GENETIC STRUCTURE OF KENYAN POPULATIONS OF BIOMPHALARIA PFEIFFERI (GASTROPODA: PLANORBIDAE)

Biomphalaria Pfeifferi is widespread in Africa, tolerates adiverse array of habitats, and is highly susceptible to schistosomeinfection. As such, it is the most important host of Schistosomamansoni in the Old World. Representatives of Biomphalaria pfeifferifrom twelve localities in southern Kenya were examined usingstarch gel electrophoresis in order to describe the populationstructure. Ten to fifteen loci were resolved in each population.Genotypic frequencies obtained for 10 loci have been used tocalculate Nei' genetic distances. Nine eastern populations wererelatively similar to each other (D 0.016), but divergent fromwestern populations (D 0.178). Two of three western populationswere quite similar (D = 0.001), the third being more divergent(D 0.258). Genotypic frequencies showed a substantial departurefrom Hardy-Weinberg equilibrium, due to a marked deficiencyof heterozygotes. Calculation of F-statistics revealed evidenceof non-random mating as well as considerable differentiationamong localities. Biomphalaria pfeifferi differs from otherBiomphalaria species studied to date in exhibiting evidenceof non-random mating. Higher levels of inbreeding could alterthe pattern of response to selection imposed by parasites suchas Schistosoma mansoni. (Received 18 May 1989; accepted 30 October 1989)     

ALLOZYME VARIATION AMONG POPULATIONS OF BIOMPHALARIA PFEIFFERI (KRAUSS, 1848) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong the freshwater, hermaphroditic snail Biomphalaria pfeifferiin Cameroon. Four of 19 loci studied were polymorphic. The allozymesfor two loci, aspartate aminotransferase-1 (AAT-1) and isocitratedehydrogenase (IDH), showed distribution patterns related toclimatic zones. AAT-1¹⁰⁰ and IDH¹⁰⁰ occurred predominantly inthe Tropical Climatic Zone, located in the northern half ofthe country. AAT-1¹⁴⁰ and IDH⁹⁰ were common in the EquatorialZone. Intrapopulation variations occurred in 5 of 19 populationsampled. Only one population, polymorphic for AAT-1 locus, waspanmictic. The other 4 populations were not in Hardy-Weinbergequilibrium. The deficiency of heterozygotes in those populationsis unusual for a member of the genus Biomphalaria and may bedue to a predominance of self-fertilization. Other explanationsinclude ecological factors or the presence of segregated sub-populationsthat do not outcross. This latter hypothesis is supported bythe total absence of heterozygotes for PGM alleles. (Received 24 August 1989; accepted 14 September 1989)     

ALLOZYME VARIATION AMONG POPULATIONS OF BIOMPHALARIA CAMERUNENSIS (BOETTGER, 1941) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong populations of the freshwater, hermaphroditic snail Biomphalariacamerunensis in Cameroon. Five of 19 loci studied in 15 enzymesystems were polymorphic. Intrapopulation variation was evidentin 8 of 12 populations sampled and heterozygotes were presentin 6 of these. Two populations with large sampled sets werepanmictic. Although results for the remaining 4 populationswith heterozygotes present were inconclusive, our findings supportthe hypothesis that Biomphalaria are out-breeders. The allelesfor two loci, asparate aminotransferase-1 (AAT-1) and 6-phosphogluconatedehydrogenase (6-PGD), showed distribution patterns that suggestthey may be limited by climate or habitat. The slow migratingAAT-1 was the only allele found in the mountainous regions ofthe southwest. This region falls under the Cameroon-type climateand is the wettest region in the country. The faster allelewas restricted to the lowland rain forest of the south. For6-PGD the fast allele was found throughout the range of B. camerunensis,but the slower allele was found only in the mountainous regionswithin the Cameroon-type climate. (Received 27 April 1989; accepted 9 September 1989)     

MANIPULATION OF BIOMPHALARIA GLABRATA (SAY) (GASTROPODA: PLANORBIDAE) HAEMOCYTES IN VITRO

The propensity of molluscan haemocytes to clump irreversiblyupon mixing in suspension, together with their strong adherenceto many substrates, compromises many efforts to obtain plasma-freeor treated cells for subsequent use in experimental protocols.An anti-clumping buffer developed for use with haemocytes fromthe gastropod Lymnaea stagnalis was tested for its effectivenesswith cells from Biom-phalaria glabrata. While we were unableto obtain sufficient numbers of viable haemocytes for subsequentexperimentation after centrifugation, this divalent cation-chelatingbuffer induced rounding of adherent cells monolayered on glass.Thus, cells washed in physiological buffers could subsequentlybe retrieved as monodispersed suspensions. Such haemocytes retainedsome phagocytic activity, with levels of uptake improved whencells were held in buffer with excess divalent cations beforeaddition of target particles. (Received 7 September 1992; accepted 6 January 1993)     

