Sex and polymorphism as strategies in host-pathogen interactions

Evidence from automixis with inverted meiosis for the maintenance of sex by loss of complementation

The adaptive value of sexual reproduction is still debated. A short-term disadvantage of asexual reproduction is loss of heterozygosity, which leads to the unmasking of recessive deleterious mutations. The cost of this loss of complementation is predicted to be higher than the twofold cost of meiosis for most types of asexual reproduction. Automixis with terminal fusion of sister nuclei is especially vulnerable to the effect of loss of complementation. It is found, however, in some taxa including oribatid mites, the most prominent group of ancient asexuals. Here, I show that automixis with terminal fusion is stable if it is associated with inverted meiosis and that this appears to be the case in nature, notably in oribatid mites. The existence of automixis with terminal fusion, and its co-occurrence with inverted meiosis, therefore, is consistent with the hypothesis that loss of complementation is important in the evolution of sexual reproduction.

Consumption, contact and copulation: how pathogens have shaped human psychological adaptations

Disgust is an emotion intimately linked to pathogen avoidance. Building on prior work, we suggest disgust is an output of programmes that evolved to address three separate adaptive problems: what to eat, what to touch and with whom to have sex. We briefly discuss the architecture of these programmes, specifying their perceptual inputs and the contextual factors that enable them to generate adaptive and flexible behaviour. We propose that our sense of disgust is the result of these programmes and occurs when information-processing circuitries assess low expected values of consumption, low expected values of contact or low expected sexual values. This conception of disgust differs from prior models in that it dissects pathogen-related selection pressures into adaptive problems related to consumption and contact rather than assuming just one pathogen disgust system, and it excludes moral disgust from the domain of disgust proper. Instead, we illustrate how low expected values of consumption and contact as well as low expected sexual values can be used by our moral psychology to provide multiple causal links between disgust and morality.This article is part of the Theo Murphy meeting issue 'Evolution of pathogen and parasite avoidance behaviours'.

THE GEOGRAPHY OF COEVOLUTION: COMPARATIVE POPULATION STRUCTURES FOR A SNAIL AND ITS TREMATODE PARASITE

Gene flow and the genetic structure of host and parasite populations are critical to the coevolutionary process, including the conditions under which antagonistic coevolution favors sexual reproduction. Here we compare the genetic structures of different populations of a freshwater New Zealand snail (Potamopyrgus antipodarum) with its trematode parasite (Microphallus sp.) using allozyme frequency data. Allozyme variation among snail populations was found to be highly structured among lakes; but for the parasite there was little allozyme structure among lake populations, suggesting much higher levels of parasite gene flow. The overall pattern of variation was confirmed with principal component analysis, which also showed that the organization of genetic differentiation for the snail (but not the parasite) was strongly related to the geographic arrangement of lakes. Some snail populations from different sides of the Alps near mountain passes were more similar to each other than to other snail populations on the same side of the Alps. Furthermore, genetic distances among parasite populations were correlated with the genetic distances among host populations, and genetic distances among both host and parasite populations were correlated with "stepping-stone" distances among lakes. Hence, the host snail and its trematode parasite seem to be dispersing to adjacent lakes in a stepping-stone fashion, although parasite dispersal among lakes is clearly greater. High parasite gene flow should help to continuously reintroduce genetic diversity within local populations where strong selection might otherwise isolate "host races." Parasite gene flow can thereby facilitate the coevolutionary (Red Queen) dynamics that confer an advantage to sexual reproduction by restoring lost genetic variation.

THE MAINTENANCE OF SEX BY PARASITISM AND MUTATION ACCUMULATION UNDER EPISTATIC FITNESS FUNCTIONS

The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load, while the Red Queen hypothesis holds that sex is adaptive as a defense against coevolving pathogens. We used computer simulations to examine the combined and separate effects of selection against deleterious mutations and host-parasite coevolution on the spread of a clone into an outcrossing sexual population. The results suggest that the two processes operating simultaneously may select for sex independent of the exact shape of the function that maps mutation number onto host fitness.

