Studies on the relationship between Schistosoma and their intermediate hosts. V. The genus Bulinus and Schistosoma bovis from Iringa, Tanzania

The relationship between an isolate of Schistosoma bovis from Iringa, Tanzania, and various species of the host snail genus Bulinus from East Africa was studied using the total cercarial production per 100 exposed snails over a period of 4 weeks following patency as an index of the compatibility. All populations of Bulinus forskalii and B. africanus tested exhibited a high level of susceptibility while the populations of B. truncatus and B. globosus tested were either refractory or of low to moderately low susceptibility. All populations of B. abyssinicus, B. canescens, B. nasutus and B. tropicus tested were refractory. It is suggested that B. africanus is the most important host snail for S. bovis in East Africa, that B. forskalii at least locally may contribute significantly to the transmission and that B. truncatus and B. globosus only play a limited role in the transmission.     

Identification of snails within the Bulinus africanus group from East Africa by multiplex SNaPshot trade mark analysis of single nucleotide polymorphisms within the cytochrome oxidase subunit I

Identification of populations of Bulinus nasutus and B. globosus from East Africa is unreliable using characters of the shell. In this paper, a molecular method of identification is presented for each species based on DNA sequence variation within the mitochondrial cytochrome oxidase subunit I (COI) as detected by a novel multiplexed SNaPshotTM assay. In total, snails from 7 localities from coastal Kenya were typed using this assay and variation within shell morphology was compared to reference material from Zanzibar. Four locations were found to contain B. nasutus and 2 locations were found to contain B. globosus. A mixed population containing both B. nasutus and B. globosus was found at Kinango. Morphometric variation between samples was considerable and UPGMA cluster analysis failed to differentiate species. The multiplex SNaPshotTM assay is an important development for more precise methods of identification of B. africanus group snails. The assay could be further broadened for identification of other snail intermediate host species.     

Bulinus guernei (Mollusca: Gastropoda) of West Africa: taxonomic status and role as host for schistosomes

The basommatophoran freshwater snail Bulinus guernei (Dautzenberg, 1890; type-locality in Sénégal, West Africa) has been treated as a full species, subspecies of B. truncatus (Audouin) or synonym of the latter. Improved knowledge of B. guernei is desirable as this snail has been implicated as a host in Senegambia for Schistosoma haematobium , the blood parasite causing human urinary schistosomiasis (bilharzia). Snail samples from 18 localities were studied: from The Gambia (3), Sénégal (9), Mali (1), Mauritania (1) and, to represent B. truncatus , Morocco (2) and Egypt (2), Characters investigated were morphological (shell, radula and certain soft parts), biochemical (egg proteins and enzymes from body tissue extracts) and cytological (chromosome number). Multivariate analyses (principal components analysis and canonical variate analysis) were performed on data from the shell and radula. Comparison with the holotype shell of B. guernei confirmed that this taxon is represented by recently collected specimens from Senegambia. Variation in the total material examined did not appear to warrant making any taxonomic subdivision, and thus the taxa guernei and truncatus appear to be conspecific. Further sampling is desirable, particularly to determine the possible value of variation in the enzyme HBDH in defining infraspecific units. Experimental infection showed some individual B. guernei to be a compatible host for 5. haematobium (isolate from Sudan; transmitted locally by B. truncatus ). Snails identified as B. guernei apparently have little importance in the transmission of S. haematobium in the Sénégal river basin at present, but such snails could constitute a health hazard if a compatible form of the parasite were to be introduced into this region of Africa.     

Schistosoma haematobium in Lake Malaŵi: susceptibility and molecular diversity of the snail hosts Bulinus globosus and B. nyassanus

Intermediate hosts of Schistosoma haematobium, the causative agent of urinary schistosomiasis, in Lake Mala?i include: Bulinus globosus, a member of the B. africanus group and B. nyassanus, a diploid member of the B. truncatus/tropicus species complex. We compared genetic variability between isolates of S. haematobium from the southern part of the lake (Cape Maclear), where both B. globosus and B. nyassanus play a role as intermediate hosts, and isolates from the northern part, where only B. globosus is host. Data show that the S. haematobium isolates from these two areas of Lake Mala?i cannot be distinguished using nuclear or mitochondrial sequences and are capable of cross-infections.     

