Sex proportions of Haemoproteus blood parasites and local mate competition

Host B chromosomes as potential sex ratio distorters of intestinal nematode infrapopulations in the yellow-necked mouse (Apodemus flavicollis)

The yellow-necked mouse, Apodemus flavicollis, can be considered as a model for genetic polymorphism produced by the frequent presence of supernumerary or B chromosomes (Bs). Host genetic background is rarely taken into account in studies of parasite sex ratio. The main aim of this study was to investigate the range of infrapopulation sex ratios for nematode parasites of the yellow-necked mouse and to determine which factors most influence variation in parasite sex ratios. Six nematode species found in the collected yellow-necked mice were analysed. We confirmed the predominant pattern of female-biased sex ratios in vertebrate parasite infrapopulations. The presence of B chromosomes in host genomes played an important role in infrapopulations of Heligmosomoides polygyrus, Syphacia stroma and Trichuris muris, as hosts with B chromosomes carried a higher proportion of males. The relative increase of males in infrapopulations could result from a shift in parasite life history strategy, induced by adaptation to the specific host genotypes (Bs present). In a meta-analysis with previously published data, the sex determination system was demonstrated to play a significant role in nematode sex ratio variation, as well as specific life history patterns, such as the place of egg hatching.

Nematode parasites and leukocyte profiles of Northern Leopard Frogs, Rana pipiens: location, location, location

Globally, amphibians face a variety of anthropogenic stresses that include exposure to contaminants such as agricultural pesticides. Pesticides may negatively affect amphibian immune systems, concomitantly increasing susceptibility to parasitism. We quantified nematodes and evaluated leukocyte profiles of Northern Leopard Frogs (Rana pipiens Schreber, 1782) collected from five wetlands in southwestern Quebec, Canada, that spanned a gradient of pesticide exposure. Three taxa of nematode parasites (Rhabdias ranae Walton, 1929, genus Oswaldocruzia Travassos, 1917, and genus Strongyloides Grassi, 1879) were sufficiently numerous for detailed evaluation. When all frogs were pooled, frog size was negatively correlated with nematode species richness, abundances of each of the three nematode species, and densities of three different leukocytes. When all frogs were pooled, there was strong evidence of both negative and positive associations between pairs of parasite species. However, none of the previous relationships was significant within wetlands. Our results reveal strong spatial organization of amphibian–parasite communities and illustrate the importance of controlling for sampling locale in evaluating host–parasite associations. Finally, although several response variables varied significantly among wetlands, causes of this variation did not appear to be related to variation in nematode parasitism or pesticide exposure.

Plastic parasites: sophisticated strategies for survival and reproduction?

Adaptive phenotypic plasticity in life history traits, behaviours, and strategies is ubiquitous in biological systems. It is driven by variation in selection pressures across environmental gradients and operates under constraints imposed by trade-offs. Phenotypic plasticity has been thoroughly documented for multicellular taxa, such as insects, birds and mammals, and in many cases the underlying selective pressures are well understood. Whilst unicellular parasites face many of the same selective pressures and trade-offs, plasticity in their phenotypic traits has been largely overlooked and remains poorly understood. Here, we demonstrate that evolutionary theory, developed to explain variation observed in the life-history traits of multicellular organisms, can be applied to parasites. Though our message is general - we can expect the life-histories of all parasites to have evolved phenotypic plasticity - we focus our discussion on malaria parasites. We use an evolutionary framework to explain the trade-offs that parasites face and how plasticity in their life history traits will be expressed according to changes in their in-host environment. Testing whether variation in parasites traits is adaptive will provide new and fundamental insights into the basic biology of parasites, their epidemiology and the processes of disease during individual infections.

Evolutionary biology: parasite, know thyself

Studies of sex allocation provide some of the best evidence for Darwinian adaptation in nature. A new study of malaria parasites provides striking support for this cornerstone of evolutionary biology, with important implications for both evolutionary and medical biology.

Sex ratio adjustment and kin discrimination in malaria parasites

Malaria parasites and related Apicomplexans are the causative agents of the some of the most serious infectious diseases of humans, companion animals, livestock and wildlife. These parasites must undergo sexual reproduction to transmit from vertebrate hosts to vectors, and their sex ratios are consistently female-biased. Sex allocation theory, a cornerstone of evolutionary biology, is remarkably successful at explaining female-biased sex ratios in multicellular taxa, but has proved controversial when applied to malaria parasites. Here we show that, as predicted by theory, sex ratio is an important fitness-determining trait and Plasmodium chabaudi parasites adjust their sex allocation in response to the presence of unrelated conspecifics. This suggests that P. chabaudi parasites use kin discrimination to evaluate the genetic diversity of their infections, and they adjust their behaviour in response to environmental cues. Malaria parasites provide a novel way to test evolutionary theory, and support the generality and power of a darwinian approach.