EVOLUTIONARY GENETICS OF ALLORECOGNITION IN THE COLONIAL HYDROID HYDRACTINIA SYMBIOLONGICARPUS

Many sedentary, clonal marine invertebrates compete intensively with conspecifics for habitable space. Allorecognition systems mediate the nature and outcome of these intraspecific competitive interactions, such that the initiation of agonistic behavior and the potential for intergenotypic fusion depend strongly on the relatedness of the contestants. The dependence of these behaviors on relatedness, along with the extraordinary precision with which self can be discriminated from nonself, suggest that allorecognition systems are highly polymorphic genetically. However, allotypic specificity of this sort could be produced by any number of genetic scenarios, ranging from relatively few loci with abundant allelic variation to numerous loci with relatively few alleles per locus. At this point, virtually nothing is known of the formal genetics of allorecognition in marine invertebrates; consequently, the evolutionary dynamics of such systems remain poorly understood. In this paper, we characterize the formal genetics of allorecognition in the marine hydrozoan Hydractinia symbiolongicarpus. Hydractinia symbiolongicarpus colonizes gastropod shells occupied by hermit crabs. When two or more individuals grow into contact, one of three outcomes ensues: fusion (compatibility), transitory fusion (a temporary state of compatibility), and rejection (incompatibility, often accompanied by the production of agonistic structures termed hyperplastic stolons). Observed patterns of compatibility between unrelated, half-sib pairs, and full-sib pairs show that unrelated and half-sib pairs under laboratory culture have a very low probability of being fusible, whereas full sibs have a roughly 30% rate of fusion in experimental pairings. The genetic simulations indicate that roughly five loci, with 5-7 alleles per locus, confer specificity in this species. In ecological terms, the reproductive ecology of H. symbiolongicarpus should promote the cosettlement of kin, some of which should be full sibs, and some half sibs. Thus, there is potential for kin selection to play a major role in the evolution of the H. symbiolongicarpus allorecognition system. In genetic terms, this system conforms to theoretical predictions for a recognition system selected to distinguish among classes of kin, in addition to self from nonself.

GENETIC STRUCTURE OF COEXISTING SEXUAL AND CLONAL SUBPOPULATIONS IN A FRESHWATER SNAIL (POTAMOPYRGUS ANTIPODARUM)

We examined clonal diversity and the distribution of both clonal and sexual genotypes in a single population of freshwater snails (Potamopyrgus antipodarum) in which diploid sexual individuals and triploid parthenogens coexist. A genetic analysis of individuals from three habitat zones in Lake Alexandrina, New Zealand revealed extremely high clonal diversity: 165 genotypes among 605 clonal individuals. The frequency of triploid clonal individuals increased with increasing depth in the lake, and most of the individual clones were habitat specific, suggesting that differences among habitats are important in structuring the clonal subpopulation. There were also high levels of clonal diversity within habitats, suggesting frequent origins of habitat-specific clones. In contrast, diploid sexual individuals were proportionately more common in the shallow regions of the lake (where infection by trematode larvae is highest), and there was no significant spatial structure in the sexual subpopulation. We suggest that habitat specialization by clones, as well as parasite-mediated selection against common clones, are important factors affecting the structure of this mixed population of sexual and clonal snails.

GENETIC RELATIONSHIPS BETWEEN PARASITE VIRULENCE AND TRANSMISSION IN THE RODENT MALARIA PLASMODIUM CHABAUDI

Many parasites evolve to become virulent rather than benign mutualists. One of the major theoretical models of parasite virulence postulates that this is because rapid within-host replication rates are necessary for successful transmission (parasite fitness) and that virulence (damage to the host) is an unavoidable consequence of this rapid replication. Two fundamental assumptions underlying this so-called evolutionary trade-off model have rarely been tested empirically: (1) that higher replication rates lead to higher levels of virulence; and (2) that higher replication rates lead to higher transmission. Both of these relationships must have a genetic basis for this evolutionary hypothesis to be relevant. These assumptions were tested in the rodent malaria parasite, Plasmodium chabaudi, by examining genetic relationships between virulence and transmission traits across a population of eight parasite clones isolated from the wild. Each clone was injected into groups of inbred mice in a controlled laboratory environment, and replication rate (measured by maximum asexual parasitemia), virulence (measured by live-weight loss and degree of anemia in the mouse), and transmission (measured by density of sexual forms, gametocytes, in the blood and proportion of mosquitoes infected after taking a blood-meal from the mouse) were assessed. It was found that clones differed widely in these traits and these clone differences were repeatable over successive blood passages. Virulence traits were strongly phenotypically and genetically (i.e., across clones) correlated to maximum parasitemia thus supporting the first assumption that rapid replication causes higher virulence. Transmission traits were also positively phenotypically and genetically correlated to parasitemia, which supports the second assumption that rapid replication leads to higher transmission. Thus, two assumptions of the parasite-centered trade-off model of the evolution of virulence were shown to be justified in malaria parasites.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. V. A FIELD TEST OF THE SIB-COMPETITION LOTTERY HYPOTHESIS

