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.     

Clonal reproduction shapes evolution in the lizard malaria parasite Plasmodium floridense

The preponderant clonal evolution hypothesis (PCE) predicts that frequent clonal reproduction (sex between two clones) in many pathogens capable of sexual recombination results in strong linkage disequilibrium and the presence of discrete genetic subdivisions characterized by occasional gene flow. We expand on the PCE and predict that higher rates of clonal reproduction will result in: (1) morphologically cryptic species that exhibit (2) low within‐species variation and (3) recent between‐species divergence. We tested these predictions in the Caribbean lizard malaria parasite Plasmodium floridense using 63 single‐infection samples in lizards collected from across the parasite's range, and sequenced them at two mitochondrial, one apicoplast, and five nuclear genes. We identified 11 provisionally cryptic species within P. floridense, each of which exhibits low intraspecific variation and recent divergence times between species (some diverged approximately 110,000 years ago). Our results are consistent with the hypothesis that clonal reproduction can profoundly affect diversification of species capable of sexual recombination, and suggest that clonal reproduction may have led to a large number of unrecognized pathogen species. The factors that may influence the rates of clonal reproduction among pathogens are unclear, and we discuss how prevalence and virulence may relate to clonal reproduction.     

Prolonged clonal growth: escape route or route to extinction?

Many plant species have the capability to reproduce sexually as well as clonally. The balance between clonal reproduction and sexual reproduction varies between different species. It was estimated that 66.5% of all central European flora may form independent but genetically identical daughter plants. Also within species there is great variation in the ratio clonal/sexual reproduction. Clonal reproduction can be considered as an alternative life cycle loop that allows persistence of a species in the absence of the ability to complete the normal life cycle (i.e. seed production, germination and recruitment). Plant populations exhibiting prolonged clonal growth have been referred to as 'remnant populations'. A remnant population in general is defined as "a population capable of persistence during extended time periods despite a negative population growth rate (λ<1) due to longlived life stages and life cycles, including loops, that allow population persistence without completion of the whole life cycle". Here we argue that prolonged and nearly exclusive clonal growth through environmental suppression of sexual reproduction can ultimately lead to local sexual extinction and to monoclonal populations of a species, and that this may imply significant consequences for population viability. Especially obligate or mainly outcrossing clonal plant species may be vulnerable for sexual extinction. We argue that the consequences of reduced sexual recruitment in clonally propagating plants may be understudied and underestimated and that a re-evaluation of current ideas on clonality may be necessary.     

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.     

The population genetics of clonal and partially clonal diploids

The consequences of variable rates of clonal reproduction on the population genetics of neutral markers are explored in diploid organisms within a subdivided population (island model). We use both analytical and stochastic simulation approaches. High rates of clonal reproduction will positively affect heterozygosity. As a consequence, nearly twice as many alleles per locus can be maintained and population differentiation estimated as F(ST) value is strongly decreased in purely clonal populations as compared to purely sexual ones. With increasing clonal reproduction, effective population size first slowly increases and then points toward extreme values when the reproductive system tends toward strict clonality. This reflects the fact that polymorphism is protected within individuals due to fixed heterozygosity. Contrarily, genotypic diversity smoothly decreases with increasing rates of clonal reproduction. Asexual populations thus maintain higher genetic diversity at each single locus but a lower number of different genotypes. Mixed clonal/sexual reproduction is nearly indistinguishable from strict sexual reproduction as long as the proportion of clonal reproduction is not strongly predominant for all quantities investigated, except for genotypic diversities (both at individual loci and over multiple loci).     

A cost of Wolbachia-induced sex reversal and female-biased sex ratios: decrease in female fertility after sperm depletion in a terrestrial isopod

A number of parasites are vertically transmitted to new host generations via female eggs. In such cases, host reproduction is an intimate component of parasite fitness and no cost of the infection on host reproduction is expected to evolve. A number of these parasites distort host sex ratios towards females, thereby increasing either parasite fitness or the proportion of the host that transmit the parasite. In terrestrial isopods (woodlice), Wolbachia bacteria are responsible for sex reversion and female-biased sex ratios, changing genetic males into functional neo-females. Although sex ratio distortion is a powerful means for parasites to increase in frequency in host populations, it also has potential consequences on host biology, which may, in turn, have consequences for parasite prevalence. We used the woodlouse Armadillidium vulgare to test whether the interaction between Wolbachia infection and the resulting excess of females would limit female fertility through the reduction in sperm number that they receive from males. We showed that multiple male mating induces sperm depletion, and that this sperm depletion affects fertility only in infected females. This decrease in fertility, associated with male mate choice, may limit the spread of Wolbachia infections in host populations.     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

