Role of parasite load and differential habitat preferences in maintaining the coexistence of sexual and asexual competitors in fish of the Cobitis taenia hybrid complex

In the context of the paradoxical ubiquity of sex, we tested whether stable coexistence of sexual and asexual fish of the genus Cobitis is mediated by parasites, as asexual fish suffer more from parasitic infections because of their lower genetic variability [the Red Queen hypothesis (RQH)], or by partial niche shift of the two strains differing in mode of reproduction. We did not find a clear correlation between infection risk with a helminth parasite and the proportion of sexuals, and we found similar infection rates among sexual females and co‐occurring asexuals in general, including the most frequent clone in particular. These results suggest that the mechanisms of the RQH are not directly engaged in stabilizing this asexual complex. On the other hand, the temporally stable gradient in sexual/asexual proportions along the river correlated with gradients in environmental parameters (physicochemical water parameters, velocity, and shading of the habitat) and turnover in the fish assemblage structure. Sexual and asexual forms thus appear to prefer different habitats. The Cobitis teania asexual complex thus contributes to the view that persistence of sex may, as in many taxa, be driven by case‐specific processes. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 220–235.     

Genetic variation in sexual and clonal lineages of a freshwater snail

Sexual reproduction within natural populations of most plants and animals continues to remain an enigma in evolutionary biology. That the enigma persists is not for lack of testable hypotheses but rather because of the lack of suitable study systems in which sexual and asexual females coexist. Here we review our studies on one such organism, the freshwater snail Potamopyrgus antipodarum (Gray). We also present new data that bear on hypotheses for the maintenance of sex and its relationship to clonal diversity. We have found that sexual populations of the snail are composed of diploid females and males, while clonal populations are composed of a high diversity of triploid apomictic females. Sexual and asexual individuals coexist in stable frequencies in many ‘mixed’ populations; genetic data indicate that clones from these mixed populations originated from the local population of sexual individuals without interspecific hybridization. Field data show that clonal and sexual snails have completely overlapping life histories, but individual clonal genotypes are less variable than individuals from the sympatric sexual population. Field data also show segregation of clones among depth‐specific habitat zones within a lake, but clonal diversity remains high even within habitats. A new laboratory experiment revealed extensive clonal variation in reproductive rate, a result which suggests that clonal diversity would be low in nature without some form of frequency‐dependent selection. New results from a long‐term field study of a natural, asexual population reveal that clonal diversity remained nearly constant over a 10‐year period. Nonetheless, clonal turnover occurs, and it occurs in a manner that is consistent with parasite‐mediated, frequency‐dependent selection. Reciprocal cross‐infection experiments have further shown that parasites are more infective to sympatric host snails than to allopatric snails, and that they are also more infective to common clones than rare clones within asexual host populations. Hence we suggest that sexual reproduction in these snails may be maintained, at least in part, by locally adapted parasites. Parasite‐mediated selection possibly also contributes to the maintenance of local clonal diversity within habitats, while clonal selection may be responsible for the distribution of clones among habitats. © 2003 The Linnean Society of London. Biological Journal of the Linnean Society 2003, 79 , 165–181.     

Ecology,Wolbachia infection frequency and mode of reproduction in the parasitoid wasp Tetrastichus coeruleus (Hymenoptera: Eulophidae)

Whereas sexual reproduction may facilitate adaptation to complex environments with many biotic interactions, simplified environments are expected to favour asexual reproduction. In agreement with this, recent studies on invertebrates have shown a prevalence of asexual species in agricultural (simplified) but not in natural (complex) environments. We investigated whether the same correlation between reproductive mode and habitat can be found in different populations within one species. The parasitoid wasp Tetrastichus coeruleus forms an ideal model to test this question, since it occurs both in natural and agricultural environments. Further, we investigated whether Wolbachia infection caused parthenogenesis in female‐biased populations. In contrast to the general pattern, in Dutch and French natural areas, we found Wolbachia‐infected, highly female‐biased populations that reproduce parthenogenetically. In contrast, populations on Dutch agricultural fields were not infected with Wolbachia, showed higher frequencies of males and reproduced sexually. However, we also found a female‐only, Wolbachia‐infected population on agricultural fields in north‐eastern United States. All Wolbachia‐infected populations were infected with the same Wolbachia strain. At this moment, we do not have a convincing explanation for this deviation from the general pattern of ecology and reproductive mode. It may be that asparagus agricultural fields differ from other crop fields in ways that favour sexual reproduction. Alternatively, Wolbachia may manipulate life history traits in its host, resulting in different fitness pay‐offs in different habitats. The fixation of Wolbachia in the United States populations (where the species was introduced) may be due to founder effect and lack of uninfected, sexual source populations.     