Sexually and asexually derived tillers of Anthoxanthum odoratum were planted directly in the field to test the hypothesis that competition among groups of sexual and asexual siblings favors the maintenance of sexual reproduction in populations. The results showed a substantial fitness advantage for sexual tillers. However, in contrast with the models, the advantage of sex did not increase with increasing numbers of colonists in the patch, there were multiple survivors among colonists, and an advantage was observed even for singly planted tillers. When a truncation-selection scheme was imposed ex post facto on the data, the relative performance of sexual tillers was similar to that predicted by the Bulmer (1980) model, suggesting that sib-competition models fail due to the violation of the assumption of truncation selection. The advantage of sex was not correlated with the presence of other species, total percentage cover, or species diversity, although sites where sex was favored were physically clustered.

Parasites and sex: Catching the red queen

One version of the Red Queen hypothesis suggests that sexual reproduction may be an advantage in a coevolutionary arms race. Antagonistic biotic interactions, especially those between parasite and host, are thought to represent a sufficient evolutionary force to counterbalance the supposed inefficiency of sexual reproduction. Recent experimental studies demonstrate negative frequency-dependent selection, increased parasite load in parthenogenetic races relative to sympatric sexual conspecifics and correlations between recombination rate and frequency of parasitic chromosomes. These studies provide strong empirical evidence that there is an important role for parasites in maintaining sex.

Preserving genes: a model of the maintenance of genetic variation in a metapopulation under frequency-dependent selection

Understanding how genetic variability is maintained in natural populations is of both theoretical and practical interest. In particular, the subdivision of populations into demes linked by low levels of migration has been suggested to play an important role. But the maintenance of genetic variation in populations is also often linked to the maintenance of sexual reproduction: any force that acts to maintain sex should also act to maintain variation. One theory for the maintenance of sex, the Red Queen, states that sex and variation are maintained by antagonistic coevolutionary interactions – especially those between hosts and their harmful parasites – that give rise to negative frequency-dependent selection. In this paper I present a model to examine the relationships between population subdivision, negative frequency-dependent selection due to parasites, the maintenance of sex, and the preservation of alleles from fixation. The results show strong interactions between migration rates, negative frequency-dependent selection, and the maintenance of variability for sexual and asexual populations.

Factors affecting the evolution of virulence: nematode parasites of fig wasps as a case study

The natural history of fig-pollinating wasps and their associated species-specific nematodes allows the measurement of many parameters which are relevant to testing hypotheses concerning host-parasite ecology and evolution. Within fig wasps species, it is possible to estimate lifetime reproductive success of foundress wasps as a function of presence or absence of nematode parasitism (virulence). Across species, there is a wide range of host population structures which, in turn, results in a range of opportunities for either horizontal or vertical nematode transmission. Therefore, estimates of virulence can be related to opportunities for transmission across a group of closely related hosts and their parasites. Further, the dynamics of the nematode infections over ecological and short-term evolutionary timescales can be monitored, giving added insight into the interpretation of the virulence estimates. Moreover, several scales of longer term evolutionary relationships are either known directly from fossil evidence or can be inferred from molecular data, providing deeper temporal context for the observed patterns. This combination of attributes permits detailed testing of hypotheses concerning the factors that potentially influence the evolution of virulence in host-parasite systems, and further, population and simulation models of the system that incorporate the parameter estimates can clarify the interpretation of how those factors act. There is little evidence suggesting that intermediate and long-term evolutionary relationships explain current levels of virulence. That is, it appears that virulence can change rapidly relative to speciation events, and that the nematodes do not tend to become ‘benign over time’. Instead, it appears that host population structure can influence the evolution of parasite virulence by affecting the relative opportunities for horizontal to vertical transmission, which, in turn, influences the relative costs and benefits of virulence to the nematodes. At one level, increased opportunities for horizontal transmission decouple the reproductive interests of the individual nematodes from those of the individual hosts that they are directly parasitizing, thereby reducing the cost of virulence to individual nematodes. At another level, increased opportunities of horizontal transmission also increases the relative frequency of hosts infected by multiple strains of nematodes. This promotes the evolution of more virulent forms by increasing the relative importance of within-host competition among nematode strains, thereby favouring strains that ‘eat more host sooner’. An interesting property of the fig-nematode systems is that the proportion of infected hosts does not change dramatically through time. This finding implies that there can be considerable negative effects on survival of infected hosts in addition to the previously documented reductions in fecundity of infected foundresses, because the latter are insufficient to account for the observed stability of wasp infection rates.