THE EFFECT OF SODIUM CHLORIDE CONCENTRATION ON GROWTH AND EGG LAYING OF HELISOMA DURYI, BIOMPHALARIA ALEXANDRINA AND BULINUS TRUNCATUS (GASTROPODA: PLANORBIDAE)

Helisoma duryi, Biomphalaria alexandrine and Bulinus truncatuswere maintained in media with seven different sodium chlorideconcentrations ranging from 0.6 to 32 mM. Growth of H. duryiwas significantly reduced only in the 32 mM NaCl treatment,while no significant effects were observed in the net reproductiverate or in egg-laying rates. Bulinus truncatus showed optimalgrowth within the range of 1 to 8 mM NaCl and the net reproductiveand egg-laying rates were reduced in the 32 mM NaCl treatment.Growth of B. alexandrina was reduced in the 16 and 32 mM NaCltreatments, while the net reproductive rate was reduced in the16 and 32 mM NaCl treatments, while the net reproductive ratewas reduced in media with more than 2.0 mM NaCl. Hatching ofB. truncatus eggs was not affected by the high NaCl concentrations,while the hatchability of H. duryi eggs was reduced in the 32mM treatment. Survival of newly-hatched H. duryi was best inthe 1 mM NaCl treatment while for B. truncatus survival wasreduced only in the 32 mM treatment. (Received 28 March 1988; accepted 7 April 1989)     

THE BIOMPHALARIA HAVANENSIS COMPLEX (GASTROPODA: PLANORBIDAE) IN CUBA: A MORPHOLOGICAL AND GENETIC STUDY

Conchological, anatomical and genetic characteristics werecompared among several populations of the Biomphalaria havanensis complexfrom Cuba and other localities in the Caribbean region. Two morphometricmeasures (height and diameter) distinguished two closely similarmorphs of the shell. The reproductive system also separated thesemorphs, particularly upon comparison of sizes of the penis sheathand the preputium. The two morphological groups differed in9 to 12 fixed alleles according to population. Further allozymicdata analyzed in the present work confirmed this difference.The results strongly support the assumption that the B. havanensis complexincludes two distinct species: B. havanensis, for which thetype locality is the swampy area near the vicinity of Havana,Cuba and Biomphalaria sp. a species which commonly occurs inCuba and in Dominican Republic. The taxonomic identity of Biomphalariasp. is discussed. (Received 28 January 2000; accepted 15 August 2000)     

THE GROWTH OF PELTODORIS ATROMACULATA BERGH, 1880 (GASTROPODA: NUDIBRANCHIA) IN THE LABORATORY

The growth of Peltodons atromaculata Bergh, 1880, measured astotal dorsal body area, was followed in the laboratory for 15months. The growth of dorsal brown patches was compared to thetotal area of the mantle, and was shown to be approximatelyconstant over time in the different animals. This numericalapproach can be useful to quantify the body pigmentation clinepreviously described for Mediterranean specimens. P atromaculatahas an annual life-cycle in the laboratory, reaching its maximumsize before opposition. Post-spawning mortality followed a decreasein size for all specimens. Mantle margin autotoray in P atromaculataseems not to be related to defence against predation, but itis a process that takes place before death in laboratory. (Received 1 December 1994; accepted 28 April 1995)     

ALLOZYME VARIATION AMONG POPULATIONS OF BULINUS FORSKALII (EHRENBERG, 1831) (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme electrophoresis was used to study genetic diversityamong populations of the freshwater, hermaphroditic snail Bulinusforskalii in Cameroon. Three of 15 loci studied in 13 enzymesystems were polymorphic. Intrapopulation variation occurredin 8 of 32 populations sampled and heterozygotes were presentin 2 of these. Neither of these populations were in Hardy-Weinbergequilibrium. These findings are in agreement with reports thathave demonstrated a low genetic diversity in this very widelydistributed, eurytopic species and that have indicated thisspecies reproduces principally by self-fertilization. The restricteddistributions of rarer alleles in three loci may be due to anarrower habitat range for those phenotypes. For isocitratedehydrogenase and phosphoglucoisomerase, the faster alleleswere found throughout the range of B. forskalii extending fromthe Sahelian regions of the north to rain forest of the south.The slower, rarer alleles for these loci were restricted tothe equatorial rain forest regions. For hydroxybutyrate dehydrogenase,the opposite was true for the rarer allele, which was restrictedto a small region in the arid, tropical climate. (Received 27 April 1989; accepted 5 September 1989)     

