Biology subjects

Loker, E. S.

Publications and source records attributed to Loker, E. S..

6 recordsLinked to original sources

bioRxiv2024

Immune targets for schistosomiasis control identified by a genome-wide association study of East African snail vectors

Schistosomiasis, afflicting >260 million people worldwide, could be controlled by preventing infection of freshwater snail vectors. Intestinal schistosomiasis, caused by Schistosoma mansoni, occurs predominantly in Sub-Saharan Africa and is vectored by Biomphalaria sudanica and related Biomphalaria species. Despite their importance in transmission, very little genomic work has been initiated in African snails, thus hindering development of novel control strategies. To identify genetic factors influencing snail resistance to schistosomes, we performed a pooled genome-wide association study (pooled-GWAS) on the offspring of B. sudanica collected from a persistent hotspot of schistosomiasis in Lake Victoria, Kenya, and exposed to sympatric S. mansoni. Results of the pooled-GWAS were used to develop an amplicon panel to validate candidate loci by genotyping individual snails. This validation revealed two previously uncharacterized, evolutionarily dynamic regions, SudRes1 and SudRes2, that were significantly associated with resistance. SudRes1 includes receptor-like protein tyrosine phosphatases and SudRes2 includes a class of leucine-rich repeat-containing G-protein coupled receptors, both comprising diverse extracellular binding domains, suggesting roles in pathogen recognition. No loci previously tied to schistosome resistance in other snail species showed any association with compatibility suggesting that loci involved in the resistance of African vectors differ from those of neotropical vectors. Beyond these two loci, snail ancestry was strongly correlated with schistosome compatibility, indicating the importance of population structure on transmission dynamics and infection risk. These results provide the first detail of the innate immune system of the major schistosome vector, B. sudanica, informing future studies aimed at predicting and manipulating vector competence. Significance StatementAlthough schistosomiasis-associated morbidity and mortality have reduced significantly due to chemotherapy, interruption of transmission, a WHO goal, requires complimentary novel snail vector-focused interventions. We performed a genome-wide association study of snails exposed to schistosomes in an endemic area of high transmission in Kenya. We identified two snail genomic regions that were associated with snail immunity to schistosomes, and which had not previously been tied to parasite infection. The characterized protein structures are plausibly consistent with a role in host-parasite interaction. Therefore, they are new, potential targets for schistosomiasis control. These results show the need for focused research on transmission-relevant vectors and their genetic variants that could have a large impact on schistosome transmission dynamics and human health risk.

genomics↗

bioRxiv2023

The genome and transcriptome of the snail Biomphalaria sudanica s.l.: Immune gene diversification and highly polymorphic genomic regions in an important African vector of Schistosoma mansoni

BackgroundControl and elimination of schistosomiasis is an arduous task, with current strategies proving inadequate to break transmission. Exploration of genetic approaches to interrupt Schistosoma mansoni transmission, the causative agent for human intestinal schistosomiasis in sub-Saharan Africa and South America, has led to genomic research of the snail vector hosts of the genus Biomphalaria. Few complete genomic resources exist, with African Biomphalaria species being particularly underrepresented despite this being where the majority of S. mansoni infections occur. Here we generate and annotate the first genome assembly of Biomphalaria sudanica sensu lato, a species responsible for S. mansoni transmission in lake and marsh habitats of the African Rift Valley. Supported by whole-genome diversity data among five inbred lines, we describe orthologs of immune-relevant gene regions in the South American vector B. glabrata and present a bioinformatic pipeline to identify candidate novel pathogen recognition receptors (PRRs). ResultsDe novo genome and transcriptome assembly of inbred B. sudanica originating from the shoreline of Lake Victoria (Kisumu, Kenya) resulted in a haploid genome size of [~]944.2 Mb (6732 fragments, N50=1.067 Mb), comprising 23,598 genes (BUSCO=93.6% complete). The B. sudanica genome contains orthologues to all described immune genes/regions tied to protection against S. mansoni in B. glabrata. The B. sudanica PTC2 candidate immune genomic region contained many PRR-like genes across a much wider genomic region than has been shown in B. glabrata, as well as a large inversion between species. High levels of intra-species nucleotide diversity were seen in PTC2, as well as in regions linked to PTC1 and RADres orthologues. Immune related and putative PRR gene families were significantly over-represented in the sub-set of B. sudanica genes determined as hyperdiverse, including high extracellular diversity in transmembrane genes, which could be under pathogen-mediated balancing selection. However, no overall expansion in immunity related genes were seen in African compared to South American lineages. ConclusionsThe B. sudanica genome and analyses presented here will facilitate future research in vector immune defense mechanisms against pathogens. This genomic/transcriptomic resource provides necessary data for the future development of molecular snail vector control/surveillance tools, facilitating schistosome transmission interruption mechanisms in Africa.