Differences between selection on sex versus recombination in red queen models with diploid hosts

The Red Queen hypothesis argues that parasites generate selection for genetic mixing (sex and recombination) in their hosts. A number of recent papers have examined this hypothesis using models with haploid hosts. In these haploid models, sex and recombination are selectively equivalent. However, sex and recombination are not equivalent in diploids because selection on sex depends on the consequences of segregation as well as recombination. Here I compare how parasites select on modifiers of sexual reproduction and modifiers of recombination rate. Across a wide set of parameters, parasites tend to select against both sex and recombination, though recombination is favored more often than is sex. There is little correspondence between the conditions favoring sex and those favoring recombination, indicating that the direction of selection on sex is often determined by the effects of segregation, not recombination. Moreover, when sex was favored it is usually due to a long-term advantage whereas short-term effects are often responsible for selection favoring recombination. These results strongly indicate that Red Queen models focusing exclusively on the effects of recombination cannot be used to infer the type of selection on sex that is generated by parasites on diploid hosts.

Sexual reproduction facilitates the adaptation of parasites to antagonistic host environments: Evidence from empirical study in the wheat-Mycosphaerella graminicola system

Most eukaryotes use sexual reproduction to transmit genetic information from generation to generation despite the advantages offered by asexual reproduction. One theory to explain the origin and maintenance of sexual reproduction hypothesises that sexual recombination generates genetic variation that allows faster adaptation to fluctuating and/or stressful environments. We used a combination of ecological, molecular genetic, statistical and experimental evolution approaches to test this hypothesis in an agricultural plant-pathogen system. We inoculated wheat hosts with 10 strains of the fungal pathogen Mycosphaerella graminicola in a field experiment and estimated the contributions of sexual reproduction, asexual reproduction and immigration to the genetic composition of fungal populations sampled from moderately resistant and susceptible hosts through the course of an epidemic cycle. We found that a significant proportion of the M. graminicola population in the late phase of the epidemic originated from sexual reproduction among isolates that had been introduced into the field plots at the beginning of the epidemic. Recombinants were recovered at a higher frequency on the moderately resistant plant host Madsen than on the susceptible host Stephens. By the end of the growing season, we estimated that approximately 13% of the strains sampled from the resistant host were recombinants, compared with 9% in the samples collected from the susceptible host. We also found that pathogen strains originating from the resistant cultivar displayed higher levels of fitness, virulence and fungicide tolerance than those originating from the susceptible cultivar. Our results provide empirical support for the hypothesis that sexual reproduction facilitates the evolution of parasites to overcome host resistance.