POPULATION DYNAMICS OF THE FRESHWATER SNAIL BIOMPHALARIA PFEIFFERI IN EASTERN ZAIRE

The population dynamics of the freshwater snail Biomphalariapfeifferi, an intermediate host of the parasitic worm Schistosomamansoni, was studied in the Virunga stream in Eastern Zaïre. Methods are developed to estimate age-specific survivorshipfrom regular quadrat-sampling data without assuming a constantmortality rate, and to approximate age-specific fecundity whenthe fecundity function is known in the laboratory. The population dynamics in the field was found to be very differentfrom that in the laboratory: it is basically discontinuous,with one main generation per year; a massive mortality occursafter hatching; fecundity is reduced considerably; and thereare marked seasonal variations in both survivorship and fecundity.Time-dependent demographic parameters were estimated by monthinstead of by cohort. They show that the environmental conditionsare favourable to an increase of the population only duringa short period of the year. The intrinsic rate of natural increase,r, was found to be well correlated with the changes in egg density. The environmental factors determining the population dynamicsin the Virunga are discussed; current speed seems critical. (Received 4 February 1987;     

GENETIC AND MORPHOLOGICAL VARIATION OF POPULATIONS BELONGING TO THE BULINUS TRUNCATUS/TROPICUS COMPLEX (GASTROPODA; PLANORBIDAE) IN SOUTH WESTERN ZIMBABWE

Aquatic snails from south western Zimbabwe belonging to theBulinus trunscatus/tropicus complex vary widely in shell formsuggesting that more than one taxon could be present. This possibilitywas investigated by making observations on snail samples from13 populations from the Plumtree area, in respect of allozymevariation (5 polymorphic loci), shell morphology (9 variables),copulatory organ and chromosome number. Comparative data wereobtained from snails from north western Zimbabwe identifieddefinitely as B. tropicus. Analysis of the genetic structurerevealed a high degree of polymorphism (P) ranging from 0.29–0.80among populations from Plumtree and expected heterozygosity(H_e) from 0.02–0.22. No enzymatic diagnostic loci werefound which could differentiate the different morphs or populationsand discriminant function analysis on the morphological datashowed an overlap of morphs among populations. Snails analyzedfor chromosome number were all diploid (2n = 36). Snails exposedto Schistosoma haematobium mira-cidia were all refractory. Thisinformation supports the view of a single species, B. tropicus,which is differentiated due to migration barriers and whereenvironmental variables might be implicated in the morphometricdivergence. (Received 31 July 1995; accepted 15 January 1998)     

COMPARATIVE FITNESS AND REPRODUCTIVE ISOLATION BETWEEN TWO BULINUS GLOBOSUS(GASTROPODA: PLANORBIDAE) POPULATIONS

Bulinus globosus, an intermediate host of schisto-somes, isa self-fertile hermaphrodite species whose regular mating systemis cross-fertilization. Fitness of selfing individuals originatingfrom two geographically remote populations (Elevi, Ivory Coast;Daikaina, Niger) was scored. Individuals were then paired eitherwith individuals from their own population, or with individualsfrom the other population. Electrophoretic analysis showed thatpairing ‘female’ Elevi and ‘male’ Daikainaindividuals results in only a low level of outcrossing, whereasthe reverse pairings produced almost only outcrossed offspring.This could reflect unilateral partial isolation between thetwo populations. However, analysis of fitness values withinand among groups of self-fertilizing and cross-fertilizing individuals,as well as electrophoretic data, suggest that selfing (or partialselfing) is the regular mating system in Elevi. Such experimentsare relevant to the biological control of schistosiomasis byintroducing resistant snails from allopatric populations, becausethey allow an assessment of the fate of introduced resistantgenes. (Received 30 December 1991; accepted 13 March 1992)     

DISTRIBUTION OF THE SCHISTOSOME VECTOR SNAIL, BIOMPHALARIA STRAMINEA (PULMONATA: PLANORBIDAE) IN HONG KONG

Biomphalaria straminea (Dunker) is a snail intermediate hostof Schistosoma mansoni Sambon which has invaded Hong Kong inrecent years from its home range in Brazil. This study examinesits distribution pattern in terms of habitat type and hydrologicalfactors. The data revealed very strong colonizing power, bothin terms of number of sites occupied as well as numerical dominanceat these sites. An inability to colonize lotic habitats mayhave been related to low levels of dissolved minerals and nutrientsbut the effect of current speed on colonization has yet to beresolved. Principal component analysis revealed that hydrological factorsimportant for colonization included high levels of dissolvedcalcium, magnesium, iron, chloride, total nitrogen, sulphateand phosphate. pH had only a secondary effect. The significanceof these findings is discussed with special reference to thelikelihood of introduction of B. straminea elsewhere, and possiblemanagement strategies. (Received 16 December 1988; accepted 12 February 1989)     