Development of reverse-transcription PCR techniques to analyse the density and sex ratio of gametocytes in genetically diverse Plasmodium chabaudi infections

We have developed cross-genotype and genotype-specific quantitative reverse-transcription PCR (qRT-PCR) assays to detect and quantify the number of parasites, transmission stages (gametocytes) and male gametocytes in blood stage Plasmodium chabaudi infections. Our cross-genotype assays are reliable, repeatable and generate counts that correlate strongly (R(2)s>90%) with counts expected from blood smears. Our genotype-specific assays can distinguish and quantify different stages of genetically distinct parasite clones (genotypes) in mixed infections and are as sensitive as our cross-genotype assays. Using these assays we show that gametocyte density and gametocyte sex ratios vary during infections for two genetically distinct parasite lines (genotypes) and present the first data to reveal how sex ratio is affected when each genotype experiences competition in mixed-genotype infections. Successful infection of mosquito vectors depends on both gametocyte density and their sex ratio and we discuss the implications of competition in genetically diverse infections for transmission success.

Host cell preference and variable transmission strategies in malaria parasites

Malaria and other haemosporin parasites must undergo a round of sexual reproduction in their insect vector in order to produce stages that can be transmitted to vertebrate hosts. Consequently, it is crucial that parasites produce the sex ratio (proportion of male sexual stages) that will maximize the number of fertilizations and thus, transmission to new vertebrate hosts. There is some evidence to show that, consistent with evolutionary theory, the sex ratios of malaria parasites are negatively correlated to their inbreeding rate. However, recent theory has shown that when fertilization success is compromised, parasites should respond by increasing their investment in sexual stages or by producing a less female biased sex ratio than predicted by their inbreeding rate alone. Here, we show that two species of rodent malaria, Plasmodium chabaudi and Plasmodium vinckei petteri , adopt different strategies in response to host anaemia, a factor thought to compromise transmission success: P. chabaudi increases investment in sexual stages, whereas P. vinckei produces a less female biased sex ratio. We suggest that these different transmission strategies may be due to marked species differences in host cell preference.

Changes in Haemoproteus sex ratios: fertility insurance or differential sex lifespan?

There is little direct evidence of the fitness effects of changes in malaria gametocyte sex ratio. Gametocyte sex ratios in haemospororin parasites (phylum Apicomplexa) are usually female skewed. However, in some cases and especially in Haemoproteus parasites, less female-biased and even male-biased sex ratios are encountered. The 'fertility insurance hypothesis' tries to explain these biases as an evolutionary strategy to facilitate gamete encounter. Thus, the hypothesis predicts that, if there is a reduction in gametocyte density (intensity of infection) or other factors preventing gametes from meeting, a change to a higher proportion of male gametocytes may be favoured. By contrast, a change in sex ratio may be caused by other non-adaptive mechanisms, for example differential survival of the gametocytes of each sex. We study within-host changes in Haemoproteus majoris sex ratios following an experimental reduction in the density of the parasites in the blood in a breeding population of blue tits (Parus caeruleus). Medication with the antimalarial drug primaquine induced a significant reduction in Haemoproteus gametocyte infection intensity in two different breeding seasons and under two different doses of medication. Sex ratios became male skewed following the experimental treatment in agreement with the predictions of the 'fertility insurance' hypothesis. Also in support of the hypothesis, a significant change towards male-biased sex ratios emerged for non-medicated birds in one year, probably owing to the natural immune reduction of the density of the parasites in the blood. The alternative possibility that changes are caused by different lifespans of gametocytes is not supported by changes in sex ratios in control hosts, where new production and release of gametocytes occur.

Gametocytogenesis: the puberty of Plasmodium falciparum

The protozoan Plasmodium falciparum has a complex life cycle in which asexual multiplication in the vertebrate host alternates with an obligate sexual reproduction in the anopheline mosquito. Apart from the apparent recombination advantages conferred by sex, P. falciparum has evolved a remarkable biology and adaptive phenotypes to insure its transmission despite the dangers of sex. This review mainly focuses on the current knowledge on commitment to sexual development, gametocytogenesis and the evolutionary significance of various aspects of gametocyte biology. It goes further than pure biology to look at the strategies used to improve successful transmission. Although gametocytes are inevitable stages for transmission and provide a potential target to fight malaria, they have received less attention than the pathogenic asexual stages. There is a need for research on gametocytes, which are a fascinating stage, responsible to a large extent for the success of P. falciparum.

