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1. Contrasting effects of host tree isolation on population connectedness in two tropical epiphytic bromeliads

   https://doi.org/10.1002/ajb2.1391  

   Amici, Autumn A. ; Nadkarni, Nalini M. ; DiBlasi, Emily ; Seger, Jon ( December 2019 , American Journal of Botany)

   Conversion of primary forests to pastures is a major cause of habitat fragmentation in the tropics. Fragmentation is expected to impede gene flow for many plant species that are restricted to remaining forest fragments. Epiphytes may be especially vulnerable to this effect of forest fragmentation because they depend on host trees. However, trees that remain in pastures may enhance connectivity across the landscape for epiphyte species that can thrive on such trees. To investigate this possibility, we studied the genetic structures of two such species on isolated pasture trees and surrounding forest, in relation to their local abundances in different habitat types and aspects of their reproductive biology including pollen and seed dispersal agents, and looked for evidence of increased or diminished gene flow.

   We used microsatellite markers to assess geographic patterns of genetic diversity and differentiation in two epiphytic bromeliads,Catopsis nitidaandWerauhia tonduziana,in the Monteverde region of Costa Rica.

   About 85% of theF_STvalue forCatopsis nitidawas found among pastures within regions, while forWeruahia tonduziana,about 80% of theF_STvalue was contributed by differences between regions, indicating much more gene flow within regions, relative toC. nitida.

   Although there was substantial genetic differentiation among epiphyte populations, those on isolated pasture trees were not substantially less diverse than those in adjacent forests, suggesting that pasture trees may serve as “stepping stones” that help these species maintain their genetic connectedness and diversity at larger geographic scales.

    

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2. Host plant‐related genomic differentiation in the European cherry fruit fly, Rhagoletis cerasi

   https://doi.org/10.1111/mec.15239  

   Bakovic, Vid ; Schuler, Hannes ; Schebeck, Martin ; Feder, Jeffrey L. ; Stauffer, Christian ; Ragland, Gregory J. ( October 2019 , Molecular Ecology)

   Elucidating the mechanisms and conditions facilitating the formation of biodiversity are central topics in evolutionary biology. A growing number of studies imply that divergent ecological selection may often play a critical role in speciation by counteracting the homogenising effects of gene flow. Several examples involve phytophagous insects, where divergent selection pressures associated with host plant shifts may generate reproductive isolation, promoting speciation. Here, we use ddRADseq to assess the population structure and to test for host‐related genomic differentiation in the European cherry fruit fly,Rhagoletis cerasi(L., 1758) (Diptera: Tephritidae). This tephritid is distributed throughout Europe and western Asia, and has adapted to two different genera of host plants,Prunusspp. (cherries) andLoniceraspp. (honeysuckle). Our data imply that geographic distance and geomorphic barriers serve as the primary factors shaping genetic population structure across the species range. Locally, however, flies genetically cluster according to host plant, with consistent allele frequency differences displayed by a subset of loci betweenPrunusandLoniceraflies across four sites surveyed in Germany and Norway. These 17 loci display significantly higher F_STvalues between host plants than others. They also showed high levels of linkage disequilibrium within and betweenPrunusandLoniceraflies, supporting host‐related selection and reduced gene flow. Our findings support the existence of sympatric host races inR. cerasiembedded within broader patterns of geographic variation in the fly, similar to the related apple maggot,Rhagoletis pomonella, in North America.

    

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3. Transcriptomics of host-specific interactions in natural populations of the parasitic plant purple witchweed ( Striga hermonthica )

   https://doi.org/10.1017/wsc.2019.20  

   Lopez, Lua ; Bellis, Emily S. ; Wafula, Eric ; Hearne, Sarah J. ; Honaas, Loren ; Ralph, Paula E. ; Timko, Michael P. ; Unachukwu, Nnanna ; dePamphilis, Claude W. ; Lasky, Jesse R. ( July 2019 , Weed Science)