STARVATION, DESICCATION AND USE OF ALLOSPERM IN THE HERMAPHRODITE FRESHWATER SNAIL BIOMPHALARIA GLABRATA (GASTROPODA: PULMONATA)

Pairs of albino and pigmented snails were used to test the hypothesisthat the hermaphrodite freshwater gastropod Biomphalaria glabratawas able to conserve and use allosperm, despite periods of desiccationand starvation. In laboratory experiments, lots of twenty snailswere subject to 0, 5, 10, 15 AND 20 days starvation; similarlots of 20 snails were subjected to 0, 2, 4, 6, 8, 10 weeksof desiccation at R.H. 100%. After all periods of starvationand desiccation, albino parents were still producing significantnumbers of pigmented offspring, suggesting preferential cross-fertilizationusing stored allosperm. (Received 30 September 1993; accepted 23 December 1993)     

COPULATION AS FEMALES AND USE OF ALLOSPERM IN THE FRESHWATER SNAIL GENUS BULINUS (GASTROPODA:PLANORBIDAE)

Each of 8 snails in 2 groups of Bulinus (Physopsis) globosus,1 group raised in isolation and 1 group raised in community,were paired for 14 consecutive days with a male-acting partnersnail. In each group, the experimental snails, which were notallowed to act as males, were able to copulate as females onapproximately 94% of the days paired. Two copulations as female,with the same male partner, occurred on 50% of the days thatthe snails were paired, in the 2 groups combined. Non-receptivefemale behaviour by the experimental snails occurred frequently,and copulation was prevented by such behaviour during 6 pairings,3 in each group. Young B. (P.) africanus first copulated as females when theywere 31–33 days old. The accessory sex glands of the femalereproductive tracts of these young female-acting snails containedmoderate to large amounts of secretion. B. (P.) africanus, which were raised in pairs, laidcross-fertilizedeggs in isolation for an average of 76 days, and 1559 eggs/snailwere deposited before cross-fertilization ceased. Cross-fertilizedeggs were produced for as long as 120 days. After 1 copulation as female, virgin B. (P.) africanus laidcross-fertilized eggs for an average of 78 days and deposited3654 eggs/snail before crossfertilization ceased. Cross-fertilizedeggs were produced for as long as 113 days. After 2 copulationsas female, 1 copulation on each of 2 consecutive days, virginB. (P.) africanus laid cross-fertilized eggs for an averageof 102 days and produced 4397 eggs/snail before cross-fertilizationceased. Cross-fertilized eggs were produced for as long as 123days. Snails which were homozygous for an allele governing mantlepigment pattern were raised with a partner which was homozygousfor a different pigment pattern. Young produced in a 4-day periodafter the snails were isolated were 100% heterozygous. The snailswere then rearranged into pairs with a partner of the same genotypefor 4 days, during which time 26% of the young produced werehomozygous. The snails were again isolated for 4 days, and 49%of the young produced during this 4-day period were homozygous.The results of this experiment strongly suggest that multipleoutcrossing occurred. In B. (P.) africanus, stored allosperm were used to fertilizeeggs after 1, 4 and 7 weeks of starvation; after 1 and 4 weeksof 15°C low temperature and 4 weeks of 15°C + 4 weeksof 10°C low temperature; and after 1 and 4 weeks of desiccation.After 8 weeks of desiccation, 2 of 3 surviving snails reproducedby self-fertilization and 1 snail did not reproduce. Too fewsnails survived 8 weeks of desiccation for a conclusion to bereached on the ability of allosperm to survive. (Received 1 June 1984;     

THE EFFECT OF WATER QUALITY ON OVIPOSITION IN BIOMPHALARIA GLABRATA (SAY, 1818) (PLANORBIDAE), AND A DESCRIPTION OF THE STAGES OF THE EGG-LAYING PROCESS

