for Texas high school students. Finally, Hammer discussed bumblebee research and bee declines with undergraduates at UT Austin, through two guest lectures. Overall, these outreach and teaching efforts enhancedpublic understanding of the important of pollinators, and interest in insect conservation. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Efforts to understand how stressors impact bumblebee gut microbiota began with thermal stress. A variety of culture-based and in vivo assays were used to measure the thermal sensitivities of bumblebee symbionts. This work led to the discovery that bee symbionts are generally very robust to heat stress, with potential signatures of local adaptation to the host's thermal niche. Furthermore, it was shown that symbionts are sensitive to low temperatures and that this impairs their ability to colonize bumblebees if the environment is too cool. These findings were published in Proceedings of the Royal Society B (Hammer et al. 2021) and have implications for effects of climate change and other stressors on bumblebee populations. Specifically, they show that unlike in other insect groups,symbionts are unlikely to mediate the effect of heat stress on bumblebees. However, stressors that disrupt thermoregulatory behavior, such as neonicotinoid pesticides, may have knock-on effects on symbiont colonization. Plans for testing additional stressors were disrupted by covid (see Changes/Problems). However, preliminary data were obtained showing that i) Serratia, a common environmental bacterium, is pathogenic to bumblebees; ii) Serratia can invade biofilms of core bumblebee gut bacteria in in vitro assays. These data will be used to support future experiments testing the sources and transmission routes of bumblebee-associated Serratia. Serratia is frequently found in both honeybees and bumblebees; the data acquired to date are a first step in evaluating its potentialimpacts onbumblebee population health. Finally, an experiment was conducted testing whether stressors associated with aging impact the bumblebee gut microbiota. In the wild, bumblebees are frequently found to have disrupted gut microbiota, and the causes ofdisruption are not understood. This experiment used a variety of molecular techniques to demonstrate that aging is not a likely explanation, as gut microbiotaand immunity are highly stable over the lifespan, in contrast to other animals. (A manuscript is currently being prepared for submission). Overall, findings from this work weigh against thermal stress or aging as major causes of microbiota disruption in bumblebees, and point to bacterial pathogen transmission as a potential driver that should be evaluated in future research.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2021
Citation:
Hammer TJ, Le E, Moran NA. 2021 Thermal niches of specialized gut symbionts: the case of social bees. Proc. R. Soc. B 288: 20201480. https://doi.org/10.1098/rspb.2020.1480
- Type:
Journal Articles
Status:
Published
Year Published:
2021
Citation:
Hammer, T.J., Le, E., Martin, A.N., Moran, N.A. 2021. The gut microbiome of bumblebees. Insectes Sociaux 68, 287-301.
- Type:
Journal Articles
Status:
Published
Year Published:
2021
Citation:
Silva Cerqueira, A.E., Hammer, T.J., Moran, N.A., Cristiano Santana, W., Megumi Kasuya, M.C., Can�do da Silva, C. 2021. Extinction of anciently associated gut bacterial symbionts in a clade of stingless bees. The ISME Journal 15, 2813-2816.
- Type:
Journal Articles
Status:
Published
Year Published:
2021
Citation:
Hammer, T.J., De Clerck-Floate, R., Tooker, J.F., Price, P.W., Miller, D.G., Connor, E.F. 2021. Are bacterial symbionts associated with gall induction in insects? Arthropod-Plant Interactions 15, 1-12.
- Type:
Book Chapters
Status:
Submitted
Year Published:
2022
Citation:
Hammer, T.J. Why many animals eat plants without help from microbes. Submitted to the Smithsonian Tropical Research Institute, Contributions of Barro Colorado National Monument.