Desiccation‐tolerance in bdelloid rotifers facilitates spatiotemporal escape from multiple species of parasitic fungi

Asexuality has major theoretical advantages over sexual reproduction. Nevertheless, obligately asexual metazoan lineages seldom endure over evolutionary time. The Red Queen hypothesis posits that their limited capacity to generate genetic novelty leads to extermination by rapidly evolving parasites and pathogens. At first glance, rotifers of the class Bdelloidea appear to contradict this view: they have reproduced asexually for over 30 Myr without being overwhelmed by parasites. However, there are special ecological conditions under which Red Queen models can accommodate this unusual outcome. If hosts disperse rapidly within a structured metapopulation during a parasite‐free life stage, then in principle they can become spatiotemporally decoupled from coevolving antagonists, and persist without sex. Intriguingly, bdelloid rotifers form dormant propagules when desiccated, which disperse easily by wind. In previous experiments, 7 days of desiccation and wind dispersal removed a fungal parasite from populations of one bdelloid species, allowing them to disperse independently. Here, I extend this finding to two additional bdelloid species and five more fungal parasites, and demonstrate its robustness under various desiccation regimes, and in the presence of multiple parasites. Results support the hypothesis that the unusual physiology of anciently asexual bdelloid rotifers helps them escape fungal parasites in space and time. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 564–574.     

The Ratchet and the Red Queen: the maintenance of sex in parasites

The adaptive significance of sexual reproduction remains as an unsolved problem in evolutionary biology. One promising hypothesis is that frequency‐dependent selection by parasites selects for sexual reproduction in hosts, but it is unclear whether such selection on hosts would feed back to select for sexual reproduction in parasites. Here we used individual‐based computer simulations to explore this possibility. Specifically, we tracked the dynamics of asexual parasites following their introduction into sexual parasite populations for different combinations of parasite virulence and transmission. Our results suggest that coevolutionary interactions with hosts would generally lead to a stable coexistence between sexual parasites and a single parasite clone. However, if multiple mutations to asexual reproduction were allowed, we found that the interaction led to the accumulation of clonal diversity in the asexual parasite population, which led to the eventual extinction of the sexual parasites. Thus, coevolution with sexual hosts may not be generally sufficient to select for sex in parasites. We then allowed for the stochastic accumulation of mutations in the finite parasite populations (Muller's Ratchet). We found that, for higher levels of parasite virulence and transmission, the population bottlenecks resulting from host–parasite coevolution led to the rapid accumulation of mutations in the clonal parasites and their elimination from the population. This result may explain the observation that sexual reproduction is more common in parasitic animals than in their free‐living relatives.     

EVOLUTION OF ASEXUALITY VIA DIFFERENT MECHANISMS IN GRASS THRIPS (THYSANOPTERA: Aptinothrips)

Asexual lineages can derive from sexual ancestors via different mechanisms and at variable rates, which affects the diversity of the asexual population and thereby its ecological success. We investigated the variation and evolution of reproductive systems in Aptinothrips, a genus of grass thrips comprising four species. Extensive population surveys and breeding experiments indicated sexual reproduction in A. elegans, asexuality in A. stylifer and A. karnyi, and both sexual and asexual lineages in A. rufus. Asexuality in A. stylifer and A. rufus coincides with a worldwide distribution, with sexual A. rufus lineages confined to a limited area. Inference of molecular phylogenies and antibiotic treatment revealed different causes of asexuality in different species. Asexuality in A. stylifer and A. karnyi has most likely genetic causes, while it is induced by endosymbionts in A. rufus. Endosymbiont‐community characterization revealed presence of Wolbachia, and lack of other bacteria known to manipulate host reproduction. However, only 69% asexual A. rufus females are Wolbachia‐infected, indicating that either an undescribed endosymbiont causes asexuality in this species or that Wolbachia was lost in several lineages that remained asexual. These results open new perspectives for studies on the maintenance of mixed sexual and asexual reproduction in natural populations.     

