Action competitive d'Helisoma duryi,wetherby (Planorbidae: Helisomatinae) a l'egard de divers mollusques vecteurs de trematodoses humaines et animales. II. Biomphalaria pfeifferi,krauss, Bulinus truncatus sericinus,jickeli et limnea natalensis,krauss

The authors studied biological competition between some african snails (Biomphalaria pfeifferi, Bulinus truncatus sericinus, Limnea natalensis) and Helisoma duryi in the laboratory.The results are irregular and less successful than with the closely related species, Biomphalaria glabrata.

avec la collaboration technique de R. THOMASSET     

A high incidence of clustered microsatellite mutations revealed by parent-offspring analysis in the African freshwater snail, <Emphasis Type="BoldItalic">Bulinus forskalii</Emphasis> (Gastropoda,Pulmonata)

Genotyping of 11 microsatellites in 432 offspring from 28 families of the hermaphroditic, freshwater snail Bulinus forskalii detected 10 de novo mutant alleles. This gave an estimated mutation rate of 1.1 × 10^–3 per locus per gamete per generation. There was a trend towards repeat length expansion and, unlike most studies, multi-step mutations predominated, suggesting that the microsatellite mutation process does not conform to a strict stepwise mutation model. Interestingly, the ten mutant alleles appear to have arisen from only six independent germline mutation events within the microsatellite array, with seven of them residing in three mutational clusters. Our results extend observations of clustered microsatellite mutations to another taxonomic group and type of mating system, self-fertile gastropods, and provide compelling evidence of premeiotic germline mutations, a phenomenon that could greatly impact upon our understanding of mutation dynamics but which has received little attention.     

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.     

Snail predation by larvae of Sepedon scapularis Adams (Diptera: Sciomyzidae), a potential biocontrol agent of snail intermediate hosts of schistosomiasis in South Africa

Malacophagous larvae of the fly Sepedon scapularis Adams were shown experimentally to be effective predators of three species of aquatic pulmonate snails tested as prey: Bulinus africanus (Krauss) an important intermediate host of Schistosoma haematobium (Bilharz), Bulinus tropicus (Krauss) and the invasive species Physa acuta Draparnaud. Survival of S. scapularis larvae from instar to instar was negatively affected by the size of prey snails, since larvae tended to be asphyxiated by the mucous secretions of the snails, or by the larval hydrofuge hairs becoming entangled in snail faeces. In experiments to test the choice of S.scapularis larvae for different species and sizes of snails, B.africanus was significantly disfavoured compared with the other two snail species. Small snails (< 3 mm) were more frequently killed by all three larval instars of S.scapularis. First instars killed few, if any, large snails (> 7 mm), whereas second and third instars preyed effectively on all sizes of snails. Third instars killed significantly more snails than younger instars. For larvae offered only one species of snail as prey, the mean total number of snails killed per larva during its entire development was 49 B.africanus, 45 B.tropicus or 34 P.acuta. It is concluded that S.scapularis is potentially useful as a biological control agent for use against indigenous Bulinus and exotic Physa snails.     

Interrelationship of molluscicidal concentration and temperature on the respiration of Bulinus truncatus

The oxygen consumption of Bulinus truncatus subjected to two molluscicides was tested at different temperatures and different concentrations. The most effective combination for field application was discussed.     

Identification of Schistosoma haematobium, S. bovis and S. curassoni by multivariate analysis of cercarial papillae indices

The disposition of cercarial papillae of 68 pre-identified Schistosoma species was established. All the cercariae originated from Africa and Madagascar and were either obtained from natural or experimental infections, and belonged to three species Schistosoma haematobium, S. bovis and S. curassoni. Discriminant analysis was based on nine characters: average values, skewness and kurtosis of three cercarial indices (AD, AL and U) for each sample or isolate. AD, AL correspond respectively to the relative distance between dorsal and lateral papillae. U corresponds to the total number of tail stem papillae. With the exception of two cases of the 68 (one of them corresponding to cercariae shed by a non-African experimentally infected snail), the method enabled discrimination of S. haematobium, S. bovis and S. curassoni.     

Growth and oviposition of the freshwater pulmonate Bulinns tropicus (Gastropoda) reared in the laboratory under reversed, extreme and irregular ultradian light-dark cycles

The effect of irregular photoperiods on various growth parameters and oviposition of a tropical freshwater pulmonate, Bulinus tropicus , has been investigated. Over a period of 6 weeks the growth (measured as an increase in wet weight) of juvenile snails was unaffected by transfer to reversed and to random non-circadian light-dark regimens, although oviposition was considerably retarded in the latter experimental group and its time of onset was delayed relative to the control group held in a 12 h light-12 h dark regimen. Snails exposed to constant light grew more, and their oviposition rate (number of eggs snail^-1 week^-1) was equal to that of the control group, but these two parameters were found to be considerably reduced in snails kept in constant darkness. The food consumption, assimilation and food conversion efficiency of snails in random and reversed light-dark regimens were comparable to the control groups, whereas animals in constant light ate more and those in constant darkness ate less. The weekly mean assimilation rate and food conversion efficiency were greater in the latter group, however.