Heavy metals modify costs of reproduction and clonal growth in the stoloniferous herb Potentilla anserina

We examined costs of sexual reproduction and clonal propagation, and their consequences for resource allocation in the clonal stoloniferous herb, Potentilla anserina, a typical pioneer species in disturbed areas. We used heavy-metal treatment in soil to create unfavourable growing conditions, because costs of reproduction are more likely to be expressed when resources are limited. We also studied whether heavy metals affect the plasticity of clonal growth form that enables the plants to avoid poor growing conditions. Ramets collected from field were grown in a greenhouse under the heavy-metal treatment consisting of a control and two levels of heavy-metals added in soil. Clonal propagation was costly in terms of total biomass of flowering ramets. Also the costs of sexual reproduction were detected in flowering ramets. Contrary to our predictions, the costs of flower production were visible in the control but not in the heavy-metal contaminated plants. Only the flowering ramets were able to produce longer stolons under heavy-metal stress, but the stolon biomass was not affected by heavy metals. Results indicate that clonal propagation and sexual reproduction may be costly for P. anserina. However, the costs are modified by heavy-metal contamination.Co-ordinating editor: J. Tuomi     

Sexual healing: mating induces a protective immune response in bumblebees

The prevalence of sexual, as opposed to clonal, reproduction given the many costs associated with sexual recombination has been an enduring question in evolutionary biology. In addition to these often discussed costs, there are further costs associated with mating, including the induction of a costly immune response, which leaves individuals prone to infection. Here, we test whether mating results in immune activation and susceptibility to a common, ecologically important, parasite of bumblebees. We find that mating does result in immune activation as measured by gene expression of known immune genes, but that this activation improves resistance to this parasite. We conclude that although mating can corrupt immunity in some systems, it can also enhance immunity in others.     

Parasite-mediated selection in experimental Daphnia magna populations

Abstract It has been suggested that parasites are a strong selecting force for their hosts and therefore may alter the outcome of competition among host genotypes. We tested the extent to which parasite-mediated selection by different parasite species influenced competition among clones of the cyclic parthenogen Daphnia magna . We monitored clone frequency changes in laboratory microcosm populations consisting of 21 D. magna clones. Parasite treatments (two microsporidians, Glugoides intestinalis and Ordospora colligata ) and a parasite-free control treatment were followed over a nine-month period. A further treatment with the bacterium Pasteuria ramosa failed. We found significant differences in clonal success among the treatments: the two parasite treatments differed from the control treatment and from each other. Additionally, we measured the clone-specific population carrying capacity, competitive ability against tester clones, and reproductive success of infected and uninfected females to test whether they correlate with clonal success in the microcosms. The clone-specific competitive ability was a good predictor of clonal success in the microcosms, but clonal carrying capacity and host reproductive success were not. Our study shows that parasite-mediated selection can strongly alter the outcome of clonal competition. The results suggest that parasites may influence microevolution in Daphnia populations during periods of asexual reproduction.     

Population genetics of complex life-cycle parasites: an illustration with trematodes

Accurate inferences on population genetics data require a sound underlying theoretical null model. Organisms alternating sexual and asexual reproduction during their life-cycle have been largely neglected in theoretical population genetic models, thus limiting the biological interpretation of population genetics parameters measured in natural populations. In this article, we derive the expectations of those parameters for the life-cycle of monoecious trematodes, a group comprising several important human and livestock parasites that obligatorily alternate sexual and asexual reproduction during their life-cycle. We model how migration rates between hosts, sexual and asexual mutation rates, adult selfing rate and the variance in reproductive success of parasites during the clonal phase affect the amount of neutral genetic diversity of the parasite (effective population size) and its apportionment within and between definitive hosts (using F-statistics). We demonstrate, in particular, that variance in reproductive success of clones, a parameter that has been completely overlooked in previous population genetics models, is very important in shaping the distribution of the genetic variability both within and among definitive hosts. Within definitive hosts, the parameter F(IS) (a measure of the deviation from random mating) is decreased by high variance in clonal reproductive success of larvae but increased by high adult self-fertilisation rates. Both clonal multiplication and selfing have similar effects on between-host genetic differentiation (F(ST)). Migration occurring before and after asexual reproduction can have different effects on the patterns of F(IS), depending on values of the other parameters such as the mutation rate. While the model applies to any hermaphroditic organism alternating sexual and clonal reproduction (e.g. many plants), the results are specifically discussed in the light of the limited population genetic data on monoecious trematodes available to date and their previous interpretation. We hope that our model will encourage more empirical population genetics studies on monoecious trematodes and other organisms with similar life-cycles.     