Even more extreme fertility insurance and the sex ratios of protozoan blood parasites

Theory developed for malaria and other protozoan parasites predicts that the evolutionarily stable gametocyte sex ratio (z*; proportion of gametocytes that are male) should be related to the inbreeding rate (f) by the equation z*=(1-f)/2. Although this equation has been applied with some success, it has been suggested that in some cases a less female biased sex ratio can be favoured to ensure female gametes are fertilized. Such fertility insurance can arise in response to two factors: (i) low numbers of gametes produced per gametocyte and (ii) the gametes of only a limited number of gametocytes being able to interact. However, previous theoretical studies have considered the influence of these two forms of fertility insurance separately. We use a stochastic analytical model to address this problem, and examine the consequences of when these two types of fertility insurance are allowed to occur simultaneously. Our results show that interactions between the two types of fertility insurance reduce the extent of female bias predicted in the sex ratio, suggesting that fertility insurance may be more important than has previously been assumed.

Malaria transmission, sex ratio and erythrocytes with two gametocytes

Transmission of haemospororin parasites (phylum Apicomplexa) needs the fertilization of at least one female by one male gamete within the bloodmeal of a suitable vector. Male and female gamete precursors (gametocytes) in Plasmodium and Haemoproteus parasites are normally alone inside the erythrocytes of the vertebrate host, but they also occur in male-female pairs in single erythrocytes. These paired gametocytes could enhance transmission success by facilitating the encounter between the female and male gametes when inside the midgut of the vector. Further study of these particular infections could provide new insights into the biology of and control strategies for haemospororin parasites.

Fertility insurance and the sex ratios of malaria and related hemospororin blood parasites

The sex ratio (z*; proportion of gametocytes that are male) of malaria and related hemospororin blood parasites has been predicted to be related to the inbreeding rate (f) by the simple equation z* = (1 - f)/2. Although there is some empirical support for this prediction, there are several cases where the sex ratio is less female biased or more variable than expected. Here, we present a theoretical model that may be able to explain some of these discrepancies. We show that if low gametocyte densities lead to a danger that female gametes may not encounter any male gametes, then natural selection favors a less female-biased sex ratio as a form of 'fertility insurance' to ensure that female gametes are mated. This model can be applied to a number of situations. In particular, (1) empirical data suggest that the number of gametocyes per blood meal can be low enough to favor fertility insurance in some Plasmodium infections in humans and (2) our model predicts facultative shifting toward less-biased sex ratios in response to immune pressure that reduces gametocyte or gamete survival or mobility, consistent with some recent experimental data from Plasmodium species of birds and mice.

Plasmodium sex determination and transmission to mosquitoes

In order to be transmitted by their mosquito vector, malaria parasites undergo sexual reproduction, which occurs between specialized male and female parasites (gametes) within the blood meal in the mosquito. Nothing was known about how Plasmodium determines the sex of its gametocytes (gamete precursors), which are produced in the vertebrate host. Recently, erythropoietin, the vertebrate hormone controlling erythropoiesis in response to anaemia, was implicated in Plasmodium sex determination in animal models of malaria. This review examines the available information and addresses the relevance of such a sex determining mechanism for Plasmodium falciparum transmission to mosquitoes, with special reference to low gametocytaemias.

Evolution of gametocyte sex ratios in malaria and related apicomplexan (protozoan) parasites

'Survival of the fittest' is usually interpreted to mean that natural selection favours genes that maximize their transmission to the next generation. Here, we discuss recent applications of this principle to the study of gametocyte sex ratios in malaria and other apicomplexan parasites. Sex ratios matter because they are an important determinant of fitness and transmission success -- and hence of disease epidemiology and evolution. Moreover, inbreeding rates can be estimated from gametocyte sex ratios. The sex ratio is also an excellent model trait for testing the validity of important components of what is being marketed as 'Darwinian medicine'.

