Search

Of the estimated 55 Hawaiian honeycreepers (subfamily Carduelinae) only 17 species remain, nine of which the International Union for Conservation of Nature considers endangered. Among the most pressing threats to honeycreeper survival is avian malaria, caused by the introduced blood parasite Plasmodium relictum, which is increasing in distribution in Hawai'i as a result of climate change. Preventing further honeycreeper decline will require innovative conservation strategies that confront malaria from multiple angles. Research on mammals has revealed strong connections between gut microbiome composition and malaria susceptibility, illuminating a potential novel approach to malaria control through the manipulation of gut microbiota. One honeycreeper species, Hawai'i 'amakihi (Chlorodrepanis virens), persists in areas of high malaria prevalence, indicating they have acquired some level of immunity. To investigate if avian host-specific microbes may be associated with malaria survival, we characterized cloacal microbiomes and malaria infection for 174 'amakihi and 172 malaria-resistant warbling white-eyes (Zosterops japonicus) from Hawai'i Island using 16S rRNA gene metabarcoding and quantitative polymerase chain reaction. Neither microbial alpha nor beta diversity covaried with infection, but 149 microbes showed positive associations with malaria survivors. Among these were Escherichia and Lactobacillus spp., which appear to mitigate malaria severity in mammalian hosts, revealing promising candidates for future probiotic research for augmenting malaria immunity in sensitive endangered species., (© 2022 John Wiley & Sons Ltd. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

Background: Plasmodium parasites that cause bird malaria occur in all continents except Antarctica and are primarily transmitted by mosquitoes in the genus Culex. Culex quinquefasciatus, the mosquito vector of avian malaria in Hawai'i, became established in the islands in the 1820s. While the deadly effects of malaria on endemic bird species have been documented for many decades, vector-parasite interactions in avian malaria systems are relatively understudied., Methods: To evaluate the gene expression response of mosquitoes exposed to a Plasmodium infection intensity known to occur naturally in Hawai'i, offspring of wild-collected Hawaiian Cx. quinquefasciatus were fed on a domestic canary infected with a fresh isolate of Plasmodium relictum GRW4 from a wild-caught Hawaiian honeycreeper. Control mosquitoes were fed on an uninfected canary. Transcriptomes of five infected and three uninfected individual mosquitoes were sequenced at each of three stages of the parasite life cycle: 24 h post feeding (hpf) during ookinete invasion; 5 days post feeding (dpf) when oocysts are developing; 10 dpf when sporozoites are released and invade the salivary glands., Results: Differential gene expression analyses showed that during ookinete invasion (24 hpf), genes related to oxidoreductase activity and galactose catabolism had lower expression levels in infected mosquitoes compared to controls. Oocyst development (5 dpf) was associated with reduced expression of a gene with a predicted innate immune function. At 10 dpf, infected mosquitoes had reduced expression levels of a serine protease inhibitor, and further studies should assess its role as a Plasmodium agonist in C. quinquefasciatus. Overall, the differential gene expression response of Hawaiian Culex exposed to a Plasmodium infection intensity known to occur naturally in Hawai'i was low, but more pronounced during ookinete invasion., Conclusions: This is the first analysis of the transcriptional responses of vectors to malaria parasites in non-mammalian systems. Interestingly, few similarities were found between the response of Culex infected with a bird Plasmodium and those reported in Anopheles infected with human Plasmodium. The relatively small transcriptional changes observed in mosquito genes related to immune response and nutrient metabolism support conclusions of low fitness costs often documented in experimental challenges of Culex with avian Plasmodium., (© 2022. The Author(s).)