Clonal diversity driven by parasitism in a freshwater snail

One explanation for the widespread abundance of sexual reproduction is the advantage that genetically diverse sexual lineages have under strong pressure from virulent coevolving parasites. Such parasites are believed to track common asexual host genotypes, resulting in negative frequency‐dependent selection that counterbalances the population growth‐rate advantage of asexuals in comparison with sexuals. In the face of genetically diverse asexual lineages, this advantage of sexual reproduction might be eroded, and instead sexual populations would be replaced by diverse assemblages of clonal lineages. We investigated whether parasite‐mediated selection promotes clonal diversity in 22 natural populations of the freshwater snail Melanoides tuberculata. We found that infection prevalence explains the observed variation in the clonal diversity of M. tuberculata populations, whereas no such relationship was found between infection prevalence and male frequency. Clonal diversity and male frequency were independent of snail population density. Incorporating ecological factors such as presence/absence of fish, habitat geography and habitat type did not improve the predictive power of regression models. Approximately 11% of the clonal snail genotypes were shared among 2–4 populations, creating a web of 17 interconnected populations. Taken together, our study suggests that parasite‐mediated selection coupled with host dispersal ecology promotes clonal diversity. This, in return, may erode the advantage of sexual reproduction in M. tuberculata populations.     

EVALUATION OF ELEVATED PLOIDY AND ASEXUAL REPRODUCTION AS ALTERNATIVE EXPLANATIONS FOR GEOGRAPHIC PARTHENOGENESIS IN EUCYPRIS VIRENS OSTRACODS

Transitions from sexual to asexual reproduction are often coupled with elevations in ploidy. As a consequence, the importance of ploidy per se for the maintenance and spread of asexual populations is unclear. To examine the effects of ploidy and asexual reproduction as independent determinants of the success of asexual lineages, we sampled diploid sexual, diploid asexual, and triploid asexual Eucypris virens ostracods across a European wide range. Applying nuclear and mitochondrial markers, we found that E. virens consists of genetically highly differentiated diploid sexual populations, to the extent that these sexual clades could be considered as cryptic species. All sexual populations were found in southern Europe and North Africa and we found that both diploid asexual and triploid asexual lineages have originated multiple times from several sexual lineages. Therefore, the asexual lineages show a wide variety of genetic backgrounds and very strong population genetic structure across the wide geographic range. Finally, we found that triploid, but not diploid, asexual clones dominate habitats in northern Europe. The limited distribution of diploid asexual lineages, despite their shared ancestry with triploid asexual lineages, strongly suggests that the wider geographic distribution of triploids is due to elevated ploidy rather than to asexuality.     

Microsatellite and single‐nucleotide polymorphisms indicate recurrent transitions to asexuality in a microsporidian parasite

Assessing the mode of reproduction of microparasites remains a difficult task because direct evidence for sexual processes is often absent and the biological covariates of sex and asex are poorly known. Species with geographically divergent modes of reproduction offer the possibility to explore some of these covariates, for example, the influence of life‐history traits, mode of transmission and life‐cycle complexity. Here, we present a phylogeographical study of a microsporidian parasite, which allows us to relate population genetic structure and mode of reproduction to its geographically diverged life histories. We show that in microsporidians from the genus Hamiltosporidium, that use the cladoceran Daphnia as host, an epidemic population structure has evolved, most probably since the last Ice Age. We partially sequenced three housekeeping genes (alpha tubulin, beta tubulin and hsp70) and genotyped seven microsatellite loci in 51 Hamiltosporidium isolates sampled within Europe and the Middle East. We found two phylogenetically related asexual parasite lines, one each from Fennoscandia and Israel, which share the unique ability of being transmitted both vertically and horizontally from Daphnia to Daphnia. The sexual forms cannot transmit horizontally among Daphnia, but presumably have a complex life cycle with a second host species. In spite of the similarities between the two asexual lineages, a clustering analysis based on microsatellite polymorphisms shows that asexual Fennoscandian parasites do not share ancestry with any other Hamiltosporidium that we have sampled. Moreover, allele sequence divergence at the hsp70 locus is twice as large in Fennoscandian than in Israeli parasites. Our results indicate that asexual reproduction evolved twice independently, first in Fennoscandian and more recently in the Israeli parasites. We conclude that the independent origin of asexuality in these two populations is associated with the altered parasite mode of transmission and the underlying dynamics of host populations.     