Similarity selection and the evolution of sex: revisiting the red queen

For over 25 years, many evolutionary ecologists have believed that sexual reproduction occurs because it allows hosts to change genotypes each generation and thereby evade their coevolving parasites. However, recent influential theoretical analyses suggest that, though parasites can select for sex under some conditions, they often select against it. These models assume that encounters between hosts and parasites are completely random. Because of this assumption, the fitness of a host depends only on its own genotype (“genotypic selection”). If a host is even slightly more likely to encounter a parasite transmitted by its mother than expected by random chance, then the fitness of a host also depends on its genetic similarity to its mother (“similarity selection”). A population genetic model is presented here that includes both genotypic and similarity selection, allowing them to be directly compared in the same framework. It is shown that similarity selection is a much more potent force with respect to the evolution of sex than is genotypic selection. Consequently, similarity selection can drive the evolution of sex even if it is much weaker than genotypic selection with respect to fitness. Examination of explicit coevolutionary models reveals that even a small degree of mother–offspring parasite transmission can cause parasites to favor sex rather than oppose it. In contrast to previous predictions, the model shows that weakly virulent parasites are more likely to favor sex than are highly virulent ones. Parasites have figured prominently in discussions of the evolution of sex, but recent models suggest that parasites often select against sex rather than for it. With the inclusion of small and realistic exposure biases, parasites are much more likely to favor sex. Though parasites alone may not provide a complete explanation for sex, the results presented here expand the potential for parasites to contribute to the maintenance of sex rather than act against it.

That parasites help explain why sex evolved has been disputed, but a new model incorporating similarity selection--where a host encounters parasites from its mother more often than by chance--makes its evolution more likely.

Host-parasite coevolution and selection on sex through the effects of segregation

The advantage of producing novel variation to keep apace of coevolving species has been invoked as a major explanation for the evolution and maintenance of sex (the Red Queen hypothesis). Recent theoretical investigations of the Red Queen hypothesis have focused on the effects of recombination in haploid species, finding that species interactions rarely favor the evolution of sex unless selection is strong. Yet by focusing on haploids, these studies have ignored a potential advantage of sex in diploids: generating novel combinations of alleles at a particular locus through segregation. Here we investigate models of host-parasite coevolution in diploid species to determine whether the advantages of segregation might rescue the Red Queen hypothesis as a more general explanation for the evolution of sex. We find that the effects of segregation can favor the evolution of sex but only under some models of infection and some parameter combinations, almost always requiring inbreeding. In all other cases, the effects of segregation on selected loci favor reductions in the frequency of sex. In cases where segregation and recombination act in opposite directions, we found that the effects of segregation dominate as an evolutionary force acting on sex in diploids.

Spatial and temporal patterns of parthenogenesis and parasitism in the freshwater snail Melanoides tuberculata

The Red Queen hypothesis predicts that sex should be more common in populations heavily infested with parasites, than in those without. This hypothesis was investigated in the aquatic snail Melanoides tuberculata, in which both sexual and parthenogenetic individuals exist in natural populations, and some populations are heavily infested by trematodes. The presence of fertile males and the higher genetic diversity of bisexual populations are indicative of sexual reproduction. We compared sites in 1990, 1999, and 2001, and we looked for a positive correlation between male and parasite frequencies. Male frequency was not correlated with the frequency of individuals infected by trematodes. This lack of correlation was reconfirmed in a retrospective power analysis. In a period of 9 years, male frequencies decreased but infection levels increased. These results do not support the Red Queen hypothesis. In samples with high male frequency the number of embryos was low, perhaps indicating that males may have a negative effect on embryo numbers. This effect of males on fitness could perhaps suggest that the cost of sex is fewer embryos. The reduction in embryo numbers may also represent a trade-off between mating and egg production costs.

Costly resistance to parasitism: evidence from simultaneous quantitative trait loci mapping for resistance and fitness in Tribolium castaneum

Information on the molecular basis of resistance and the evolution of resistance is crucial to an understanding of the appearance, spread, and distribution of resistance genes and of the mechanisms of host adaptation in natural populations. One potential important genetic constraint for the evolution of resistance is fitness cost associated with resistance. To determine whether host resistance to parasite infection is associated with fitness costs, we conducted simultaneous quantitative trait loci (QTL) mapping of resistance to parasite infection and fitness traits using the red flour beetle (Tribolium castaneum) and the tapeworm parasite (Hymenolepis diminuta) system in two independent segregating populations. A genome-wide QTL scan using amplified fragment length polymorphism (AFLP) markers revealed three QTL for beetle resistance to tapeworm infection. These three QTL account for 44-58% variance in beetle infection intensity. We identified five QTL for fecundity and five QTL for egg-to-adult viability, which accounted for 36-57% and 36-49%, respectively, of the phenotypic variance in fecundity and egg-to-adult viability. The three QTL conferring resistance were colocalized with the QTL affecting beetle fitness. The genome regions that contain the QTL for parasite resistance explained the majority of the variance in fecundity and egg-to-adult viability in the mapping populations. Colocalization of QTL conferring resistance to parasite infection and beetle fitness may result from the pleiotropic effects of the resistance genes on host fitness or from tight linkages between resistance genes and adverse deleterious mutations. Therefore, our results provide evidence that the genome regions conferring resistance to tapeworm infection are partially responsible for fitness costs in the resistant beetle populations.

