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André Théron

Publications and source records attributed to André Théron.

12 recordsLinked to original sources
PubMed2006

Host sex and parasite genetic diversity.

Is the genetic diversity of parasites infecting male and female hosts equal or different? This is the question we address in this paper by studying the neutral genetic variability of the plathyhelminth trematode Schistosoma mansoni within males and females of its natural murine host Rattus rattus in the marshy forest focus of Guadeloupe (French West Indies). Using seven microsatellite markers, we demonstrate that parasites from male hosts are genetically more diversified than parasites from female hosts. Three hypotheses are discussed that could explain this pattern: 1) a host sex-specific duration of cercariae recruitment; 2) a difference in the behaviour of male and female hosts that would lead to the exposure of males to a greater diversity of parasites; and 3) a host sex-biased immunocompetence that would lead to the selection of more genetically diversified individuals in male than in female rats. This finding is the first empirical evidence that each host sex may play different roles in the maintenance of parasite genetic diversity and so in their evolutionary dynamics and epidemiology.

Animals↗
PubMed2005

Speciation in parasites: a population genetics approach.

Parasite speciation and host-parasite coevolution should be studied at both macroevolutionary and microevolutionary levels. Studies on a macroevolutionary scale provide an essential framework for understanding the origins of parasite lineages and the patterns of diversification. However, because coevolutionary interactions can be highly divergent across time and space, it is important to quantify and compare the phylogeographic variation in both the host and the parasite throughout their geographical range. Furthermore, to evaluate demographic parameters that are relevant to population genetics structure, such as effective population size and parasite transmission, parasite populations must be studied using neutral genetic markers. Previous emphasis on larger-scale studies means that the connection between microevolutionary and macroevolutionary events is poorly explored. In this article, we focus on the spatial fragmentation of parasites and the population genetics processes behind their diversification in an effort to bridge the micro- and macro-scales.

Animals↗
PubMed2005

F-statistics under alternation of sexual and asexual reproduction: a model and data from schistosomes (platyhelminth parasites).

Accurate inferences on population genetics data require a sound underlying theoretical null model. Nearly nothing is known about the gene dynamics of organisms with complex life cycles precluding any biological interpretation of population genetics parameters. In this article, we used an infinite island model to derive the expectations of those parameters for the life cycle of a dioecious organism obligatorily alternating sexual and asexual reproductions as it is the case for schistosomes (plathyhelminth parasites). This model allowed us to investigate the effects of the degree of mixing among individuals coming from different subpopulations at each new generation (represented in the model by the migration rates before and after clonal reproductions) and the variance in the reproductive success of individuals during the clonal phase. We also consider the effects of different migration rates and degrees of clonal reproductive skew between male and female individuals. Results show that the variance in the reproductive success of clones is very important in shaping the distribution of the genetic variability both within and among subpopulations. Thus, higher variance in the reproductive success of clones generates heterozygous excesses within subpopulations and also increases genetic differentiation between them. Migration occurring before and after asexual reproduction has different effects on the patterns of F(IS) and F(ST). When males and females display different degrees of reproductive skew or migration rates, we observe differences in their respective population genetic structure. While results of the model apply to any organism alternating sexual and clonal reproductions (e.g. all parasitic trematodes, many plants, and all aphididae), we finally confront some of these theoretical expectations to empirical data from Schistosoma mansoni infecting Rattus rattus in Guadeloupe.

Animals↗
PubMed2005

Dispersal in a parasitic worm and its two hosts: consequence for local adaptation.

Characterizing host and parasite population genetic structure and estimating gene flow among populations is essential for understanding coevolutionary interactions between hosts and parasites. We examined the population genetic structure of the trematode Schistosoma mansoni and its two host species (the definitive host Rattus rattus and the intermediate host Biomphalaria glabrata) using microsatellite markers. Parasites were sampled from rats. The study was conducted in five sites of the Guadeloupe Island, Lesser Antilles. Mollusks display a pattern of isolation by distance whereas such a pattern is not found neither in schistosomes nor in rats. The comparison of the distribution of genetic variability in S. mansoni and its two host species strongly suggests that migration of parasites is principally determined by that of the vertebrate host in the marshy focus of Guadeloupe. However, the comparison between genetic differentiation values in schistosomes and rats suggests that the efficacy of the schistosome rat-mediated dispersal between transmission sites is lower than expected given the prevalence, parasitic load and migration rate of rats among sites. This could notably suggest that rat migration rate could be negatively correlated to the age or the infection status of individuals. Models made about the evolution of local adaptation in function of the dispersal rates of hosts and parasites suggest that rats and mollusks should be locally adapted to their parasites.

Animals↗
PubMed2004

Detection and genetic distance of resistant populations of Pseudosuccinea columella (Mollusca: Lymnaeidae) to Fasciola hepatica (Trematoda: Digenea) using RAPD markers.

Twelve natural populations of Pseudosuccinea columella snails, sampled in the western and central regions of Cuba, were analyzed using the RAPD-PCR technique to screen for resistance to Fasciola hepatica. Ten OPA primers previously shown to produce marker bands for resistance and susceptibility were tested. A new population of P. columella (El Azufre, Pinar del Río) exhibited the amplification patterns of resistant snails, and its resistant status was confirmed after experimental exposure to miracidia. No genetic variability was detected across or within the susceptible isolates. Similarly, the novel resistant isolate displayed an RAPD profile identical to the profile of two other isolates previously identified as resistant to F. hepatica. However, clear differences in RAPD banding patterns and genetic distance were observed between resistant and susceptible isolates.

