Chromosome-level genome assembly of Oncomelania hupensis: the intermediate snail host of Schistosoma japonicum

Background Schistosoma japonicum is a parasitic flatworm that causes human schistosomiasis, which is a significant cause of morbidity in China, the Philippines and Indonesia.Oncomelania hupensis (Gastropoda: Pomatiopsidae) is the unique intermediate host ofS. japonicum. A complete genome sequence ofO. hupensis will enable the fundamental understanding of snail biology as well as its co-evolution with theS. japonicum parasite. Assembling a high-quality reference genome ofO. hupehensis will provide data for further research on the snail biology and controlling the spread ofS. japonicum.Methods The draft genome was de novo assembly using the long-read sequencing technology (PacBio Sequel II) and corrected with Illumina sequencing data. Then, using Hi-C sequencing data, the genome was assembled at the chromosomal level. CAFE was used to do analysis of contraction and expansion of the gene family and CodeML module in PAML was used for positive selection analysis in protein coding sequences.Results A total length of 1.46 Gb high-qualityO. hupensis genome with 17 unique full-length chromosomes (2n = 34) of the individual including a contig N50 of 1.35 Mb and a scaffold N50 of 75.08 Mb. Additionally, 95.03% of these contig sequences were anchored in 17 chromosomes. After scanning the assembled genome, a total of 30,604 protein-coding genes were predicted. Among them, 86.67% were functionally annotated. Further phylogenetic analysis revealed thatO. hupensis was separated from a common ancestor ofPomacea canaliculata andBellamya purificata approximately 170 million years ago. Comparing the genome ofO. hupensis with its most recent common ancestor, it showed 266 significantly expanded and 58 significantly contracted gene families (P < 0.05). Functional enrichment of the expanded gene families indicated that they were mainly involved with intracellular, DNA-mediated transposition, DNA integration and transposase activity.Conclusions Integrated use of multiple sequencing technologies, we have successfully constructed the genome at the chromosomal-level ofO. hupensis. These data will not only provide the compressive genomic information, but also benefit future work on population genetics of this snail as well as evolutional studies betweenS. japonicum and the snail host.

First records of Triatoma rubrofasciata(De Geer,1773)(Hemiptera,Reduviidae)in Foshan,Guangdong Province,Southern China

Background:Triatomines,also known as kissing bugs,which are found throughout the world and especially in Latin America,are well known natural vectors that transmit American trypanosomiasis,also called Chagas disease.In China,the presence of two species of Triatoma(Triatoma rubrofasciata and T.sinica)was recorded in the past.Due to the growing population and the increasing risk of the global spread of Chagas disease,triatomines became a potential public health nuisance,and in 2016,we started monitoring triatomine activities in southern China.Methods:Triatomine specimens were collected by the National Institute of Parasitic Diseases,Chinese Center for Disease Control and Prevention,and identified by their morphological characteristics under a dissecting microscope.In addition to morphological analysis,the genomic DNA of the specimens was extracted,and the mitochondrial 16S rRNA,the cytochrome b(CytB)gene and the nuclear ribosomal 28S rRNA gene were PCRamplified to analyze and confirm the species genetically.Results:One female adult insect and one male adult insect were collected in a dwelling in the rural area of Shunde County,Foshan City,Guangdong Province,China(22°42′44.63″N,113°08′45.34″E).The results from the morphological and genetic analyses indicated that these triatomines were T.rubrofasciata.Conclusions:This is the first time that the occurrence of T.rubrofasciata has been confirmed in Foshan City,Guangdong Province in southern China.Further studies are needed to reach a clearer understanding of the ecology of this species of triatomine,since it has been found to be naturally infected by Trypanosoma cruzi and T.conorhini and there is evidence of its domiciliation capabilities.

Small-scale spatial analysis of intermediate and definitive hosts of Angiostrongylus cantonensis