Variation in life-history traits among Urtica dioica populations with different history in parasitism by the holoparasitic plant Cuscuta europaea

Several studies demonstrate that natural enemies (e.g. parasites) have profound negative effects on the life-history traits of their hosts. If the host can compensate for the negative effects of parasitic infection by altering its life history, these modifications may partly form the basis of resistance or tolerance against parasites. Thus, parasites may be of considerable importance in shaping the evolution of life-history traits of their hosts. To examine if previous parasitism is associated with differences in life-history traits of the host, I conducted a common garden experiment with Urtica dioica plants originating from eight populations of which four were unparasitized, and four parasitized by the holoparasitic plant, Cuscuta europaea. A field survey indicated no differences between unparasitized and parasitized populations in, for example, the number of plant species and nutrient levels in the soil. Thus, it seems reasonable to assume that differences in life-history traits between the two population types in the common garden would reflect the effects of previous selection by the parasite. In the common garden, plants from parasitized populations started to flower later and allocated less biomass to asexual reproduction (measured as the production of stolons, i.e. clonal propagation) compared to plants from unparasitized populations. These results thus indicate that selection by the parasite may have favoured later onset of flowering, and may have selected against asexual reproduction.     

What generates the diversity of Wolbachia—arthropod interactions?

Wolbachia are strictly endocellular, vertically transmitted bacteria associated with insects and crustaceans. This group of parasites modify their hosts' reproduction so as to increase their own fitness. This paper reviews the variability of these parasitic alterations and their consequences for host biology and populations. Wolbachia induce cytoplasmic incompatibility (a characteristic apparently specific to Wolbachia) in several insects and one isopod crustacean; parthenogenesis (thelytoky) in haplo-diploid insects; feminization in various isopods. The consequences of these phenomena on speciation, population dynamics and genetic polymorphism are discussed. The variability of the mechanisms of host sex determination is one important factor responsible for the diversity of Wolbachia-host interactions. However, parasite characteristics, such as the capacity to disturb host mitosis, and the ability to be horizontally transferred between hosts, also appear to play a role in this diversity.     

Exceptional cryptic diversity and multiple origins of parthenogenesis in a freshwater ostracod

The persistence of asexual reproduction in many taxa depends on a balance between the origin of new asexual lineages and the extinction of old ones. This turnover determines the diversity of extant asexual populations and so influences the interaction between sexual and asexual modes of reproduction. Species with mixed reproduction, like the freshwater ostracod (Crustacea) morphospecies Eucypris virens, are a good model to examine these dynamics. This species is also a geographic parthenogen, in which sexual females and males co-exist with asexual females in the circum-Mediterranean area only, whereas asexual females occur all over Europe. A molecular phylogeny of E. virens based on the mitochondrial COI and 16S fragments is presented. It is characterised by many distinct clusters of haplotypes which are either exclusively sexual or asexual, with only one exception, and are often separated by deep branches. Analysis of the phylogeny reveals an astonishing cryptic diversity, which indicates the existence of a species complex with more than 40 cryptic taxa. We therefore suggest a revision of the single species status of E. virens. The phylogeny indicates multiple transitions from diverse sexual ancestor populations to asexuality. Although many transitions appear to be ancient, we argue that this may be an artefact of the existence of unsampled or extinct sexual lineages.     

Pollen profile,spatial structure,and access to sex in asexual hermaphrodites

Classical cost‐of‐sex models predict the rapid fixation of asexual reproduction. Coexistence of sexuals and asexuals is common among hermaphrodite plants, however, providing asexuals with access to sex via their male function; some of the sexually reproduced progeny they sire will be asexual. The ability of asexuals to sire progeny is often hindered by the production of poor quality pollen. Using cellular automata, it is shown that decreases in pollen quality in asexuals can greatly increase the period of coexistence of sexuals and asexuals and, consequently, the cumulative contribution of sex to asex. Extensive periods of coexistence are only likely, however, if pollen and seed are dispersed locally, in which case coexistence over thousands of generations can be achieved. It is argued that, with local dispersal, the negative relationship between pollen quality and the period of coexistence of sex and asex will result in patterns of geographic parthenogenesis in which asexuals that coexist with sexuals will exhibit a poor male function, whereas asexuals with a very efficient male function will occur in exclusively asexual populations. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 954–966.     