The effect of Schistosoma haematobium infection on the growth and fecundity of three sympatric species of bulinid snails

Three species of bulinid snails were monitored to determine the effect of infection with 2 sympatric strains of Schistosoma haematobium on longevity, growth, and reproductive output, from the onset of cercarial production until death. Bulinus senegalensis was least affected by infection, with total fecundity reduced by less than 34%. Infected Bulinus truncatus showed an acceleration in growth accompanied by a 63% reduction in fecundity, although the majority of snails continued to oviposit at a low level. The longest-lived snails in this study, Bulinus globosus, showed decreased growth and survival when infected. In addition a significant proportion of infected individuals of this species failed to oviposit, and those that retained some reproductive capacity produced fewer embryos than controls. Total fecundity of B. globosus was reduced almost 90% by infection with S. haematobium, yet long-term monitoring of individuals showed that reproductive activity recovered when parasite productivity was low. Results from this and similar studies indicate that the time of infection by a trematode in relation to reproductive maturity of the molluscan host is important in determining the subsequent effects on host growth and fecundity.     

BULINUS AFRICANUS-GROUP SPECIES IN WEST AFRICA DIFFERENTIATED BY MORPHOMETRIC ANALYSIS (GASTROPODA, PLANORBIDAE)

In the present study 241 shells from snails belonging to theBulinus africanus-group in West Africa, were examined in a morphometricanalysis including ten characters. Principal component analysisand stepwise discriminant function analysis were used for analysisof the data. It was shown that Bulinus umbilicatus, B. jousseaumeiand B. globosus are clearly distinct on morphometric shell characters.Besides a general differentiation between the three species,the characters necessary for differentiation between them twoby two are found. The knowledge of the geographic distributionof B. umbilicatus and B. jousseaumei is significantly improved. (Received 18 March 1988; accepted 4 May 1988)     

Infectivity of Echinostoma friedi miracidia to different snail species under experimental conditions

The infectivity of Echinostoma friedi (Trematoda: Echinostomatidae) miracidia was studied experimentally in a range of laboratory-reared snails that coexist in the same natural locality, namely Radix peregra, Lymnaea fuscus, L. truncatula (Lymnaeidae), Gyraulus chinensis, Helisoma duryi (Planorbidae) and Physella acuta (Physidae), and snails from different geographical origins acting naturally or experimentally as intermediate hosts of Schistosoma spp., namely Planorbarius metidjensis (from Málaga, Spain), Biomphalaria glabrata (Guadeloupe), B. alexandrina (Egypt) (Planorbidae), Bulinus cernicus (Mauritius), B. globosus (Zambia), B. natalensis (South Africa) and B. truncatus (Niger) (Bulinidae). Six species of snails were found to be susceptible, with the rate of infection ranging from 0 to 36.7%. The highest infection was detected in R. peregra. The low host specificity of E. friedi might have an epidemiological significance as a requisite for a recent establishment in a new geographical area.     

The cercarial output from three Nigerian bulinids infected with two strains of Schistosoma haematobium

The daily cercarial output of two Nigerian strains of Schistosoma haematobium in sympatric Bulinus truncatus, B. globosus and B. senegalensis was measured at weekly intervals from the start of emission to the snails' death. In all cases cercariae were released throughout the life of the host, with no cases of "self cure". Patterns of output through the course of infections in B. truncatus and B. senegalensis were similar to those reported for S. haematobium by other workers, with daily production of cercariae rising to a peak within a few weeks of the onset of shedding, then declining until the host's death. In the longer lived B. globosus production was significantly higher, but declined to very low levels after the initial peak; in some individuals cercarial output remained very low, while others showed a second period of high cercarial emission. The relative compatibility of each host-parasite combination is discussed.     

Species-specific primers discriminate schistosome intermediate hosts: unambiguous PCR diagnosis of Bulinus forskalii group taxa (Gastropoda: Planorbidae)

The morphological definition of taxa has proved difficult within the Bulinus forskalii group, which includes intermediate hosts of medically important Schistosoma species in West Africa. Although B. forskalii and B. senegalensis transmit different schistosome species they are conchologically similar and their distributions overlap. Randomly amplified polymorphic DNA (RAPD) allows differentiation of sibling species in the genus Bulinus, but RAPDs are difficult to standardize, impairing their value as a taxonomic tool. Hence, RAPD products diagnostic for either B. senegalensis or B. forskalii from West Africa were cloned, sequenced and a panel of species-specific primers designed. Sequencing of RAPD products identified a homology in two apparently independent RAPD loci, a problem where RAPDs are indiscriminately scored for phylogenetic analyses. Specificity of primers was confirmed by widespread sampling throughout each species' range. This approach produced a simple, robust, unambiguous PCR-based species identification strategy for this difficult group.     