genomics↗

bioRxiv2022

Bulinus snails in the Lake Victoria Basin in Kenya: systematics and their role as hosts for schistosomes

The planorbid gastropod genus Bulinus consists of 38 species that vary in their ability to vector Schistosoma haematobium (the causative agent of human urogenital schistosomiasis), other Schistosoma species, and non-schistosome trematodes infecting animals. Relying on sequence-based identifications of bulinids (partial cox1 and 16S) and Schistosoma (cox1 and ITS), we examined Bulinus species in the Lake Victoria Basin in Kenya for naturally acquired infections with Schistosoma species. We collected 6,133 bulinids from 11 sites between 2014-2021, 226 (3.7%) of which harbored Schistosoma infections. We found 4 Bulinus taxa from Lake Victoria (B. truncatus, B. tropicus, B. ugandae, and B. cf. transversalis), and an additional 4 from other habitats (B. globosus, B. productus, B. forskalii, and B. scalaris). S. haematobium infections were found in B. globosus and B. productus (with infections in the former predominating) whereas S. bovis infections were identified in B. globosus, B. productus, B. forskalii, and B. ugandae. No nuclear/mitochondrial discordance potentially indicative of S. haematobium/S. bovis hybridization was detected. We highlight the presence of Bulinus ugandae as a distinct lake-dwelling taxon closely related to B. globosus yet, unlike all other members of the B. africanus species group, is likely not a vector for S. haematobium, though it does exhibit susceptibility to S. bovis. Other lake-dwelling bulinids also lacked S. haematobium infections, supporting the possibility that they all lack compatibility with local S. haematobium, thereby preventing widespread transmission of urogenital schistosomiasis in the lakes waters. We support B. productus as a distinct species from B. nasutus, B. scalaris as distinct from B. forskalii, and add further evidence for a B. globosus species complex with three lineages represented in Kenya alone. This study serves as an essential prelude for investigating why these patterns exist and whether the underlying biological mechanisms may be exploited for the purpose of limiting schistosome transmission. Author SummarySchistosomiasis is a neglected tropical disease caused by members of the genus Schistosoma. Every schistosome species is dependent on a particular species, or array of species, of intermediate snail host(s) for transmission. In the Lake Victoria Basin in Kenya, two related schistosome species (Schistosoma haematobium and Schistosoma bovis) utilize multiple species within the genus Bulinus as intermediate hosts. Discerning which bulinid species vector the agent of human schistosomiasis, which are vectors for bovine schistosomiasis, which are vectors for both, and identifying the habitats for each, is critical to understanding local transmission patterns. Confident snail species identification is a prerequisite for implicating a snail in transmission. Certain closely related bulinids cannot be confidently distinguished between using morphological criteria and previously published misidentifications have obscured the literature regarding where human schistosomiasis transmission occurs. This study used sequence-based methods to identify local bulinid species and to identify schistosomes from infected snails. Our results implicate two bulinid species in the transmission of S. haematobium and four species in the transmission of S. bovis. Both S. haematobium associated species were found exclusively in streams and dams in the Lake Victoria Basin thereby seemingly keeping the shores of Lake Victoria free of S. haematobium transmission.