Recombination and loss of complementation: a more than two-fold cost for parthenogenesis

Certain types of asexual reproduction lead to loss of complementation, that is unmasking of recessive deleterious alleles. A theoretical measure of this loss is calculated for apomixis, automixis and endomitosis in the cases of diploidy and polyploidy. The effect of the consequent unmasking of deleterious recessive mutations on fitness is also calculated. Results show that, depending on the number of lethal equivalents and on the frequency of recombination, the cost produced by loss of complementation after few generations of asexual reproduction may be greater than the two-fold cost of meiosis. Maintaining complementation may, therefore, provide a general short-term advantage for sexual reproduction. Apomixis can replace sexual reproduction under a wide range of parameters only if it is associated with triploidy or tetraploidy, which is consistent with our knowledge of the distribution of apomixis.

Loss of complementation and the logic of two-step meiosis

Meiosis is usually a two-step process: two divisions preceded by a duplication. One-step meiosis, a single division without prior replication, is a more logical way to produce haploid gametes; moreover, one-step meiosis leads to higher variabilty in the progeny than two-step meiosis. Yet one-step meiosis is very rare in nature, and may not even exist at all. I suggest that this is because one-step meiosis, in contrast to two-step meiosis, can be easily invaded and replaced by asexual reproduction. I discuss why other existing peculiar forms of division leading to the production of haploid gametes, but not one-step meiosis, have the same effect as two-step meiosis.

Quantitative trait loci for susceptibility to tapeworm infection in the red flour beetle

Parasites have profound effects on host ecology and evolution, and the effects of parasites on host ecology are often influenced by the magnitude of host susceptibility to parasites. Many parasites have complex life cycles that require intermediate hosts for their transmission, but little is known about the genetic basis of the intermediate host's susceptibility to these parasites. This study examined the genetic basis of susceptibility to a tapeworm (Hymenolepis diminuta) in the red flour beetle (Tribolium castaneum) that serves as an intermediate host in its transmission. Quantitative trait loci (QTL) mapping experiments were conducted with two independent segregating populations using amplified fragment length polymorphism (AFLP) markers and randomly amplified polymorphic DNA (RAPD) markers. A total of five QTL that significantly affected beetle susceptibility were identified in the two reciprocal crosses. Two common QTL on linkage groups 3 and 6 were identified in both crosses with similar effects on the phenotype, and three QTL were unique to each cross. In one cross, the three main QTL accounted for 29% of the total phenotypic variance and digenic epistasis explained 39% of the variance. In the second cross, the four main QTL explained 62% of the variance and digenic epistasis accounted for only 5% of the variance. The actions of these QTL were either overdominance or underdominance. Our results suggest that the polygenic nature of beetle susceptibility to the parasites and epistasis are important genetic mechanisms for the maintenance of variation within or among beetle strains in susceptibility to tapeworm infection.

Sex-specific genetic structure: new trends for dioecious parasites

In dioecious parasite species, genetic structure can differ between sexes, as recently demonstrated for the digenetic trematode Schistosoma mansoni and the ectoparasitic tick Ixodes ricinus. This article presents some of the methods that allow detecting such a pattern in natural populations. The proximate and ultimate factors that potentially generate a sex-specific genetic structure are discussed, as are evolutionary and epidemiological consequences for dioecious parasites and vectors.