Transmission of Schistosoma bovis in Mkulwe (Mbozi District, Mbeya region, southern highlands of Tanzania)

Various populations of laboratory bred bulinid snails were exposed to miracidia of Schistosoma bovis from Mbozi. The parasite is naturally transmitted by Bulinus globosus in the area. Laboratory infection revealed a good relationship with B. forskalii and B. globosus from Mbozi and a population of B. forskalii from Dar es Salaam (infection rates 100%, 63.6% and 41.7% respectively). Populations of B. globosus and B. nasutus from Dar es Salaam were refractory. It appears that both snail species (B. globosus and B. forskalii) present in Mbozi district transmit S. bovis.     

A molecular phylogenetic analysis of Bulinus (Gastropoda: Planorbidae) with conserved nuclear genes

Jørgensen, A., Madsen, H., Nalugwa, A., Nyakaana, S., Rollinson, D., Stothard, J. R. & Kristensen, T. K. A molecular phylogenetic analysis of Bulinus (Gastropoda: Planorbidae) with conserved nuclear genes. —Zoologica Scripta, 40, 126–136. Mutational saturation of inspected DNA loci and topological incongruence in the phylogenetic inferences have previously confounded attempts to resolve the evolutionary relationships within the freshwater snail genus Bulinus. Traditionally, the 37 species of Bulinus are placed within the four species groups and the evolutionary divergence between groups is substantial. With an intention to shed new light on species group relationships, the present study was designed to investigate the basal divergences in the phylogeny of Bulinus using highly conserved nuclear genes. The resolved phylogeny inferred that the four species groups of Bulinus were monophyletic and Shimodaira‐Hasegawa topology tests found them to be significantly supported. The Bulinus truncatus/tropicus species complex and Bulinus wrighti (Bulinus reticulatus group) formed a well‐supported sister‐group relationship. The Bulinus africanus species group was the sister‐group to the clade (Bulinus truncatus/tropicus + B. wrighti) with the Bulinus forskalii species group as the sister‐group to these taxa. The sister‐group relationship between Indoplanorbis and Bulinus was non‐significant and the basal clade support of Bulinus improved upon exclusion of Indoplanorbis. The finding of basal long branches of Bulinus species originating from Madagascar strongly suggests the presence of additional cryptic species and an evolutionary scenario influenced by this island’s geological vicariance from the African mainland. Speciation by polyploidy was inferred to have evolved within a clade in the Bulinus truncatus/tropicus species complex. Although the monophyletic status of each species group was firmly supported, it was difficult to establish species group concepts equally across the variations and place this precisely in a specific temporal framework.     

Susceptibility of freshwater snails to the amphistome Calicophoron microbothrium and the influence of the species on susceptibility of Bulinus tropicus to Schistosoma haematobium and Schistosoma mattheei infections

The susceptibility of Bulinus tropicus, B. globosus, Biomphalana pfeifferi, Lymnaea natalensis, and Melanoides tuberculata to Calicophoron microbothrium was examined. Bulinus tropicus had the highest prevalence (65.0%), followed by B. pfeifferi (37.5%), B. globosus (6.8%), and M. tuberculata (5.9%). Lymnaea natalensis was refractory to infection. Bulinus tropicus snails infected with C. microbothrium alone or coinfected with either Schistosoma haematobium or S. mattheei 0, 7, 14, and 21 days after exposure to C. microbothrium produced C. microbothrium cercariae only.     