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.     

Effects of sheep grazing and temporal variability on population dynamics of the clonal herb <Emphasis Type="Italic">Geranium sylvaticum</Emphasis> in an alpine habitat

An improved understanding of population-level consequences of grazing on plants can be facilitated by an assessment of grazing effects on all stages in the life-cycle. In this study, 6 years of demographic data for three populations of the perennial herb Geranium sylvaticum were analysed. We examined the effects of sheep grazing (high sheep density, low sheep density and no sheep) and interannual climatic variability on vital rates and population growth rates (λ). Grazing did not affect survival or flowering rates, but reduced rates of growth and increased rates of clonal reproduction. At the population level, high contributions from retrogression and clonal reproduction buffered reduced rates of growth and stasis, and no consistent differences in λ between populations exposed to different sheep densities were found. Instead, large between-year variability in λ, independent of sheep density, was detected, related to variation in the local summer climate. The results indicated, however, that grazing effects on λ were more severe in unfavourable than in normal years. Our study highlights that increased clonal reproduction rates functioned as a tolerance mechanism towards grazing in this herb, which forms a mechanism to explain how moderate population responses to grazing in some herbs can arise.     

Population genetic consequences of extreme variation in sexual and clonal reproduction in an aquatic plant

Most plants combine sexual reproduction with asexual clonal reproduction in varying degrees, yet the genetic consequences of reproductive variation remain poorly understood. The aquatic plant Butomus umbellatus exhibits striking reproductive variation related to ploidy. Diploids produce abundant viable seed whereas triploids are sexually sterile. Diploids also produce hundreds of tiny clonal bulbils, whereas triploids exhibit only limited clonal multiplication through rhizome fragmentation. We investigated whether this marked difference in reproductive strategy influences the diversity of genotypes within populations and their movement between populations by performing two large-scale population surveys (n = 58 populations) and assaying genotypic variation using random amplified polymorphic DNA (RAPDs). Contrary to expectations, sexually fertile populations did not exhibit higher genotypic diversity than sterile populations. For each cytotype, we detected one very common and widespread genotype. This would only occur with a very low probability (< 10-7) under regular sexual recombination. Compatibility analysis also indicated that the pattern of genotypic variation largely conformed to that expected with predominant clonal reproduction. The potential for recombination in diploids is not realized, possibly because seeds are outcompeted by bulbils for safe sites during establishment. We also failed to find evidence for more extensive movement of fertile than sterile genotypes. Aside from the few widespread genotypes, most were restricted to single populations. Genotypes in fertile populations were very strongly differentiated from those in sterile populations, suggesting that new triploids have not arisen during the colonization of North America. The colonization of North America involves two distinct forms of B. umbellatus that, despite striking reproductive differences, exhibit largely clonal population genetic structures.     

Queen developmental time and fitness consequences for queens of clonal social parasitic honeybees (A. m. capensis) and its host A. m. scutellata

Summary. The social parasitic honeybees of South Africa (Apis mellifera capensis) consist of a single clonal lineage, which has been selected for traits related to worker reproduction. Viable queens of this parasitic clonal lineage have never been observed. We tested if it is possible to rear queens from eggs of the social parasitic workers. In a competitive situation, using larvae of the parasitic clonal lineage and of the host, we tested the discriminatory ability of host colonies (A. m. scutellata) between parasitic and non-parasitic larvae. We found evidence for a reduced fitness of queens reared from the social parasite lineage, resulting from a longer developmental time. The results are discussed in the light of a fitness trade-off between queen and worker caste.Received 22 July 2004; revised 22 December 2004; accepted 5 January 2005.     