With the overall goal of developing a method to reliably induce ovipositionin the freshwater pulmonate Biomphalaria glabrata, the effectsof water quality on female reproductive physiology were examined.Groups of snails were subjected to controlled experimental conditionsconsisting of a daily regimen of feeding and water change. Aftera period of acclimatiz-ation, egg mass (EM) output under theseconditions was relatively stable, and snails laid a majority(82.5%) of their EM during the initial 4 h following daily waterchange. When this regimen was perturbed by halting water changefor 24 h (dirty-water treatment), EM output was significantly inhibited.When water change was resumed, EM output returned to previouslevels within 4 h post-water change (PWC). This dirty-water treatmentfollowed by water change also resulted in a significant increasein mean EM size during the 4 h PWC when compared to controls.To better describe the events preceding egg-laying in B. glabrata,we then used these experimental manipulations to induce ovipositionin groups of snails, and dissected them during the 4 h followingwater change. Observations of the reproductive tracts of stimulatedsnails allowed us to divide the egg-laying process, from ovulationto oviposition, into discrete stages, after de Jong-Brink, Koop,Roos & Bergamin-Sassen (1982). Stage I was characterized bythe presence of ova in the hermaphroditic duct and carrefour,and fertilized, packaged eggs in the oviduct and muciparousgland. Stage II was characterized by the presence of packagedeggs in the othecal gland embedded in a mucous layer, constitutingthe egg mass to be laid on the substratum. No packaging eventswere occurring in the carrefour/albumen gland region duringthis stage. When snails were dissected immediately after oviposition(Stage III), unpackaged ova were observed in the hermaphroditicduct, carrefour, and oviduct. The mean time it took for snailsto reach Stage III was 120 6 49 min (SD), and this value wasstatistically different from the mean time to Stages I and II,showing that our induction protocol results in a temporal progressionthrough the egg- laying process. Gonadal oocyte density (oocytes/mm²of ovotestis) was quantified as a function of these stages ofthe reproductive cycle, and was found to be significantly lowerduring Stage II (fully formed egg mass in othecal gland) thanall other stages examined. Taken together, these results showthat female reproductive activity can be experimentally controlledthrough the manipulation of water quality, and that such a protocolis a valuable tool for addressing specific questions regardingthe reproductive physiology of B. glabrata. The implicationsof these results as they pertain to the regulation of femalereproductive activity in B. glabrata are discussed. (Received 15 January 1999; accepted 17 May 1999)     

PATTERNS OF SEXUAL ROLES ADOPTED BY THE SCHISTOSOME-VECTOR SNAIL BIOMPHALARIA GLABRATA (PLANORBIDAE)

Relatively little is known about the mating behaviour of hermaphroditefreshwater snails, many of which transmit the Schistosoma trematodesamong humans in developing countries. Knowledge of the breedingbiology of these snails could help in the design of schistosomecontrol programmes, as well as possibly contributing to ourunderstanding of the evolution of simultaneous hermaphroditismin animaL. Here we describe an experiment investigating thepatterns of sexual roles adopted by the Schistosoma mansoni-vectotsnail, Btmphalaria glabrata. During observations on groups offreely interacting snails, no individuals copulated significantlymore often in the male than in the female role, or vice versa.Only one individual showed a pattern of alternating sexual rolesover successive matings that differed significantly from a randomsequence of roles. There was no evidence for reciprocal copulationwith one particular partner, either between consecutive matings(unless they were temporarily isolated from other snails) orbetween non-consecutive matings (separated by copulations withother conspccifics). We discuss these results in the contextof sex allocation and ESS mating strategy theories. ^*Present address (or correspondence 1 G VERNON. Bioscan (UK).Standingford House, Cave Street, St. Clements, Oxford OX4 IBA. (Received 5 October 1995; accepted 6 November 1995)     

ALLOZYME COMPARISONS AMONG SPECIES OF THE BULINUS FORSKALII GROUP (GASTROPODA: PLANORBIDAE) IN CAMEROON

Allozyme comparisons were made for adult Bulinus forskalii,B. camerunensis, and B. senegalensis from populations in Cameroonusing starch gel electrophoresis. Fifteen loci were examinedbut no differences were found between B. forskalii and B. camerunensis,bringing into question the validity of B. camerunensis as aseparate species. Bulinus senegalensis differed from B. forskaliiand B. camerunensis in allozymes for acid phosphatase, alpha-glycero-phosphatedehydxogenase, hydroxybutyrate dehy-drogenase, and phosphoglucomutase.No differences were found, as reported elsewhere, between B.forskalii and B. senegalensis for aspartate amino-transferase,isocitrate debydrogenase, phosphogluco-isomerase, and xanthineoxidise. No polymorphism was seen in B. camerunensis or variousB. senegalensis populations examined. Three alleles for isocitratedehydrogenase were observed for B. forskalii. Mixed populationsof B. forskalii and B. senegalensis were found at four sitesbut no evidence of hybridization between these species was found. (Received 9 May 1988; accepted 30 August 1988)