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1. Direct and indirect effects of forest microclimate on pathogen spillover

   https://doi.org/10.1002/ecy.2686  

   Dillon, Whalen W. ; Meentemeyer, Ross K. ( April 2019 , Ecology)

   Disease dynamics are governed by variation of individuals, species, and environmental conditions across space and time. In some cases, an alternate reservoir host amplifies pathogen loads and drives disease transmission to less competent hosts in a process called pathogen spillover. Spillover is frequently associated with multi‐host disease systems where a single species is more tolerant of infection and more competent in pathogen transmission compared to other hosts. Pathogen spillover must be driven by biotic factors, including host and community characteristics, yet biotic factors interact with the abiotic environment (e.g., temperature) to create disease. Despite its fundamental role in disease dynamics, the influence of the abiotic environment on pathogen spillover has seldom been examined. Improving our understanding of disease processes such as pathogen spillover hinges on disentangling the effects of interrelated biotic and abiotic factors over space and time. We applied 10 yr of fine‐scale microclimate, disease, and tree community data in a path analysis to investigate the relative influence of biotic and abiotic factors on pathogen spillover for the emerging infectious forest disease sudden oak death (SOD). Disease transmission inSODis primarily driven by the reservoir host California bay laurel, which supports high foliar pathogen loads that spillover onto neighboring oak trees and create lethal canker infections. The foliar pathogen load and susceptibility of oaks is expected to be sensitive to forest microclimate conditions. We found that biotic factors of pathogen load and tree diversity had relatively stronger effects on pathogen spillover compared to abiotic microclimate factors, with pathogen load increasing oak infection and tree diversity reducing oak infection. Abiotic factors still had significant effects, with greater heat exposure during summer months reducing pathogen loads and optimal pathogen conditions during the wet season increasing oak infection. Our results offer clues to possible disease dynamics under future climate change where hotter and drier or warmer and wetter conditions could have opposing effects on pathogen spillover in theSODsystem. Disentangling direct and indirect effects of biotic and abiotic factors affecting disease processes can provide key insights into disease dynamics including potential avenues for reducing disease spread and predicting future epidemics.

    

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2. Natural resistance to worms exacerbates bovine tuberculosis severity independently of worm coinfection

   https://doi.org/10.1073/pnas.2015080118  

   Ezenwa, Vanessa O. ; Budischak, Sarah A. ; Buss, Peter ; Seguel, Mauricio ; Luikart, Gordon ; Jolles, Anna E. ; Sakamoto, Kaori ( January 2021 , Proceedings of the National Academy of Sciences)

   Pathogen interactions arising during coinfection can exacerbate disease severity, for example when the immune response mounted against one pathogen negatively affects defense of another. It is also possible that host immune responses to a pathogen, shaped by historical evolutionary interactions between host and pathogen, may modify host immune defenses in ways that have repercussions for other pathogens. In this case, negative interactions between two pathogens could emerge even in the absence of concurrent infection. Parasitic worms and tuberculosis (TB) are involved in one of the most geographically extensive of pathogen interactions, and during coinfection worms can exacerbate TB disease outcomes. Here, we show that in a wild mammal natural resistance to worms affects bovine tuberculosis (BTB) severity independently of active worm infection. We found that worm-resistant individuals were more likely to die of BTB than were nonresistant individuals, and their disease progressed more quickly. Anthelmintic treatment moderated, but did not eliminate, the resistance effect, and the effects of resistance and treatment were opposite and additive, with untreated, resistant individuals experiencing the highest mortality. Furthermore, resistance and anthelmintic treatment had nonoverlapping effects on BTB pathology. The effects of resistance manifested in the lungs (the primary site of BTB infection), while the effects of treatment manifested almost entirely in the lymph nodes (the site of disseminated disease), suggesting that resistance and active worm infection affect BTB progression via distinct mechanisms. Our findings reveal that interactions between pathogens can occur as a consequence of processes arising on very different timescales.

    

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3. microRNA regulation in an ancient obligate endosymbiosis

   https://doi.org/10.1111/mec.14464  

   Feng, Honglin ; Wang, Lingyu ; Wuchty, Stefan ; Wilson, Alex C. C. ( January 2018 , Molecular Ecology)