molecular biology↗

bioRxiv2019

Systematics and geographical distribution of Galba species, a group of cryptic and worldwide freshwater snails

Cryptic species can present a significant challenge to the application of systematic and biogeographic principles, especially if they are invasive or transmit parasites or pathogens. Detecting cryptic species requires a pluralistic approach in which molecular markers facilitate the detection of coherent taxonomic units that can then be analyzed using various traits (e.g., internal morphology) and crosses. In asexual or self-fertilizing species, the latter criteria are of limited use. We studied a group of cryptic freshwater snails (genus Galba) from the family Lymnaeidae that have invaded almost all continents, reproducing mainly by self-fertilization and transmitting liver flukes to humans and livestock. We aim to clarify the systematics, distribution and phylogenetic relationships of these species with an integrative approach that includes morphology (shell and reproductive anatomy), molecular markers, wide-scale sampling across America, and data retrieved from GenBank (to include Old World samples). Our phylogenetic analysis suggests that the genus Galba originated ca. 22 Myr ago and today comprises six clusters of species. Four of these clusters (G. truncatula, G. cubensis/viator, G. humilis and G. schirazensis) are morphologically cryptic and constitute species or species complexes with wide variation in their genetic diversity, geographic distribution and invasiveness. The other two clusters constitute a single species or a species complex (Galba cousini/meridensis) that demonstrate more geographically restricted distributions and exhibit an alternative morphology more phylogenetically derived than the cryptic one. Further genetic studies are required to clarify the status of both G. cousini/meridensis and G. cubensis/viator. We emphasize that no Galba species should be identified without molecular markers and that additional sampling is required, especially in North America, Eurasia and Africa to clarify remaining questions in systematics and biogeography. We also discuss several hypotheses that can explain crypsis in Galba, such as convergence and morphological stasis, and hypothesize a role for stabilizing selection in amphibious (rather than strictly freshwater) habitats.

evolutionary biology↗

bioRxiv2018

The in vivo transcriptome of Schistosoma mansoni in two prominent vector species, Biomphalaria pfeifferi and B. glabrata

BackgroundThe full scope of the genes expressed by schistosomes during intramolluscan development has yet to be characterized. Understanding the gene products deployed by larval schistosomes in their snail hosts will provide insights into their establishment, maintenance, asexual reproduction, ability to castrate their hosts, and their prolific production of human-infective cercariae. Using the Illumina platform, the intramolluscan transcriptome of Schistosoma mansoni was investigated in field-derived specimens of the prominent vector species Biomphalaria pfeifferi at 1 and 3 days post infection (d) and from snails shedding cercariae. These S. mansoni samples were derived from the same snails used in our complementary B. pfeifferi transcriptomic study. We supplemented this view with microarray analyses of S. mansoni from B. glabrata at 2d, 4d, 8d, 16d, and 32d.\n\nPrincipal FindingsTranscripts representing at least 7,740 (66%) of known S. mansoni genes were expressed during intramolluscan development, with the greatest number expressed in snails shedding cercariae. Many transcripts were constitutively expressed throughout development featuring membrane transporters, and metabolic enzymes involved in protein and nucleic acid synthesis and cell division. Several proteases and protease inhibitors were expressed at all stages, including some proteases usually associated with cercariae. Transcripts associated with G-protein coupled receptors, germ cell perpetuation, and stress responses and defense were well represented. We noted transcripts homologous to planarian anti-bacterial factors, several neural development or neuropeptide transcripts including neuropeptide Y, and receptors that may be associated with schistosome germinal cell maintenance and that could also impact host reproduction. In at least one snail the presence of larvae of another digenean species (an amphistome) was associated with repressed S. mansoni transcriptional activity.\n\nConclusions/SignificanceThis in vivo study, particularly featuring field-derived snails and schistosomes, provides a distinct view from previous studies of development of cultured intramolluscan stages from lab-maintained organisms. We found many highly represented transcripts with suspected or unknown functions, with connection to intramolluscan development yet to be elucidated.\n\nAUTHOR SUMMARYSchistosoma mansoni is one of the most important schistosome species causing the neglected tropical disease human intestinal schistosomiasis. By focusing on S. mansoni in vivo with its broadly distributed sub-Saharan African snail intermediate host, Biomphalaria pfeifferi, we uncover new insights and basic knowledge of this host-parasite relationship that are critical for understanding schistosomiasis transmission. We show that in vivo studies, particularly using field-derived specimens, provides a distinct view from the uniformed transcriptional responses traditionally seen from in vitro studies on S. mansoni and Biomphalaria snails. With the growing consensus that we need to supplement chemotherapy with other control methods, understanding how S. mansoni interacts with its obligatory snail host becomes integral for future planning of control programs. The data provided within provides specific analysis on how the schistosomes successfully protect themselves from host defenses and the necessary transcriptional responses required for its amplifying asexual proliferation that result in human-infective cercariae.

