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PLoS One
2009 Aug 04;48:e6494. doi: 10.1371/journal.pone.0006494.
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Genome-wide transcriptional response of Silurana (Xenopus) tropicalis to infection with the deadly chytrid fungus.
Rosenblum EB, Poorten TJ, Settles M, Murdoch GK, Robert J, Maddox N, Eisen MB.
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Emerging infectious diseases are of great concern for both wildlife and humans. Several highly virulent fungal pathogens have recently been discovered in natural populations, highlighting the need for a better understanding of fungal-vertebrate host-pathogen interactions. Because most fungal pathogens are not fatal in the absence of other predisposing conditions, host-pathogen dynamics for deadly fungal pathogens are of particular interest. The chytrid fungus Batrachochytrium dendrobatidis (hereafter Bd) infects hundreds of species of frogs in the wild. It is found worldwide and is a significant contributor to the current global amphibian decline. However, the mechanism by which Bd causes death in amphibians, and the response of the host to Bd infection, remain largely unknown. Here we use whole-genome microarrays to monitor the transcriptional responses to Bd infection in the model frog species, Silurana (Xenopus) tropicalis, which is susceptible to chytridiomycosis. To elucidate the immune response to Bd and evaluate the physiological effects of chytridiomycosis, we measured gene expression changes in several tissues (liver, skin, spleen) following exposure to Bd. We detected a strong transcriptional response for genes involved in physiological processes that can help explain some clinical symptoms of chytridiomycosis at the organismal level. However, we detected surprisingly little evidence of an immune response to Bd exposure, suggesting that this susceptible species may not be mounting efficient innate and adaptive immune responses against Bd. The weak immune response may be partially explained by the thermal conditions of the experiment, which were optimal for Bd growth. However, many immune genes exhibited decreased expression in Bd-exposed frogs compared to control frogs, suggesting a more complex effect of Bd on the immune system than simple temperature-mediated immune suppression. This study generates important baseline data for ongoing efforts to understand differences in response to Bd between susceptible and resistant frog species and the effects of chytridiomycosis in natural populations.
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Figure 1. Summary of significant changes in gene expression between Bd-exposed and naïve control frogs.Numbers of genes with increased and decreased expression in infected frogs are shown for the skin (the site of infection) and the liver (an immunologically and physiologically important organ). Data are summarized for two time-points: early (3 days after initial exposure to Bd) and late (when clinical symptoms of chytridiomycosis are evident).
Figure 2. Validation of chip-based gene expression patterns with RT-PCR.Average expression is shown for several genes of interest relative to an endogenous control (beta actin). Mean and standard deviation are shown for biological replicates. In all cases RT-PCR results were in the same direction as array-based results and of comparable (or greater) magnitude.
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