Female biased sex-ratio in Schistosoma mansoni after exposure to an allopatric intermediate host strain of Biomphalaria glabrata (original) (raw)
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Genotypic variation in host response to infection affects parasite reproductive rate
International Journal for Parasitology, 2016
Parasite fitness is largely influenced by a variation in host response due to the host's genetic background. 32 Here we investigated the impact of host genotype on pathogen success in the snail vector of its castrating 33 parasite, Schistosoma mansoni. We infected five inbred lines of Biomphalaria glabrata with two infection 34 doses and followed their growth, reproductive output and parasite production throughout the course 35 of infection. There was no difference in resistance to infection among inbred lines, but lines varied in their 36 responses to infection and the numbers of parasites produced. Snails did not compensate for castration by 37 increasing their fecundity during the early phase of infection (fecundity compensation). However, some 38 lines were able to delay parasite shedding for up to 30 weeks, thus prolonging reproduction before the 39 onset of castration. Here we propose this strategy as a novel defense against castrating pathogens in 40 snails. Gigantism, a predicted outcome of castration due to energy reallocation, occurred early in infec-41 tion (<15 weeks) and was not universal among the snail lines. Lines that did not show gigantism were 42 also characterised by a high parasite production rate and low survivorship, perhaps indicating energy 43 reallocation into parasite production and costly immune defense. We observed no differences in total 44 parasite production among lines throughout the entire course of infection, although lines differed in their 45 parasite reproductive rate. The average rate of parasite production varied among lines from 1300 to 2450 46 cercariae within a single 2 h shedding period, resulting in a total production of 6981-29,509 cercariae 47 over the lifetime of a single snail. Regardless of genetic background, snail size was a strong predictor 48 of parasite reproduction: each millimetre increase in snail size at the time of the first shed resulted in 49 up to 3500 more cercariae over the lifetime of the snail. The results of this study provide a detailed pic-50 ture of variation in hosts' responses to infection and the resulting impacts on parasite fitness, further 51 defining the intricacies of snail-schistosome compatibility. 52
Parasitology Research, 2007
Genetic variability is often predicted to enhance host fitness in the face of parasitism, yet this idea is rarely tested in an experimental setting, particularly with animal hosts. To assess this question, we used a relatively resistant line of snail hosts (Biomphalaria glabrata) to generate inbred and outcrossed progeny that were then either exposed or sham-exposed to the trematode parasite, Schistosoma mansoni. Results showed no difference in prevalence between the groups; however, large differences appeared in other host life history traits, particularly reproduction. Outcrossed progeny produced large numbers of eggs relative to inbred progeny especially in the face of infection. Furthermore, eggs produced by outcrossed snails took less time to hatch and exhibited greater hatching success compared to their inbred counterparts. Parasite reproduction demonstrated the opposite trend, with fewer parasites emerging from outcrossed snails compared to inbred individuals. This work shows that the introduction of genetic variation into inbred snail populations can have important implications for the viability of host populations and disease transmission.
Experimental Parasitology, 2008
For parasites that require multiple hosts to complete their development, genetic interplay with one host may impact parasite transmission and establishment in subsequent hosts. In this study, we used microsatellite loci to address whether the genetic background of snail intermediate hosts influences life-history traits and transmission patterns of dioecious trematode parasites in their definitive hosts. We performed experimental Schistosoma mansoni infections utilizing two allopatric populations of Biomphalaria glabrata snails and assessed intensities and sex ratios of adult parasites in mouse definitive hosts. Our results suggest that the genetic background of hosts at one point in a parasite's life cycle can influence the intensities and sex ratios of worms in subsequent hosts.
Interspecific antagonism and virulence in hosts exposed to two parasite species
Journal of Invertebrate Pathology, 2007
Co-infection of host organisms by multiple parasite species has evolutionary consequences for all participants in the symbiosis. In this study, we co-exposed aquatic-snails (Biomphalaria glabrata) to two of their trematode parasites, Schistosoma mansoni and Echinostoma caproni. In co-exposed snails, E. caproni prevalence was 63% compared to only 23% for S. mansoni. Co-exposed E. caproni-infected snails exhibited reduced fecundity, higher mortality, and higher parasite reproduction (higher virulence) compared to hosts exposed to echinostomes alone. Conversely, co-exposed S. mansoni-infected snails released fewer parasites and produced greater numbers of eggs compared to hosts exposed to S. mansoni alone. These results suggest that co-exposure not only influences the establishment (presence or absence) of particular parasite species, but also impacts host life history, parasite reproduction, and the virulence of the interaction.