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Progress 06/15/19 to 06/14/20
Outputs
Target Audience:I presented my NIFA-funded research at "BOMBUSS", a meeting for scientists working on bumblebee research, agricultural applications, andconservation in North America. Other presentations of this researchincluded a seminar at the Purdue University Department of Entomology, the Ecology Society of America meeting, and the "Virtual Symbiosis Seminar Series".I also discussed the issue of insect declines as a guest lecturer for an undergraduate course on Biodiversity at UT Austin. Changes/Problems:The Covid-19 pandemic necessitated a shift in the direction of the project, which hadpredominantly involved laboratory-based experiments. Althoughour laboratory is now available for limited use, time restrictions, difficulties with purchasing and ordering, and limitations on personnel (e.g., no undergraduate researchers) make laboratory-based projects less feasible. Furthermore, any long-term experiments are subject to potential disruptions, should university policy change. In light of this, the PD and supervisor Moran decided to initiate a predominantly field-based and computational component of the project that is within the scope and goals of the original project proposal. Specifically, a comparative study of microbiota function of different bumble bee species, and their interaction with environmental conditions, is underway. Building off of the laboratory-based work to date on thermal responses, the objective is to test whether bee adaptation to resource limitation and heat stress has involved functional changes in the microbiota. Replicate individuals and populations of over a dozen U.S.Bombusspecies occupying distinct habitat types were sampled during summer 2020. Metagenomic sequencing and bioinformatic analyses will be used to infer the metabolic and functional capabilities of gut microbiota in these specimens. Second, I am currently writing a review paper on the microbiota of bumble bees. Although there are numerous reviews of the honey bee microbiota, no such resource exists for bumble bees, despite the fact that over 60 primary data papers have been published on the topic. Most of these were published in just the last few years, indicating substantial interest in the bee and microbiology research communities. The review will summarize existing knowledge on bumble bee microbiota, and describe applications for agriculture, bee conservation, and basic research on host-microbe interactions.Apreproposal for submission to Proc. R. Soc. B was green-lighted by the Reviews Editor. What opportunities for training and professional development has the project provided?This project has provided multiple opportunities to the Project Director fortraining and professional development. The PD worked closely with the postdoctoral supervisor Dr. Nancy Moran and gained skills in bacterial cultivation, bee rearing, and bioinformatics. Through working with Dr. Moran and members of the laboratory, thePD also strengthened professional skills in project design, presentation, and writing. Four conferences/seminars provided an opportunity to present research and network with other members of the bee and microbiome research communities. The PD also successfully applied for tenure-track faculty jobs and will be starting as an Assistant Professor at UC Irvine in Jan. 2022. Furthermore, four undergraduate students received direct mentorship from the PD during the course of this reporting period. These students gained technical skills (e.g. molecular techniques, bioinformatics, insect rearing) as well as experience in conducting research, writing, presentation, and applying for undergraduate fellowships. How have the results been disseminated to communities of interest?Results have been disseminated to a variety of scientific audiences through seminars and a research manuscript currently available as a preprint. Outreach events were originally planned for spring 2020, reaching members of the public in Central Texas and focusing on the importance of pollinators to agriculture and natural ecosystems. However, these were postponed due to Covid-19. What do you plan to do during the next reporting period to accomplish the goals?Some changes in research direction were undertaken in response to Covid-19 (see following section). Fieldwork was conducted from mid-June 2020 through August 2020. Laboratory processing of these field samples is now underway.I expect that sequence data will be generated by Jan. 2021, with bioinformatic analyses occurring through April 2021 and manuscript writing and submission in May 2021. Furthermore, areview paper on the bumble bee microbiota is also in progress, with an anticipated manuscript submission date of November 2020.
Impacts
What was accomplished under these goals?