Background:Angiostrongyliasis is a food-borne parasitic zoonosis.Human infection is caused by infection with the third-stage larvae of Angiostrongylus cantonensis.The life cycle of A.cantonensis involves rodents as definitive hosts and molluscs as intermediate hosts.This study aims to investigate on the infection status and characteristics of spatial distribution of these hosts,which are key components in the strategy for the prevention and control of angiostrongyliasis.Methods:Three villages from Nanao Island,Guangdong Province,China,were chosen as study area by stratified random sampling.The density and natural infection of Pomacea canaliculata and various rat species were surveyed every three months from December 2015 to September 2016,with spatial correlations of the positive P.canaliculata and the infection rates analysed by ArcGIS,scan statistics,ordinary least squares(OLS)and geographically weighted regression(GWR)models.Results:A total of 2192 P.canaliculata specimens were collected from the field,of which 1190 were randomly chosen to be examined for third-stage larvae of A.cantonensis.Seventy-two Angiostrongylus-infected snails were found,which represents a larval infection rate of 6.1%(72/1190).In total,110 rats including 85 Rattus norvegicus,10 R.flavipectus,one R.losea and 14 Suncus murinus were captured,and 32 individuals were positive(for adult worms),representing an infection rate of 29.1%of the definitive hosts(32/110).Worms were only found in R.norvegicus and R.flavipectus,representing a prevalence of 36.5%(31/85)and 10%(1/10),respectively in these species,but none in R.losea and S.murinus,despite testing as many as 32 of the latter species.Statistically,spatial correlation and spatial clusters in the spatial distribution of positive P.canaliculata and positive rats existed.Most of the spatial variability of the host infection rates came from spatial autocorrelation.Nine spatial clusters with respect to positive P.canaliculata were identified,but only two correlated to infection rates.The results show that corrected Akaike information criterion,R2,R2 adjusted andσ2 in the GWR model were superior to those in the OLS model.Conclusions:P.canaliculata and rats were widely distributed in Nanao Island and positive infection has also been found in the hosts,demonstrating that there was a risk of angiostrongyliasis in this region of China.The distribution of positive P.canaliculata and rats exhibited spatial correlation,and the GWR model had advantage over the OLS model in the spatial analysis of hosts of A.cantonensis.

Morphological and molecular characterization of invasive Biomphalaria straminea in southern China

Background:Schistosomiasis is a common parasitic disease designated as a neglected tropical disease by the World Health Organization.Schistosomiasis mansoni is a form of the disease that is caused by the digenean trematode Schistosoma mansoni,transmitted through Biomphalaria spp.as an intermediate host.Biomphalaria was introduced to Hong Kong,China in aquatic plants shipments coming from Brazil and the snail rapidly established its habitats in southern China.Earlier studies of Biomphalaria spp.introduced to southern China identified the snails as Biomphalaria straminea,one of the susceptible species implicated in S.mansoni transmission in South America.However,recent molecular investigations also indicated the presence of another South American species,B.kuhniana,which is refractory to infection.As such,it is important to identify accurately the species currently distributed in southern China,especially with emerging reports of active S.mansoni infections in Chinese workers returning from Africa.Methods:We combined morphological and molecular taxonomy tools to precisely identify Biomphalaria spp.distributed in Guangdong Province,southern China.In order to clearly understand the molecular profile of the species,we constructed a phylogeny using mtDNA data(COI and 16S rRNA sequences)from six populations of Biomphalaria spp.from Shenzhen City in Guangdong Province.In addition,we examined the external morphology of the shell and internal anatomy of the reproductive organs.Results:Both morphological and molecular evidences indicated a close affinity between Biomphalaria spp.populations from Guangdong and B.straminea from Brazil.The shell morphology was roughly identical in all the populations collected with rounded whorls on one side and subangulated on the other,a smooth periphery,an egg-shaped aperture bowed to one side,and a deep umbilicus.The shape and number of prostate diverticula(ranged from 11.67 to 17.67)in Guangdong populations supports its close affinity to B.straminea rather than B.kuhniana.Molecular analysis did not conflict with morphological analysis.Little genetic differentiation was observed within Biomphalaria populations collected.Phylogenetic analysis of COI and 16S rRNA haplotypes from snails collected and B.straminea sequences from Brazil and China using Bayesian inference revealed that Guangdong populations were clustered in one clade with B.straminea from Hong Kong of China and B.straminea from Brazil indicating their close affinity to each other.Conclusions:Data obtained in the current study clearly show that the populations of Biomphalaria spp.investigated are B.straminea,and we assume that those snails were either introduced via passive dispersal from Hong Kong of China or as a result of multiple introduction routes from Brazil.

Differentiating snail intermediate hosts of Schistosoma spp.using molecular approaches:fundamental to successful integrated control mechanism in Africa

Background:Snail intermediate hosts play active roles in the transmission of snail-borne trematode infections in Africa.A good knowledge of snail-borne diseases epidemiology particularly snail intermediate host populations would provide the necessary impetus to complementing existing control strategy.Main body:This review highlights the importance of molecular approaches in differentiating snail hosts population structure and the need to provide adequate information on snail host populations by updating snail hosts genome database for Africa,in order to equip different stakeholders with adequate information on the ecology of snail intermediate hosts and their roles in the transmission of different diseases.Also,we identify the gaps and areas where there is need for urgent intervention to facilitate effective integrated control of schistosomiasis and other snail-borne trematode infections.Conclusions:Prioritizing snail studies,especially snail differentiation using molecular tools will boost disease surveillance and also enhance efficient schistosomaisis control programme in Africa.