Sex allocation and population structure in apicomplexan (protozoa) parasites

Establishing the selfing, rate of parasites is important for studies in clinical and epidemiological medicine as well as evolutionary biology Sex allocation theory offers a relatively cheap and easy way to estimate selfing rates in natural parasite populations. Local mate competition (LMC) theory predicts that the optimal sex ratio (r*; defined as proportion males) is related to the selfing rate (s) by the equation r* = (1-s)/2. In this paper, we generalize the application of sex allocation theory across parasitic protozoa in the phylum Apicomplexa. This cosmopolitan phylum consists entirely of parasites, and includes a number of species of medical and veterinary importance. We suggest that LMC theory should apply to eimeriorin intestinal parasites. As predicted, data from 13 eimeriorin species showed a female-biased sex ratio, with the sex ratios suggesting high levels of selfing (0.8-1.0). Importantly, our estimate of the selfing rate in one of these species, Toxoplasma gondii, is in agreement with previous genetic analyses. In contrast, we predict that LMC theory will not apply to the groups in which syzygy occurs (adeleorins, gregarines and piroplasms). Syzygy occurs when a single male gametocyte and a single female gametocyte pair together physically or in close proximity, just prior to fertilization. As predicted, data from four adeleorin species showed sex ratios not significantly different from 0.5.

Sex determination in malaria parasites

A century ago, W. G. MacCallum identified distinct male and female forms in malaria parasites of both birds and humans. Since then, scientists have been puzzled by the high female-to-male ratios of parasites in Plasmodium infections and by the mechanism of sex determination. The sex ratio of malaria parasites was shown to become progressively more male as conditions that allow motility and subsequent fertilization by the male parasites become adverse. This resulted from an increased immune response against male gametes, which coincides with intense host erythropoietic activity. Natural and artificial induction of erythropoiesis in vertebrate hosts provoked a shift toward male parasite production. This change in parasite sex ratio led to reduced reproductive success in the parasite, which suggests that sex determination is adaptive and is regulated by the hematologic state of the host.

Mass dynamics of the spleen and other organs in geese: measures of immune relationships to helminths?

The spleen is an important organ of avian immune systems. We examined whether helminth loads were related to spleen mass in the lesser snow goose, Chen caerulescens caerulescens. On 27 collecting occasions, 744 geese were obtained at 13 different locations in a south-north gradient in midcontinental North America. The masses of the spleen, caecum, small intestine, large intestine, pancreas, heart, and crop of all geese were determined, and intestinal and caecal helminths were counted. Seventy-eight percent of geese harbored at least one helminth species. For analyses, helminths were grouped as cestodes (26% prevalence), trematodes (19% prevalence), and nematodes (70% prevalence). After sample location and time, host age, host sex, and host body size were controlled for in a multivariate analysis of covariance, nematodes were the only helminth group associated with variation in organ masses. Greater nematode loads were weakly associated with lower spleen, higher caecum, lower large intestine, and lower heart masses. When uninfected individuals were excluded from the analysis, greater nematode loads were no longer associated with variation in spleen size but were associated with higher crop mass, and greater cestode loads were associated with higher heart mass. In neither of these analyses were any other cestode-organ or trematode-organ associations significant. Geese carrying two or more helminth groups had lower spleen masses than did geese infected with no or one helminth group. When we interchanged response and explanatory variables from the preceding analyses and retained the same covariates, the same organ mass - helminth associations tended to remain significant. Nonetheless, the small variation in helminth loads explained by variation in spleen mass (or vice versa) provided only weak support for the hypothesis that intraspecifically, wild individuals with lower investment in immunity are more susceptible to nematode infections.

Sex allocation and population structure in malaria and related parasitic protozoa

Here we demonstrate how sex allocation theory, one of the best verified areas of metazoan evolutionary biology, can be successfully applied to microparasitic organisms, by relating parasite prevalence and sex ratio in the Haemosporina. Members of this taxon, which includes Plasmodium, are parasitic protozoa with obligate sexual cycles in which dioecious haploid gametes drawn from the peripheral blood of a vertebrate host fuse within a dipteran vector. Consequently mating takes place within a highly subdivided population, a condition known to promote local mate competition and inbreeding and hence the evolution of female-biased sex ratios. We used an epidemiological framework to investigate mating patterns and sex ratio evolution within natural populations of these parasites. This phenotypic approach compliments more conventional biochemical approaches to the population genetics of parasitic protozoa. Data are presented which support a theoretical relation between transmission-stage sex ratio and prevalence across parasite populations. These results are consistent with a large inter-population variation in genetic structure and argue against sweeping generalizations about the clonality or otherwise of populations of these parasitic protozoa.