   Although many insects are associated with obligate bacterial endosymbionts, the mechanisms by which these host/endosymbiont associations are regulated remain mysterious. While microRNAs (miRNAs) have been recently identified as regulators of host/microbe interactions, including host/pathogen and host/facultative endosymbiont interactions, the role miRNAs may play in mediating host/obligate endosymbiont interactions is virtually unknown. Here, we identified conserved miRNAs that potentially mediate symbiotic interactions between aphids and their obligate endosymbiont,Buchnera aphidicola. Using smallRNAsequence data fromMyzus persicaeandAcyrthosiphon pisum, we annotated 93M. persicaeand 89A. pisummiRNAs, among which 69 were shared. We found 14 miRNAs that were either highly expressed in aphid bacteriome, theBuchnera‐housing tissue, or differentially expressed in bacteriome vs. gut, a non‐Buchnera‐housing tissue. Strikingly, 10 of these 14 miRNAs have been implicated previously in other host/microbe interaction studies. Investigating the interaction networks of these miRNAs using a custom computational pipeline, we identified 103 miRNA::mRNAinteractions shared betweenM. persicaeandA. pisum. Functional annotation of the sharedmRNAtargets revealed only two over‐represented cluster of orthologous group categories: amino acid transport and metabolism, and signal transduction mechanisms. Our work supports a role for miRNAs in mediating host/symbiont interactions between aphids and their obligate endosymbiontBuchnera. In addition, our results highlight the probable importance of signal transduction mechanisms to host/endosymbiont coevolution.

    

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4. Whitebark pine facilitation at treeline: potential interactions for disruption by an invasive pathogen

   https://doi.org/10.1002/ece3.2198  

   Tomback, Diana F. ; Blakeslee, Sarah C. ; Wagner, Aaron C. ; Wunder, Michael B. ; Resler, Lynn M. ; Pyatt, Jill C. ; Diaz, Soledad ( June 2016 , Ecology and Evolution)

   In stressful environments, facilitation often aids plant establishment, but invasive plant pathogens may potentially disrupt these interactions. In many treeline communities in the northernRockyMountains of theU.S. andCanada,Pinus albicaulis, a stress‐tolerant pine, initiates tree islands at higher frequencies than other conifers – that is, leads to leeward tree establishment more frequently. The facilitation provided by a solitary (isolated)P. albicaulisleading to tree island initiation may be important for different life‐history stages for leeward conifers, but it is not known which life‐history stages are influenced and protection provided. However,P. albicaulismortality from the non‐native pathogenCronartium ribicolapotentially disrupts these facilitative interactions, reducing tree island initiation. In twoRockyMountain eastern slope study areas, we experimentally examined fundamental plant–plant interactions which might facilitate tree island formation: the protection offered byP. albicaulisto leeward seed and seedling life‐history stages, and to leeward krummholz conifers. In the latter case, we simulated mortality fromC. ribicolafor windwardP. albicaulisto determine whether loss ofP. albicaulisfromC. ribicolaimpacts leeward conifers. Relative to other common solitary conifers at treeline, solitaryP. albicaulishad higher abundance. More seeds germinated in leeward rock microsites than in conifer or exposed microsites, but the odds of cotyledon seedling survival during the growing season were highest inP. albicaulismicrosites. Planted seedling survival was low among all microsites examined. Simulating death of windwardP. albicaulisbyC. ribicolareduced shoot growth of leeward trees. Loss ofP. albicaulisto exotic disease may limit facilitation interactions and conifer community development at treeline and potentially impede upward movement as climate warms.

    

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5. Transcriptome‐based analyses of phosphite‐mediated suppression of rust pathogens Puccinia emaculata and Phakopsora pachyrhizi and functional characterization of selected fungal target genes

   https://doi.org/10.1111/tpj.13817  

   Gill, Upinder S. ; Sun, Liang ; Rustgi, Sachin ; Tang, Yuhong ; von Wettstein, Diter ; Mysore, Kirankumar S. ( February 2018 , The Plant Journal)

   Phosphite (Phi) is used commercially to manage diseases mainly caused by oomycetes, primarily due to its low cost compared with other fungicides and its persistent control of oomycetous pathogens. We explored the use of Phi in controlling the fungal pathogensPuccinia emaculataandPhakopsora pachyrhizi, the causal agents of switchgrass rust and Asian soybean rust, respectively. Phi primes host defenses and efficiently inhibits the growth ofP. emaculata,P. pachyrhiziand several other fungal pathogens tested. To understand these Phi‐mediated effects, a detailed molecular analysis was undertaken in both the host and the pathogen. Transcriptomic studies in switchgrass revealed that Phi activates plant defense signaling as early as 1 h after application by increasing the expression of several cytoplasmic and membrane receptor‐like kinases and defense‐related genes within 24 h of application. Unlike in oomycetes,RNAsequencing ofP. emaculataandP. pachyrhizidid not exhibit Phi‐mediated retardation of cell wall biosynthesis. The genes with reduced expression in either or both rust fungi belonged to functional categories such as ribosomal protein, actin,RNA‐dependentRNApolymerase, and aldehyde dehydrogenase. A fewP. emaculatagenes that had reduced expression upon Phi treatment were further characterized. Application of double‐strandedRNAs specific toP. emaculatagenes encoding glutamateN‐acetyltransferase and cystathionine gamma‐synthase to switchgrass leaves resulted in reduced disease severity uponP. emaculatainoculation, suggesting their role in pathogen survival and/or pathogenesis.

    

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