Coevolution between parasite virulence and host life‐history traits
The American Naturalist, 2002
Epidemiological models generally explore the evolution of parasite life-history traits, namely, virulence and transmission, against a background of constant host life-history traits. However, life-history models have predicted the evolution of host traits in response to parasitism. The coevolution of host and parasite lifehistory traits remains largely unexplored. We present an epidemiological model, based on resource allocation theory, that provides an analysis of the coevolution between host reproductive effort and parasite virulence. This model allows for hosts with either a fixed (i.e., genetic) or conditional (i.e., a phenotypically plastic) response to parasitism. It also considers superinfections. We show that parasitism always favors increased allocation to host reproduction, but because of epidemiological feedbacks, the evolutionarily stable host reproductive effort does not always increase with parasite virulence. Superinfection drives the evolution of parasite virulence and acts on the evolution of the host through parasite evolution, generally leading to higher host reproductive effort. Coevolution, as opposed to cases where only one of the antagonists evolves, may generate correlations between host and parasite life-history traits across environmental gradients affecting the fecundity or the survival of the host. Our results provide a theoretical framework against which experimental coevolution outcomes or field observations can be contrasted.
Parasite distribution and virulence: implications for parasite-mediated sexual selection
Behavioral Ecology and Sociobiology, 1993
The Hamilton-Zuk hypothesis states that females choosing males with more developed secondary sexual traits, i.e. "brighter" males, achieve greater fitness if variability in brightness reflects heritable variation in resistance to parasites. However, several factors will affect the likelihood that parasites play a role in sexual selection in given species. Here, using simple models, we show that because of parasite aggregation on a few hosts, only few breeding males would suffer from reductions in brightness due to parasites. Only in cases where parasites are abundant and show low levels of aggregation among their hosts would there be sufficient variability in brightness among breeding males for female choice of bright, resistant males to evolve. In addition, sufficient parasiteinduced variability in brightness among breeding males will only occur in host-parasite systems where pathology is linearly related to the number of parasites per host. The presence of males that are uninfected and bright but genetically susceptible to parasites will also influence the fitness advantages obtainedl by females choosing bright males. If genetic immunity against parasites is rare in the host population, females can probably only benefit from choosing bright males if parasites are common and little aggregated among males. These results greatly limit the generality of the Hamilt0n-Zuk hypothesis, and suggest that only a small fraction of host-parasite associations could promote the evolution of host mate choice for resistance based on brightness.
Evolution, 2007
Genotype × environment interactions can facilitate coexistence of locally adapted specialists. Interactions evolve if adaptation to one environment trades off with performance in others. We investigated whether evolution on one host genotype traded off with performance on others in long-term experimental populations of different genotypes of the protozoan Paramecium caudatum, infected with the bacterial parasite Holospora undulata. A total of nine parasite selection lines evolving on three host genotypes and the ancestral parasite were tested in a cross-infection experiment. We found that evolved parasites produced more infections than did the ancestral parasites, both on host genotypes they had evolved on (positive direct response to selection) and on genotypes they had not evolved on (positive correlated response to selection). On two host genotypes, a negative relationship between direct and correlated responses indicated pleiotropic costs of adaptation. On the third, a positive relationship suggested cost-free adaptation. Nonetheless, on all three hosts, resident parasites tended to be superior to the average nonresident parasite. Thus genotype specificity (i.e., patterns of local adaptation) may evolve without costs of adaptation, as long as direct responses to selection exceed correlated responses.
Host Sex and Local Adaptation by Parasites in a Snail‐Trematode Interaction
The American Naturalist, 2004
One of the leading theories for the evolutionary stability of sex in eukaryotes relies on parasite-mediated selection against locally common host genotypes (the Red Queen hypothesis). As such, parasites would be expected to be better at infecting sympatric host populations than allopatric host populations. Here we examined all published and unpublished infection experiments on a snailtrematode system (Potamopyrgus antipodarum and Microphallus sp., respectively). A meta-analysis demonstrated significant local adaptation by the parasite, and a variance components analysis showed that the variance due to the host-parasite interaction far exceeded the variance due to the main effects of host source and parasite source. The meta-analysis also indicated that asexual host populations were more resistant to allopatric sources of parasites than were (mostly) sexual host populations, but we found no significant differences among parasite populations in the strength of local adaptation. This result suggests that triploid asexual snails are more resistant to remote sources of parasites, but the parasite has, through coevolution, overcome the difference. Finally, we found that the degree of local adaptation did not depend on the genetic distance among host populations. Taken together, the results demonstrate that the parasites are adapted, on average, to infecting their local host populations and suggest that they may be a factor in selecting against common host genotypes in natural populations.