   Abstract Host-specific interactions can maintain genetic and phenotypic diversity in parasites that attack multiple host species. Host diversity, in turn, may promote parasite diversity by selection for genetic divergence or plastic responses to host type. The parasitic weed purple witchweed [ Striga hermonthica (Delile) Benth.] causes devastating crop losses in sub-Saharan Africa and is capable of infesting a wide range of grass hosts. Despite some evidence for host adaptation and host-by- Striga genotype interactions, little is known about intraspecific Striga genomic diversity. Here we present a study of transcriptomic diversity in populations of S. hermonthica growing on different hosts (maize [ Zea mays L.] vs. grain sorghum [ Sorghum bicolor (L.) Moench]). We examined gene expression variation and differences in allelic frequency in expressed genes of aboveground tissues from populations in western Nigeria parasitizing each host. Despite low levels of host-based genome-wide differentiation, we identified a set of parasite transcripts specifically associated with each host. Parasite genes in several different functional categories implicated as important in host–parasite interactions differed in expression level and allele on different hosts, including genes involved in nutrient transport, defense and pathogenesis, and plant hormone response. Overall, we provide a set of candidate transcripts that demonstrate host-specific interactions in vegetative tissues of the emerged parasite S. hermonthica . Our study shows how signals of host-specific processes can be detected aboveground, expanding the focus of host–parasite interactions beyond the haustorial connection. 

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4. Microbiome stability and structure is governed by host phylogeny over diet and geography in woodrats ( Neotoma spp.)

   https://doi.org/10.1073/pnas.2108787118  

   Weinstein, Sara B. ; Martínez-Mota, Rodolfo ; Stapleton, Tess E. ; Klure, Dylan M. ; Greenhalgh, Robert ; Orr, Teri J. ; Dale, Colin ; Kohl, Kevin D. ; Dearing, M. Denise ( November 2021 , Proceedings of the National Academy of Sciences)

   The microbiome is critical for host survival and fitness, but gaps remain in our understanding of how this symbiotic community is structured. Despite evidence that related hosts often harbor similar bacterial communities, it is unclear whether this pattern is due to genetic similarities between hosts or to common ecological selection pressures. Here, using herbivorous rodents in the genusNeotoma, we quantify how geography, diet, and host genetics, alongside neutral processes, influence microbiome structure and stability under natural and captive conditions. Using bacterial and plant metabarcoding, we first characterized dietary and microbiome compositions for animals from 25 populations, representing seven species from 19 sites across the southwestern United States. We then brought wild animals into captivity, reducing the influence of environmental variation. In nature, geography, diet, and phylogeny collectively explained ∼50% of observed microbiome variation. Diet and microbiome diversity were correlated, with different toxin-enriched diets selecting for distinct microbial symbionts. Although diet and geography influenced natural microbiome structure, the effects of host phylogeny were stronger for both wild and captive animals. In captivity, gut microbiomes were altered; however, responses were species specific, indicating again that host genetic background is the most significant predictor of microbiome composition and stability. In captivity, diet effects declined and the effects of host genetic similarity increased. By bridging a critical divide between studies in wild and captive animals, this work underscores the extent to which genetics shape microbiome structure and stability in closely related hosts.

    

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5. Eristalis flower flies can be mechanical vectors of the common trypanosome bee parasite, Crithidia bombi

   https://doi.org/10.1038/s41598-021-95323-w  

   Davis, Abby E. ; Deutsch, Kaitlin R. ; Torres, Alondra M. ; Mata Loya, Mesly J. ; Cody, Lauren V. ; Harte, Emma ; Sossa, David ; Muñiz, Paige A. ; Ng, Wee Hao ; McArt, Scott H. ( December 2021 , Scientific Reports)

   Flowers can be transmission platforms for parasites that impact bee health, yet bees share floral resources with other pollinator taxa, such as flies, that may be hosts or non-host vectors (i.e., mechanical vectors) of parasites. Here, we assessed whether the fecal-orally transmitted gut parasite of bees,Crithidia bombi, can infectEristalis tenaxflower flies. We also investigated the potential for two confirmed solitary bee hosts ofC. bombi,Osmia lignariaandMegachile rotundata, as well as two flower fly species,Eristalis arbustorumandE. tenax,to transmit the parasite at flowers. We found thatC. bombidid not replicate (i.e., cause an active infection) inE. tenaxflies. However, 93% of inoculated flies defecated liveC. bombiin their first fecal event, and all contaminated fecal events containedC. bombiat concentrations sufficient to infect bumble bees. Flies and bees defecated inside the corolla (flower) more frequently than other plant locations, and flies defecated at volumes comparable to or greater than bees. Our results demonstrate thatEristalisflower flies are not hosts ofC. bombi, but they may be mechanical vectors of this parasite at flowers. Thus, flower flies may amplify or diluteC. bombiin bee communities, though current theoretical work suggests that unless present in large populations, the effects of mechanical vectors will be smaller than hosts.

    

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