Philippe Brémond

Origin and diversification of the human parasite Schistosoma mansoni

Schistosoma mansoni is the most widespread of the human‐infecting schistosomes, present in 54 countries, predominantly in Africa, but also in Madagascar, the Arabian Peninsula, and the Neotropics. Adult‐stage parasites that infect humans are also occasionally recovered from baboons, rodents, and other mammals. Larval stages of the parasite are dependent upon certain species of freshwater snails in the genus Biomphalaria, which largely determine the parasites geographical range. How S. mansoni genetic diversity is distributed geographically and among isolates using different hosts has never been examined with DNA sequence data. Here we describe the global phylogeography of S. mansoni using more than 2500 bp of mitochondrial DNA (mtDNA) from 143 parasites collected in 53 geographically widespread localities. Considerable within‐species mtDNA diversity was found, with 85 unique haplotypes grouping into five distinct lineages. Geographical separation, and not host use, appears to be the most important factor in the diversification of the parasite. East African specimens showed a remarkable amount of variation, comprising three clades and basal members of a fourth, strongly suggesting an East African origin for the parasite 0.30–0.43 million years ago, a time frame that follows the arrival of its snail host. Less but still substantial variation was found in the rest of Africa. A recent colonization of the New World is supported by finding only seven closely related New World haplotypes which have West African affinities. All Brazilian isolates have nearly identical mtDNA haplotypes, suggesting a founder effect from the establishment and spread of the parasite in this large country.

The influence of mating system, demography, parasites and colonization on the population structure of Biomphalaria pfeifferi in Madagascar

Current evolutionary forces and historical processes interact to shape the distribution of neutral genetic variability within and among populations. Focusing on the genetics of recently introduced organisms offers a good opportunity to understand the relative importance of these factors. This study concerns variation at 8 polymorphic microsatellite loci in 30 populations of Biomphalaria pfeifferi. The sampling area spans most of the species’ range in Madagascar where it was probably introduced recently. Extremely low variation was found within all populations studied, which may partly result from high selfing rates. However, this cannot account for the variance of variation across populations, which is better explained by habitat openness (that reflects environmental stochasticity), the prevalence of the parasitic trematode Schistosoma mansoni and historical demography (colonization and subsequent bottlenecks). Large global differentiation was also observed, suggesting that current gene flow among populations is limited to small distances, within watersheds and to few individuals. Our data set also allowed us to test several hypotheses regarding colonization, based on bottleneck and admixture tests. The observed pattern requires at least two independent introductions from slightly differentiated genetic sources in the western part of Madagascar. Another introduction, from a very different genetic origin, should also be postulated to explain the genetic composition of eastern populations. That this introduction occurred recently suggests that the colonization of Madagascar by B. pfeifferi is an ongoing process.

Intraspecific diversity of Schistosoma haematobium in west Africa: chronobiology of cercarial emergence.

Cercarial emergence patterns were used to analyse the intraspecific variability within and between nine populations of Schistosoma haematobium collected along a transect line from the north to the South of the Ivory Coast (Africa) and using Bulinus truncatus or Bulinus globosus as intermediate snail hosts. Statistical comparison demonstrated the existence of a chronobiological polymorphism and the existence of three homogeneous groups of S. haematobium isolates with mean shedding times of the cercariae decreasing from the North to the South. The chronobiological variability observed was not correlated with the species of Bulinus (B. truncatus vs. B. globosus) implicated in the parasite transmission but with the climatic and vegetal features of the transmission area. S. haematobium from shaded sites of the forest zone (South) showed cercarial emergence patterns significantly earliest than that of S. haematobium from open sites of the savanna zone (North). Differences in sensitivity to light intensity could characterize the existence of eco-geographical races of S. haematobium one of the forest, the other from the savanna.

Hybrids between Schistosoma mansoni and S. rodhaini: characterization by isoelectric focusing of six enzymes

Results are reported of enzyme analyses by isoelectric focusing in polyacrylamide gels of individual extracts fromSchistosoma mansoni (Guadeloupe),S. rodhaini (Burundi), and their experimental hybrids (first and second generation). The distinctive patterns of lactate dehydrogenase (LDH), malate dehydrogenase (MDH), glucose-6-phosphate dehydrogenase (G6PD), acid phosphatase (AcP), phosphoglucomutase (PGM) and glucose phosphate isomerase (GPI) enable the characterization of the two parental strains and the hybrids. Particular observations, such as the existence of a polymorphism at both MDH-1 and MDH-2 loci and a sex-linked heredity for GPI, are discussed. A genetic interpretation is proposed to explain the patterns observed for MDH and GPI (with a dimeric structure) and for PGM (monomeric structure); a comparison is made with electrophoretic data available forS. mansoni andS. rodhaini.

Allelic frequency variations at the MDH-1 locus within Schistosoma mansoni strains from Guadeloupe (French West Indies): ecological interpretation.

1. An isoenzymatic study on Schistosoma mansoni from Guadeloupe has been carried out using isoelectric focusing in polyacrylamide gels. 2. Among the seven systems examined (LDH, MDH, G6PD, PGI, PGM, AcP, HK), only MDH showed variation at the MDH-1 locus and mdh-1a allele frequencies were used to characterise eight strains derived either from human or murine hosts, and representative of different transmission sites. 3. In comparison with criteria previously employed to type these strains (ecological context of their transmission sites, cercarial emergence patterns), mdh-1a frequency variations have been correlated to the degree of participation of the murine host reservoir in the parasite transmission dynamics within the different foci of Guadeloupe.

Experimental host-induced selection in Schistosoma mansoni strains from Guadeloupe and comparison with natural observations.

Allelic frequency variation at the malate dehydrogenase (E.C.1.1.1.37) polymorphic locus (Mdh-I) was analysed during several successive generations in four strains of Schistosoma mansoni from Guadeloupe, maintained experimentally on mice. A rapid evolution of the frequency of the Mdh-la allele is interpreted as being the result of an interaction between experimental drift and selection induced by the murine laboratory host. These experimental results are compared to the genetic structures observed among the corresponding natural populations of S. mansoni in Guadeloupe (West Indies). They strengthen the hypothesis of a natural host-induced selection by the murine host (Rattus rattus), which, in Guadeloupe, plays the role of host reservoir for this human schistosome.