The maintenance of sex in parasites

The maintenance of sex is an unresolved paradox in evolutionary biology, given the inherent twofold fitness advantage for asexuals. Parasitic helminths offer a unique opportunity to address this enigma. Parasites that can create novel antigenic strains are able to escape pre-existing host immunity. Viruses produce diversity through mutation with rapid clonal proliferation. The long generation times of helminth parasites prevent them from adopting this strategy. Instead, we argue that sexual reproduction enables parasitic helminths to rapidly generate strain diversity. We use both a stochastic, individual-based model and a simple analytical model to assess the selective value of sexual versus asexual reproduction in helminth parasites. We demonstrate that sexual reproduction can more easily produce and maintain strain diversity than asexual reproduction for long-lived parasites. We also show that sexual parasite populations are resistant to invasion by rare asexual mutants. These results are robust to high levels of cross-immunity between strains. We suggest that the enhancement of strain diversity, despite stochastic extinction of strains, may be critical to the evolutionary success of sex in long-lived parasites.

Sex-specific genetic structure in Schistosoma mansoni: evolutionary and epidemiological implications

We studied the population genetic structure of 360 and 1247 adult Schistosoma mansoni using seven microsatellite and seven random amplified polymorphic DNA (RAPD) markers, respectively. Parasites were collected from their natural definitive host Rattus rattus in Guadeloupe (West Indies). We found a sex-specific genetic structure, a pattern never before reported in a parasitic organism. Male genotypes were more randomly distributed among rats than female genotypes. This interpretation was consistent with a lower differentiation between hosts for males relative to females, the higher genetic similarity between females in the same host and the observed local (i.e. within-individual-host) differences in allele frequencies between the two sexes. We discuss our results using ecological and immunological perspectives on host-parasite relationships. These results change our view on the epidemiology of schistosomiasis, a serious disease affecting humans in African and American intertropical zones.

Host-parasite coevolution in a multilocus gene-for-gene system

This paper examines a mathematical model for the coevolution of parasite virulence and host resistance under a multilocus gene-for-gene interaction. The degrees of parasite virulence and host resistance show coevolutionary cycles for sufficiently small costs of virulence and resistance. Besides these coevolutionary cycles of a longer period, multilocus genotype frequencies show complex fluctuations over shorter periods. All multilocus genotypes are maintained within host and parasite classes having the same number of resistant/virulent alleles and their frequencies fluctuate with approximately equally displaced phases. If either the cost of virulence or the number of resistance loci is larger then a threshold, the host maintains the static polymorphism of singly (or doubly or more, depending on the cost of resistance) resistant genotypes and the parasite remains universally avirulent. In other words, host polymorphism can prevent the invasion of any virulent strain in the parasite. Thus, although assuming an empirically common type of asymmetrical gene-for-gene interaction, both host and parasite populations can maintain polymorphism in each locus and retain complex fluctuations. Implications for the red queen hypothesis of the evolution of sex and the control of multiple drug resistance are discussed.

Parasite adaptation to locally common host genotypes

According to the Red Queen hypothesis--which states that interactions among species (such as hosts and parasites) lead to constant natural selection for adaptation and counter-adaptation--the disproportionate evolutionary success of parasites on common host genotypes leads to correlated selection for sexual reproduction and local adaptation by the parasite population. Here we determined whether local adaptation is due to disproportionate infection of common host genotypes, and, if so, whether infection of common host genotypes is due to commonness per se, or some other aspect of these genotypes. In a reciprocal cross-inoculation experiment parasites occupying the same geographical area (sympatric) infected locally common host genotypes significantly more often than rare host genotypes, whereas parasites occupying separate geographical areas (allopatric) showed no such significant difference. A mixed source of parasites (containing F1 hybrids) also showed no difference in infection between rare and common host genotypes. These results show that local adaptation results from parasite tracking of locally common host genotypes, and, as such, a necessary condition of the Red Queen hypothesis is met.

Virulence of vector-borne pathogens. A stochastic automata model of perpetuation

To determine how virulence may be perpetuated in populations of vector-borne pathogens, we simulated their fitness in a stochastic simulation based on cellular automata. Thereby, directly transmissible pathogens that differed in virulence were permitted to compete for hosts with similarly virulent pathogens that could infect hosts remotely because they were vector-borne. Fitness was defined as the proportion of the host population infected with each pathogen at equilibrium. Virulent, directly transmitted pathogens prevailed solely when their infectivity was transient. When duration of infectivity exceeded that of host survival, the less virulent pathogen invariably prevailed. Although remotely transmitted virulent pathogens persisted somewhat longer than did virulent pathogens that were transmitted directly, they never perpetuated themselves. We conclude that populations of vector-borne pathogens may retain pathogenicity somewhat longer than do those that are directly transmitted, but that both kinds of pathogens tend to become nonvirulent.