Bumble bees are important pollinators of many crops and wild plants. Some species are declining, motivating a need to understand the threats thatbumble bees face and the mechanisms by whichstressors impact bees. Recently, bumble bees have been discovered to host a characteristic and functionally important community of gut microbes. This project investigates the potential role of microbiota in mediating bumble bee responses to stressors. A goal of this work is to improve bumble bee management and conservation, and ultimately provide microbial solutions for bumble bee pollination (e.g., probiotics). The Project Director (TJH) has conducted several laboratory-based experiments bothin vitro(i.e., gut microbes in culture media) andin vivo(in live bees) focusing on two stressors relevant to bumble bee health: pathogens, and thermalstress. Pathogens are a major concern for bees commercially bred for crop pollination, as well as for wild populations. These experiments have indicated that Serratia,a common yet understudiedbacterial pathogen, can outcompete bumble bee gut bacteriain vitroand cause mortality in beesin vivo. A second set of experiments focused on thermal stress, which is expected to increasingly impact bumble bees as climate change accelerates. Results indicated that, overall, bee gut microbes are surprisingly robust to high temperaturesand are unlikely to be theprimary driver of climate change effects. However, there is also extensive variation among gut microbes; some heat-sensitive strains may indeed be a liability as bees cope with heat stress. Furthermore, it was discovered that microbial colonization of the gut is highly cold-sensitive. In the future, thermally robust strains could be evaluated for use as bee probiotics in agriculture. Objective 1.Characterize the responses of a panel of gut microbial species and strains to stressors in vitro. Experiments during the 2019-2020 reporting period focused on two stressors: thermal stress and opportunistic bacterial pathogens. These were prioritized for study as preliminary experiments yielded promising results. With regard to thermal stress, experiments were conducted to: i) Test upper and lower thermal limits to growth for a panel of honey bee and bumble bee gut bacterial strains, 2) Measure the ability of these bacteria to recover from a heat treatment simulating bee exposure to heat stress, 3) Assay growth ratesin vitrounder two temperatures and the potential for adaptation to the local thermal environment. On the whole, bee bacteria are generally robust to high temperatures, yet there was significant strain variability in thermal limits. There was also evidence for host-specific adaptation to the honey bee versus bumble bee thermal environment. With regard to pathogen interactions, bumble bee gut bacteria were challenged in in vitrocompetition assays with the bacterial pathogenSerratia. Even under conditions favorable to the growth of bee bacteria, they were readily outcompeted by Serratia, suggesting that similar dynamics may occur in the gut environment during infection. Objective 2.Assess the effects of gut microbial variation on bee susceptibility to stressors in vivo. Experiments were carried out using laboratory-rearedBombus impatiens, purchased from asupplier of colonies used for crop pollination. One set of experiments was designed to test whether a heat exposure treatment in vivoimpacted bees via disruption of their microbiota. This was initially hampered by a difficulty in achieving robust microbial colonization of experimental bees, as has recently been corroborated in another study (Sauers and Sadd, Evolution 2019). A pilot experiment did not indicate a significant impact of heat exposure on gut microbes, a result in agreement with the aforementionedin vitrodata. A follow-up experiment found, unexpectedly, that microbial colonization is highly cold-sensitive. Specifically, bees reared at a low temperature were unable to consistently acquire high numbers of symbionts. Bees maintain high nest temperatures with specific thermoregulatory behaviors, so any disruptions of these behaviors (which have been shown to occur with neonicotinoid pesticides) may also have consequences for bee gut bacteria. Implications are further discussed inHammer et al. bioRxiv 2020 (under review at Proc R Soc B).The second set of experiments were focused on the bacterial pathogenSerratia. The initial objective of assessing microbial variation and pathogen susceptibility in vivowas, again, hampered by thehighlyvariable ability of different bacterial strains to colonize B. impatiensbees. However, these experiments did find thatSerratiainfection can cause mortality inbumble bees. AlthoughSerratiahas been repeatedly found in bumble bees, its potential pathogenicity had not been evaluated, as the vast majority of research focused on other pathogens and parasites. This result suggests that bacterial pathogens may be underappreciated contributors to bumble bee disease. Objective 3.Identify candidate molecular mechanisms underlying microbial resilience and host protection. Analyses of molecular mechanisms underlying microbial sensitivity or resilience to thermal stress are still in progress. Pilot experiments investigated the potential role of a focal molecular mechanism, the bacterial type VI secretion system (T6SS), in antagonistic interactions between bee bacteria andSerratia. As mentioned above,Serratiais able to rapidly outcompete bee bacteriain vitro. However, this does not depend on the presence or absence of the T6SS in the latter, weighing againstthe hypothesis that gut microbial protection against Serratia is mediated by T6SS.
Publications
- Type:
Journal Articles
Status:
Under Review
Year Published:
2020
Citation:
Hammer, T.J., Le, E., Moran, N.A. Thermal niches of specialized gut symbionts: the case of social bees. Under Review at Proc. R. Soc. B Biol. Sci. bioRxiv preprint: https://doi.org/10.1101/2020.06.16.155309.
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