Animals↗
PubMed2004

Sex-specific correlation between heterozygosity and clone size in the trematode Schistosoma mansoni.

The mode of reproduction (sexual and/or asexual) and the mating system determine the patterns of gene transmission and genotype formation across generations. Schistosoma mansoni is a dioecious trematode that necessarily alternates sexual and asexual reproduction during its life cycle. In a previous study of the distribution of S. mansoni genetic variability within and between definitive host individuals, we noticed that deleting multilocus genotypes from each infrapopulation so as to keep only one copy of each multilocus genotype, seemed to have a substantial effect on FIS values. More precisely, female FIS increased when repeated genotypes were removed whereas no effect was observed on male FIS. This suggested that multilocus genotypes at high frequency tended to be more heterozygous. The aim of the present study is specifically to test and analyse this phenomenon. We demonstrate that the number of repetitions per clone correlates with individual heterozygosity. This effect is however, sex-specific: only female clone size correlates with heterozygosity. We discuss this phenomenon in relation to the heterozygosity-fitness relationship and the sex-specific response to inbreeding depression.

Animals↗
PubMed2004

Test of pangamy by genetic analysis of Schistosoma mansoni pairs within its natural murine host in Guadeloupe.

Mating system plays a determinant role in the maintenance and distribution of genetic variations. It can be assessed indirectly by analyzing the distribution of the genetic variability within populations or directly by considering how mating pairs are formed. In the present study, 71 pairs of adult Schistosoma mansoni worms sampled from naturally infected rats were genotyped to investigate how male and female schistosomes paired according to their genetic relatedness. Among all samples, pangamy, the random association between males and females, could not be rejected. Whereas the schistosome mating system has been intensively studied under experimental conditions, to the best of our knowledge, our study is the first to attempt to understand the way in which males and females pair in natural conditions.

Animals↗
PubMed2003

Evidence of phenotypic differences between resistant and susceptible isolates of Pseudosuccinea columella (Gastropoda: Lymnaeidae) to Fasciola hepatica (Trematoda: Digenea) in Cuba.

Isolates of Cuban Pseudosuccinea columella susceptible and non-susceptible to Fasciola hepatica miracidial infection were compared for cellular reaction to miracidial development, shell morphometrics, mantle pigmentation pattern, and egg laying behaviour. At 24 h post-exposure, non-susceptible snails showed an early cellular host reaction to encapsulate and phagocytise the miracidium, whereas viable transforming miracidia were observed in exposed susceptible snails, indicating that non-susceptibility resulted from resistance rather than physiological unsuitability. Susceptible isolates (more than 80% susceptible to infection) showed significantly higher values of the morphometric indices shell width/length and aperture width/length than resistant isolates (100% resistant). Mantle pigmentation pattern was analysed using Visilog software; the cumulative distances between mantle spots were plotted and compared between isolates, showing a clear distinction between groups. Egg-laying behaviour also showed significant differences with susceptible snails preferring to lay their eggs on the container's walls whereas resistant snails preferred to lay their eggs on the bottom of the container.

Animals↗
PubMed2002

Fasciola hepatica (Trematoda: Digenea): its effect on the life history traits of Pseudosuccinea columella (Gasteropoda: Lymnaeidae), an uncommon interaction.

An uncommon snail-trematode interaction pattern was found for the Pseudosuccinea columella - Fasciola hepaticamodel under laboratory conditions. Week-old juveniles from two isolates showed very high rates of infection (90.0% and 93.3%) after exposure to five miracidia. They also showed differences in their life history traits when compared to a control group. However, they did not exhibit the increase in size and reduction/cessation in host fecundity which is usually observed in most of the other related snail-trematode systems. In contrast, infected juvenile P. columella showed increased egg laying after the onset of cercarial emission and there was no effect on growth. A third isolate of P. columella was found to be refractory to miracidial infection. This constitutes the first report of non-susceptibility to F. hepatica in a lymnaeid species which is usually susceptible. These non-susceptible snails exhibited lower fecundity and survival compared to non-exposed susceptible snails under identical laboratory conditions. The differences observed in terms of life span and reproduction could result from the cost of resistance for individuals from this isolate.

Age Factors↗
PubMedDate not supplied

Fasciola hepatica: identification of molecular markers for resistant and susceptible Pseudosuccinea columella snail hosts.

Protein electrophoresis, RAPD-PCR and nuclear rDNA ITS sequencing were performed to search for genetic differences between Pseudosuccinea columella snails susceptible and resistant to Fasciola hepatica infection. Of the 21 enzymatic loci analyzed in both populations, none of them exhibited neither within- or between-group variation. Such an absence of enzyme polymorphism support the hypothesis of selfing as the "prevalent" mating system for this hermaphroditic species. Conversely, the RAPD profiles displayed clear differences between susceptible and resistant isolates for 17 of the 26 primers tested while no within-group variation was detected. rDNA ITS sequence analysis from snails of each isolates showed only two bases that differed between groups accounting for a 0.17% of variation confirming that susceptible and resistant snails belong to the same species. This is the first time that a genetic variation using RAPD markers is demonstrated between susceptible and resistant lymnaeid snails vis-a-vis of F. hepatica infection in absence of experimental selection.

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