Sex, size, competition and escape-strategies of reproduction and dispersal in Lasallia pustulata (Umbilicariaceae, Ascomycetes)

The lichen Lasallia pustulata has a mixed strategy of asexual and sexual reproduction. Close-dispersed, asexual, symbiotic isidia are produced early, when the thalli are small. The asexual propagules are subsequently supplemented by far-dispersed, sexually generated ascospores when the thalli grow larger. This observation is consistent with evolutionary stable strategy (ESS) models of dispersal allocations in heterocarpic plants accordin to which the production of far-dispersed propagules should increase as clutch size and sibcompetition in the local habitat increases. The observation is also consistent with the "tangled bank" or "elbow room" hypothesis for the maintenance of sexuality, according to which sex, by generating genetic variation, represents an escape from competition in biologically saturated environments. Thus the advantage of sex is density dependent. L. pustulata grows in densely packed populations where intraspecific competition results in self-thining and the development of distinct sizehierarchies.

Epizootics of Salmonella infection in poultry may be the result of modern selective breeding practices

This paper discusses the hypothesis that a major factor in the epizootics of Salmonella infection in poultry is a declining host genetic diversity. A computer model is described which is based on models that have been previously used to investigate host-pathogen coevolution in cereal crops. It is shown that, as host genetic diversity declines, parasite diversity also declines to a lower equilibrium level. With a highly diverse host, parasite numbers decline to zero. With a homogeneous host population, after an initial decline, there is a rapid increase in parasite numbers, due to the selection of a particularly well adapted parasite strain. This simple computer simulation is used as the basis for a discussion of the literature supporting the suggestion that a major factor in the epizootic of Salmonella in poultry is related to the low genetic diversity of commercial poultry flocks.

Parasite-induced parthenogenesis in a freshwater snail: stable, persistent patterns of parasitism

The role of parasites in the evolution of host reproductive modes has gained renewed interest in evolutionary ecology. It was previously argued that obligate parthenogenesis (all-female reproduction) arose in a freshwater snail, Campeloma decisum, as a consequence of severe sperm limitation caused by an unencysted trematode, Leucochloridiomorpha constantiae. In the present study, certain conditions are examined for parasitic castration to account for the maintenance of parthenogenesis: the spatial patterns of the prevalence and intensity of infection on a broad geographical scale and its relationship to host genotype; the recovery from infection after isolation from sources of infection; age-related patterns of infections; and the effects of L. constantiae on snail fecundity.In contrast to the common pattern of the aggregated distribution of parasites within host populations, many snail populations with high prevalence and intensity of infection have non-aggregated parasite distributions. Clonal genotype of the host explained little of the variation in intensity and prevalence of infection by the parasite. Female snails maintained similar prevalence and intensity of infection after isolation, and individuals accumulated parasites throughout their lifespan, both of which suggest there is no effective immune response to infection by L. constantiae. Snail fecundity is not significantly influenced by the intensity of infection. These results suggest that L. constantiae may have represented a strong selective force against males during the initial introduction of this parasite into sexual snail populations because of the persistent nature of infection.

Diversity studies of Salmonella incidents in some domestic livestock and their potential relevance as indicators of niche width

This study attempts to determine whether or not livestock are becoming more susceptible to epidemics of salmonella infections by the analysis of published data on the annual number of reported infections in various animals. The number of incidents reported each year may be subject to a variety of biases due to temporal and geographical differences in reporting practices. This study analysed these reports by the calculation of diversity indices which are not subject to some of these potential biases. The relationship between the ecological concept of niche width and the diversity of species or types occupying that niche is discussed. The diversity of salmonella types reported in fowl has shown a highly significant decline over the 13-year period 1976-88. It is suggested that this declining diversity may be related to the declining niche width of the biotope available to this pathogen. Although speculative, this reduction in niche width could be related to a declining genetic diversity in the host animals or to an increasing intensification of animal husbandry.