Impact of a social parasite on ant host populations depends on host species,habitat and year

Parasites often affect the abundance and life‐history traits of their hosts. We studied the impact of a social parasite – a slavemaking ant – on host ant communities using two complementary field manipulations. In the first experiment, we analysed the effect of social parasite presence on host populations in one habitat. In a second experiment, conducted in two habitats, we used a cross‐fostering design, analysing the effect of sympatric and allopatric social parasites. In the first experiment, host colonies benefited to some extent from residing in parasite‐free areas, showing increased total production. Yet, in the second experiment, host colonies in plots containing social parasites were more productive, and this effect was most evident in response to allopatric social parasites. We propose several explanations for these inconsistent results, which are related to environmental variability. The discrepancies between the two habitats can be explained well by ecological variation as a result of differences in altitudes and climate. For example, ant colonies in the colder habitat were larger and, for one host species, colonies were more often polygynous. In addition, our long‐term documentation – a total of four measurements of community structure in 6 years – showed temporal variation in abundance and life‐history traits of ant colonies, unrelated to the manipulations. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 559–570.     

Extreme tolerance to environmental stress of sexual and parthenogenetic resting eggs of Eucypris virens (Crustacea,Ostracoda)

1. The freshwater ostracod (Ostracoda), Eucypris virens, is commonly found in European temporary pools, where its long‐term persistence completely relies on the build‐up of resting egg banks. Extreme tolerance of dormant eggs and seeds is widely assumed, but freshwater ostracod eggs are relatively poorly studied. The study of ostracod resting eggs is of particular relevance as it may yield the key to understanding the distribution of the sexes in many species capable of both sexual and asexual reproduction. 2. We assessed the tolerance of dried resting eggs produced by females originating from three populations with males and three all‐female E. virens populations. Hatching time and success was compared between control eggs and eggs exposed to one of seven ecologically relevant stressors: digestive enzymes, high salinity, deep freezing, hydration, UV‐B radiation, hypoxia and insecticide treatment. 3. None of the stressors reduced significantly the viability of either sexual or asexual eggs. When compared with the reproductive mode–specific controls, exposure to UV‐B radiation had a mild impact on the survival of sexual and asexual eggs (?16.8 and ?22.4%, respectively), but this was only significant for asexual eggs. These results point to an extreme tolerance of E. virens resting eggs and have important implications for the ecology and evolution of the species. 4. The timing of hatching was not affected by the stress treatment, except for UV‐B radiation. A marginally significant delay in hatching response was observed for UV‐B‐radiated eggs when compared to the overall mean, but this treatment effect was absent when compared with the reproductive mode–specific controls. 5. The populations with males produced eggs that hatched on average earlier (?1.5 days at 17 °C) and were more successful (+26%) than asexual eggs. Due to the limited number of populations and the population‐specific origin and age of the eggs, the possibility due to the differences in age and origin of the resting eggs, or to variations in local conditions, cannot be ruled out.     

The maintenance of sex: host–parasite coevolution with density‐dependent virulence

Why don’t asexual females replace sexual females in most natural populations of eukaryotes? One promising explanation is that parasites could counter the reproductive advantages of asexual reproduction by exerting frequency‐dependent selection against common clones (the Red Queen hypothesis). One apparent limitation of the Red Queen theory, however, is that parasites would seem to be required by theory to be highly virulent. In the present study, I present a population‐dynamic view of competition between sexual females and asexual females that interact with co‐evolving parasites. The results show that asexual populations have higher carrying capacities, and more unstable population dynamics, than sexual populations. The results also suggest that the spread of a clone into a sexual population could increase the effective parasite virulence as population density increases. This combination of parasite‐mediated frequency‐dependent selection, and density‐dependent virulence, could lead to the coexistence of sexual and asexual reproductive strategies and the long‐term persistence of sex.     

First case of parthenogenesis in ladybirds (Coleoptera: Coccinellidae) suggests new mechanisms for the evolution of asexual reproduction

Parthenogenesis, the development of unfertilized eggs resulting in the exclusive production of female offspring, is rare in animals relative to sexual reproduction and is mainly reported in invertebrates. It has been hypothesized that polyploidy, hybridization and endosymbiont infections are its major causal events but the mechanisms triggering asexual reproduction remain unclear. Here, we study the proximate causes at the origin of parthenogenesis in the first reported case of asexuality in the Coccinellidae (Coleoptera). The asexual populations were found in the Azores and the Mascarene archipelagos, and were identified as Nephus voeltzkowi Weise, a bisexual species widespread in sub-Saharan Africa. The specimens from both populations are diploid but present different karyotypes and heterozygosities that evoke hybrid origins, commonly associated with parthenogenesis in Coleoptera. However, the close proximity of their genomes (99.8% homology for the complete mitochondrial genome and 99.9% for the complete nuclear ribosomal cluster) together with the congruence between the mtDNA tree and the nuclear tree, and the low heterozygosity levels, suggests that the two populations are not hybrid. We propose that they belong to a single chromosomally polymorphic species undergoing Robertsonian fusions. Furthermore, specimens from both populations are infected with Wolbachia (supergroup B strain), contrary to sympatric bisexual species of the same genus. Although Wolbachia has been shown to induce parthenogenesis in haplodiploid organisms, it has been recently suggested that it could also induce parthenogenesis in hosts with other sex determination systems. Whether chromosome rearrangements and/or Wolbachia infections are post-parthenogenetic events or are at the origin of parthenogenesis still needs to be determined.     