molecular biology↗

bioRxiv2018

Transcriptional responses of Biomphalaria pfeifferi and Schistosoma mansoni following exposure to niclosamide, with evidence for a synergistic effect on snails following exposure to both stressors

BackgroundSchistosomiasis is one of the worlds most common NTDs. Successful control operations often target snail vectors with the molluscicide niclosamide. Little is known about how niclosamide affects snails, including for Biomphalaria pfeifferi, the most important vector for Schistosoma mansoni in Africa. We used Illumina technology to explore how field-derived B. pfeifferi, either uninfected or harboring cercariae-producing S. mansoni sporocysts, respond to a sublethal exposure of niclosamide. This study afforded the opportunity to determine if snails respond differently to biotic or abiotic stressors, and if they reserve unique responses for when presented with both stressors in combination. We also examined how sporocysts respond when their snail host is exposed to niclosamide.\n\nPrincipal FindingsCercariae-producing sporocysts within snails exposed to niclosamide express ~68% of the genes in the S. mansoni genome, as compared to 66% expressed by intramolluscan stages of S. mansoni in snails not exposed to niclosamide. Niclosamide does not disable sporocysts nor does it seem to provoke from them distinctive responses associated with detoxifying a xenobiotic. For B. pfeifferi, niclosamide treatment alone increases expression of several features not up-regulated in infected snails including particular cytochrome p450s and heat shock proteins, glutathione-S-transferases, antimicrobial factors like LBP/BPI and protease inhibitors, and also provokes strong down regulation of proteases. Exposure of infected snails to niclosamide resulted in numerous up-regulated responses associated with apoptosis along with down-regulated ribosomal and defense functions, indicative of a distinctive, compromised state not achieved with either stimulus alone.\n\nConclusions/SignificanceThis study helps define the transcriptomic responses of an important and under-studied schistosome vector to S. mansoni sporocysts, to niclosamide, and to both in combination. It suggests the response of S. mansoni sporocysts to niclosamide is minimal and not reflective of a distinct repertoire of genes to handle xenobiotics while in the snail host. It also offers new insights for how niclosamide affects snails.\n\nAuthorS SummarySchistosomaisis control programs often employ the use of chemical molluscicides, such as niclosamide, to control the obligatory intermediate snail hosts. Despite its widespread use, we know little about how niclosamide affects snails like Biomphalaria pfeifferi, the most important vector Schistosoma mansoni in Africa. By sequencing the transcriptomes of uninfected and S. mansoni-infected B. pfeifferi exposed to niclosamide, we analyze the snails response to both biotic and abiotic stressors. We can also examine the response of S. mansoni to niclosamide exposure during intramolluscan development. Biomphalaria pfeifferi snails exposed only to niclosamide showed unique up-regulation of stress and defense-related transcripts not seen in snails infected with a biotic, like S. mansoni infection, alone. Schistosoma mansoni-infected B. pfeifferi exposed to niclosamide were clearly unable to regulate normal metabolic and detoxification processes. Cercariae-producing sporocysts within snails exposed to niclosamide are largely unaffected and continue to produce transcripts required for cercariae production.

molecular biology↗