Host choice and penetration by Schistosoma haematobium miracidia

Schistosome parasites commonly show specificity to their intermediate mollusc hosts and the degree of specificity can vary between parasite strains and geographical location. Here the role of miracidial behaviour in host specificity of Schistosoma haematobium on the islands of Zanzibar is investigated. In choice-chamber experiments, S. haematobium miracidia moved towards Bulinus globosus snail hosts in preference to empty chambers. In addition, miracidia preferred uninfected over patent B. globosus. This preference should benefit the parasite as patent snails are likely to have mounted an immune response to S. haematobium as well as providing poorer resources than uninfected snails. Miracidia also discriminated between the host B. globosus and the sympatric, non-host species Cleopatra ferruginea. In contrast, S. haematobium did not discriminate against the allopatric Bulinus nasutus. Penetration of the host by miracidia was investigated by screening snails 24 h after exposure using polymerase chain reaction (PCR) with S. haematobium specific DraI repeat primers. There was no difference in the frequency of penetration of B. globosus versus B. nasutus. These responses to different snail species may reflect selection pressure to avoid sympatric non-hosts which represent a transmission dead end. The distribution of B. nasutus on Unguja is outside the endemic zone and so there is less chance of exposure to S. haematobium, hence there will be little selection pressure to avoid this non-host snail.     

Molecular phylogenetic investigations of Bulinus (Gastropoda: Planorbidae) in Lake Malawi with comments on the topological incongruence between DNA loci

Freshwater snails of the genus Bulinus, subfamily Bulininae, are widespread in Africa and the Middle East. Several species are intermediate hosts for schistosomes, mammalian blood flukes that cause schistosomiasis. The genus is split into four species groups and without recourse to further molecular DNA studies, phylogenetic relationships within and between species‐groups remain obscure. For example, the Bulinus truncatus/tropicus species complex, which cannot be adequately defined with morphological characters, appears to be monophyletic but increased taxon sampling is warranted for confirmation when this group appears to contain species with limited sequence divergence in either mitochondrial 16S rRNA or Cytochome Oxidase subunit I (COI) loci. In the present study, taxonomic sampling is increased through addition of several taxa, including B. nyassanus and B. succinoides both endemic to Lake Malawi. From DNA analyses, these two species appear basal within the B. truncatus/tropicus species complex which provides an interesting evolutionary insight into its origins. The relationship of the B. truncatus/tropicus species complex with the three other groups, however, is more problematic as incongruence between loci and computational methodologies exist. This phenomenon is likely due to mutational saturation of the COI as evidenced by the transition : transversion ratio which also results in placement of the discoidal Indoplanorbis exustus, a member of the Bulininae, within the ingroup. To resolve these more basal incongruencies between species groups inspection of more evolutionary conserved molecular loci is needed, for example, the nuclear histone H3 gene, and 18S and 28S rDNA. Louise V. Gersdorff Jørgensen, Thomas K. Kristensen and Henry Madsen, The Mandahl‐Barth Research Centre for Biodiversity and Health, DBL — Centre for Health Research and Development, Department of Veterinary Pathobiology, Faculty of Life Sciences, University of Copenhagen, Jægersborg Allé 1D, DK‐2920 Charlottenlund, Denmark. E‐mails: lgj@life.ku.dk , tkk@life.ku.dk , hmad@life.ku.dk J. Russell Stothard, Biomedical Parasitology Division, Department of Zoology, Natural History Museum, Cromwell Road, London SW7 5BD, UK. E‐mail: R.Stothard@nhm.ac.uk     

The role of Chaetogaster limnaei in the dynamics of trematode transmission in natural populations of freshwater snails

Chaetogaster limnaei, an oligochaete was always found infesting Bulinus (Phy.) globosus, B.(B.) forskalii and Lymnaea natalensis in Bo, Sierra Leone, living below the shell or embedded in the mucus of the foot. The worms had no preference for snails with trematode infection. The population was highest in the dry season when fewer snails were infected by trematodes. This is suggested to be a result of the protection the worms give to snails against invading miracidia.     