The transcriptional response of hosts to genetically similar pathogens can vary substantially, with important implications for disease severity and host fitness. A low pathogen load can theoretically elicit both high and low host responses, as the outcome depends on both the effectiveness of the host at suppressing the pathogen and the ability of the pathogen to evade the immune system. Here, we investigate the transcriptional response of Eurasian siskins ( Spinus spinus ) to two closely related lineages of the malaria parasite Plasmodium relictum . Birds were infected with either the high-virulent lineage P. relictum SGS1, the low-virulent sister lineage P. relictum GRW4, or sham-injected (controls). Blood samples for RNA sequencing were collected at four time points during the course of infection, totaling 76 transcriptomes from 19 birds. Hosts infected with SGS1 experienced up to 87% parasitemia and major transcriptome shifts throughout the infection, and multiple genes showed strong correlation with parasitemia. In contrast, GRW4-infected hosts displayed low parasitemia (maximum 0.7%) with a minor transcriptional response. We furthermore demonstrate that the baseline gene expression levels of hosts prior to infection were irrelevant as immunocompetence markers, as they could not predict future pathogen load. This study shows that the magnitude of the host transcriptional response can differ markedly from related parasites with different virulence, and it enables a better understanding of the molecular interactions taking place between hosts and parasites.

Parasites often have reduced genomes as their own genes become redundant when utilizing their host as a source of metabolites, thus losing their own de novo production of metabolites. Primate malaria parasites can synthesize vitamin B1 (thiamine) de novo but rodent malaria and other genome-sequenced apicomplexans cannot, as the three essential genes responsible for this pathway are absent in their genomes. The unique presence of functional thiamine synthesis genes in primate malaria parasites and their sequence similarities to bacterial orthologues, have led to speculations that this pathway was horizontally acquired from bacteria. Here we show that the genes essential for the de novo synthesis of thiamine are found also in avian Plasmodium species. Importantly, they are also present in species phylogenetically basal to all mammalian and avian Plasmodium parasites, i.e. Haemoproteus. Furthermore, we found that these genes are expressed during the blood stage of the avian malaria infection, indicating that this metabolic pathway is actively transcribed. We conclude that the ability to synthesize thiamine is widespread among haemosporidians, with a recent loss in the rodent malaria species.

The genomic architecture of organisms, including nucleotide composition, can be highly variable, even among closely-related species. To better understand the causes leading to structural variation in genomes, information on distinct and diverse genomic features is needed. Malaria parasites are known for encompassing a wide range of genomic GC-content and it has long been thought that Plasmodium falciparum, the virulent malaria parasite of humans, has the most AT-biased eukaryotic genome. Here, I perform comparative genomic analyses of the most AT-rich eukaryotes sequenced to date, and show that the avian malaria parasites Plasmodium gallinaceum, P. ashfordi, and P. relictum have the most extreme coding sequences in terms of AT-bias. Their mean GC-content is 21.21, 21.22 and 21.60 %, respectively, which is considerably lower than the transcriptome of P. falciparum (23.79 %) and other eukaryotes. This information enables a better understanding of genome evolution and raises the question of how certain organisms are able to prosper despite severe compositional constraints.

Malaria parasites (Plasmodium spp.) include some of the world's most widespread and virulent pathogens. Our knowledge of the molecular mechanisms these parasites use to invade and exploit their hosts other than in mice and primates is, however, extremely limited. It is therefore imperative to characterize transcriptome-wide gene expression from nonmodel malaria parasites and how this varies across individual hosts. Here, we used high-throughput Illumina RNA sequencing on blood from wild-caught Eurasian siskins experimentally infected with a clonal strain of the avian malaria parasite Plasmodium ashfordi (lineage GRW2). Using a bioinformatic multistep approach to filter out host transcripts, we successfully assembled the blood-stage transcriptome of P. ashfordi. A total of 11 954 expressed transcripts were identified, and 7860 were annotated with protein information. We quantified gene expression levels of all parasite transcripts across three hosts during two infection stages - peak and decreasing parasitemia. Interestingly, parasites from the same host displayed remarkably similar expression profiles during different infection stages, but showed large differences across hosts, indicating that P. ashfordi may adjust its gene expression to specific host individuals. We further show that the majority of transcripts are most similar to the human parasite Plasmodium falciparum, and a large number of red blood cell invasion genes were discovered, suggesting evolutionary conserved invasion strategies between mammalian and avian Plasmodium. The transcriptome of P. ashfordi and its host-specific gene expression advances our understanding of Plasmodium plasticity and is a valuable resource as it allows for further studies analysing gene evolution and comparisons of parasite gene expression., (© 2017 John Wiley & Sons Ltd.)