GFP‐targeting allows visualization of the apicoplast throughout the life cycle of live malaria parasites

Background information. The Plasmodium parasite, during its life cycle, undergoes three phases of asexual reproduction, these being repeated rounds of erythrocytic schizogony, sporogony within oocysts on the mosquito midgut wall and exo‐erythrocytic schizogony within the hepatocyte. During each phase of asexual reproduction, the parasite must ensure that every new daughter cell contains an apicoplast, as this organelle cannot be formed de novo and is essential for parasite survival. To date, studies visualizing the apicoplast in live Plasmodium parasites have been restricted to the blood stages of Plasmodium falciparum. Results. In the present study, we have generated Plasmodium berghei parasites in which GFP (green fluorescent protein) is targeted to the apicoplast using the specific targeting sequence of ACP (acyl carrier protein), which has allowed us to visualize this organelle in live Plasmodium parasites. During each phase of asexual reproduction, the apicoplast becomes highly branched, but remains as a single organelle until the completion of nuclear division, whereupon it divides and is rapidly segregated into newly forming daughter cells. We have shown that the antimicrobial agents azithromycin, clindamycin and doxycycline block development of the apicoplast during exo‐erythrocytic schizogony in vitro, leading to impaired parasite maturation. Conclusions. Using a range of powerful live microscopy techniques, we show for the first time the development of a Plasmodium organelle through the entire life cycle of the parasite. Evidence is provided that interference with the development of the Plasmodium apicoplast results in the failure to produce red‐blood‐cell‐infective merozoites.     

Density,parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

Many organisms can reproduce both asexually and sexually. For cyclical parthenogens, periods of asexual reproduction are punctuated by bouts of sexual reproduction, and the shift from asexual to sexual reproduction has large impacts on fitness and population dynamics. We studied populations of Daphnia dentifera to determine the amount of investment in sexual reproduction as well as the factors associated with variation in investment in sex. To do so, we tracked host density, infections by nine different parasites, and sexual reproduction in 15 lake populations of D. dentifera for 3 years. Sexual reproduction was seasonal, with male and ephippial female production beginning as early as late September and generally increasing through November. However, there was substantial variation in the prevalence of sexual individuals across populations, with some populations remaining entirely asexual throughout the study period and others shifting almost entirely to sexual females and males. We found strong relationships between density, prevalence of infection, parasite species richness, and sexual reproduction in these populations. However, strong collinearity between density, parasitism, and sexual reproduction means that further work will be required to disentangle the causal mechanisms underlying these relationships.     

Marked variation in parasite resistance between two wild populations of the Trinidadian guppy, Poecilia reticulata (Pisces: Poeciliidae)

This is the first study to demonstrate significant differences between two natural Trinidadian guppy populations in susceptibility to a pathogenic monogenean parasite, Gyrodactylus turnbulli . Following experimental infection with an isogenic laboratory culture of G. turnbulli , fish from the Upper Aripo (UA) lost parasites more slowly and carried up to three times as many parasites at peak infection than did those from the Lower Aripo (LA). The UA population appeared to be more susceptible than the LA fish, even though fish of both populations were naïve to this particular laboratory strain of G. turnbulli and had not encountered any gyrodactylid infection for at least 65 days. The parasite infection reduced the feeding response and feeding activity of UA and LA fish to a similar extent. Our findings suggest that this ectoparasite may have an important impact on the evolutionary biology of guppies (and possibly of other teleosts), particularly as the prevalence of Gyrodactylus infection in natural populations may be as high as 50%. We discuss the role of parasite infections on natural and sexual selection, the good genes model and the implications for immunogenetics in small genetically isolated host populations.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79 , 645–651.