INBREEDING DEPRESSION,NEUTRAL POLYMORPHISM,AND COPULATORY BEHAVIOR IN FRESHWATER SNAILS: A SELF-FERTILIZATION SYNDROME

This paper examines the consequences of self-fertilization on life-history traits and neutral genetic polymorphism in natural populations of three species of hermaphrodite freshwater snails: Biomphalaria straminea, Bulinus globosus, and the aphallic species Bulinus truncatus. Life-history traits (fecundity, growth, hatching rate, and survival of offspring) are compared under laboratory conditions between isolated (obligatory selfing) and paired (outcrossing possible) snails in one population of B. straminea and B. globosus and two populations of B. truncatus. The genetic polymorphism of the same four populations is analyzed using electrophoretic markers in B. straminea and B. globosus and microsatellite markers in B. truncatus. In B. truncatus and B. straminea, isolated snails have a higher fecundity than paired snails, whereas the contrary is observed in B. globosus. For all populations, no difference in hatching rate and offspring survival is detected between the two treatments. Genetic analyses using microsatellite markers conducted in B. truncatus on progeny of paired snails reveal a high selfing rate in spite of high copulation rates, highlighting the difficulties of obtaining outcrossing in highly selfing snails. The high survival of selfed offspring in B. truncatus and B. straminea indicates that inbreeding depression is limited. The extent of inbreeding depression in B. globosus is less clear. Overall, fitness decrease in this species is limited to fecundity. The extent of allozyme polymorphism is very limited whereas a much higher variability is observed with microsatellites. Biomphalaria straminea and B. truncatus populations are also characterized by very low observed heterozygosities and large heterozygote deficiencies, whereas the B. globosus population does not exhibit such a deficiency. Overall these results allow the definition of a self-fertilization syndrome in hermaphrodite freshwater snails: selfing populations (such as those of B. straminea and B. truncatus studied here) are characterized by high selfing rates in spite of copulations, limited deleterious effects of selfing, limited neutral genetic polymorphism, and large heterozygote deficiencies.     

The Commelinaceae of West Africa: A biosystematic survey

All the known West African taxa are surveyed and their variation and distribution is discussed. Chromosome numbers for most of the taxa are given, in many cases for the first time, and published cytological data for those genera occurring in West Africa is reviewed. Many of the taxa have been studied under cultivation and most under living conditions in the field. The results of these observations are considered in relation to the evolution of new variations and to the taxonomy of the groups concerned. The following new taxa are described: Aneilema beniniense subsp. leonense, C'ommelina diffusa subsp. montana, Floscopa africana subsp. petrophila and Slanfieldiella axillaris. Several nomenclatural changes have been made, the most important being the recognition that the African species Commelina gerrardi C. B. Clarke and C. vogelii C. B. Clarke are conspecific with the American C. erecta Linn, which thus becomes a pantropie species complex. Several species are recorded from West Africa for the first time including: Coleolrype kiurentii K. Schum. Commelina carsonii C. B. Clarke, C. zambesiaca C. B. Clarke, Cyanotis arachnoidea C. B. Clarke, and Palisota lagopus Mildbr. The pattern of variation in the larger species complexes in the genera Aneilema, Commelina, Cyanotis and Floscopa is considered in detail. These and other groups in the family are considered to be in an active state of evolutionary change. The factors which have stimulated these changes and the biological processes by means of which they are brought about are discussed. Attention is drawn to the suitability of this family for evolutionary studies and its value in the teaching of experimental taxonomy in the tropics.     

Developing species-specific primers to identify Bulinus truncatus and Bulinus beccari, the intermediate hosts of Schistosoma haematobium in Saudi Arabia

This work aimed to determine the inter- and intra-specific variations in populations of Bulinus truncatus and Bulinus beccari, the intermediate hosts of Schistosoma haematobium in Saudi Arabia, and to develop species-specific primers to identify these snails as a first step in the development of multiplex PCR for simultaneously identifying the snails and diagnosing its infections in a single step. Two populations of B. truncatus were collected from Asser and Bisha (A and B), and two B. beccari populations were collected from Mahial Asser and Merba (C and D). The snails' genomic DNA was extracted and amplified using 5 different primers. The primers displayed variable intra- and inter-specific differences across the populations. The largest RAPD-PCR fragments were cloned into a vector as a preparatory step for sequencing. Similarity searches for the sequenced cloned inserts revealed no similar sequences in the GenBank database or its associated databases. Specific primers used to target the B. truncatus and B. beccari genomes were designed using the Gene Runner program and based on the DNA sequences obtained from RAPD fragment sequence analyses. Using these primers for specific PCRs resulted in expected single-band PCR products of 536 bp for B. beccari and 478 bp for B. truncatus. These results will be helpful for simultaneously identifying B. truncatus and B. beccari snails and diagnosing S. haematobium infections within the snails using single step multiplex PCR.