The reproductive biology of Polytrichum formosum: clonal structure and paternity revealed by microsatellites

Using highly polymorphic microsatellite markers, we assessed clonal structure and paternity in a population of the bryophyte species Polytrichum formosum. Identical multilocus genotypes of individual shoots were almost never observed in spatially separated cushions, but were found to be highly clustered within moss cushions. Therefore, asexual reproduction through dispersal of gametophyte fragments is not very important in P. formosum. However, asexual reproduction on a very localized scale through vegetative growth of genets (branching of gametophytes via clonal growth of rhizomes) is very extensive. The patchy spatial distribution of genets and the absence of intermingling among genets suggest that this species follows a 'phalanx' clonal growth strategy. Vegetative proliferation of genets will increase their size, and, consequently, will have considerable fitness consequences for individuals in terms of increased genet longevity and reproductive output. Although paternity analysis of sporophytes confirmed male genet size, i.e. gamete production, to be an important determinant of male reproductive fitness, it also showed that the spatial distance to female genets is the predominant factor that governs male reproductive success. Moreover, we showed that male gamete dispersal distances in P. formosum are much further than generally assumed, and are in the order of metres rather than centimetres. Combining the findings, we conclude that the high genotypic diversity observed for this facultatively clonal species is most likely explained by a preponderance of sexual reproduction over clonal reproduction.     

Clonal integration in Ranunculus reptans: by‐product or adaptation?

We studied fitness consequences of clonal integration in 27 genotypes of the stoloniferous herb Ranunculus reptans in a spatially heterogeneous light environment. We grew 216 pairs of connected ramets (eight per genotype) with mother ramets in light and daughter ramets in shade. In half of the pairs we severed the stolon connection between the two ramets at the beginning of the experiment. During the experiment, 52.7% of the ramet pairs with originally intact connection physically disintegrated. We detected significant variation among genotypes in this regard. Survival of planted ramets was 13.3% higher for originally connected pairs. Moreover, there was significant variation among genotypes in survival, in the difference in survival between plant parts developing from mother and daughter ramets, and in the effect of integration on this difference. In surviving plants connection between ramets decreased size differences between mother and daughter parts. Variation among genotypes was significant in growth and reproduction and marginally significant in the effect of physiological integration on growth and reproduction. Connected daughter ramets had longer leaves and internodes than daughters in severed pairs indicating that integration stimulated plant foraging in both the vertical and the horizontal plane. Observed effects of integration on fitness components in combination with genetic variation in maintenance and effects of connection indicate that clonal integration in R. reptans has the capability to evolve, and therefore suggest that clonal integration is adaptive. If genetic variation in integration is common, future studies on clonal integration should always use defined genetic material and many clones to allow extrapolation of results to population and wider levels.     

Frequency and spatial patterning of clonal reproduction in Louisiana iris hybrid populations

Abstract.— The plant genera in which natural hybridization is most prevalent tend to be outcrossing perennials with some mechanism for clonal (i.e., asexual) reproduction. Although clonal reproduction in fertile, sexually reproducing hybrid populations could have important evolutionary consequences, little attention has been paid to quantifying this parameter in such populations. In the present study, we examined the frequency and spatial patterning of clonal reproduction in two Louisiana iris hybrid populations. Allozyme analysis of both populations revealed relatively high levels of genotypic diversity. However, a considerable amount of clonality was apparent. Nearly half of all genets (47%) in one population and more than half (61%) in the other had multiple ramets. Furthermore, both populations exhibited relatively high levels of genetic structuring, a pattern that resulted from the aggregation of clonal ramets. The occurrence of clonal reproduction in hybrid populations could not only facilitate introgression through an increase in the number of flowering ramets per genet and/or the survivorship of early generation hybrids, but might also influence the mating system of such populations. Any potential increase in the selfing rate due to cross-pollination among ramets of the same genet may, in turn, increase the likelihood of homoploid hybrid speciation.     

Effects of light on the growth and clonal reproduction of Ligularia virgaurea

Ligularia virgaurea is a perennial herb that is widely distributed in the alpine meadow on the eastern Qinghai-Tibet plateau.We investigated the patterns of growth and reproduction of L.virgaurea under two contrasting levels of light conditions for two continuous growing seasons.Our results showed that the light affects on the maximum relative growth rate,the shoot weight ratio and the root weight ratio differed between the two growing seasons.L.virgaurea reproduced initially through rhizome in the second growing season,rather than sexual reproduction.The proportion of genets with clonal reproduction decreased under shaded conditions.A minimum genet size should be attained for clonal reproduction to begin under the shaded conditions.There was a positive linear relationship between clonal reproduction and genet size.Light level affected the allocation of total biomass to clonal structures,with less allocation under the full natural irradiance than under the shaded conditions.There seemed to be a trade-off between vegetative growth and clonal reproduction under the full natural irradiance,in terms of smaller relative growth rates of genets with clonal reproduction than those without clonal reproduction.L.virgaurea emphasized clonal reproduction under the full natural irradiance,while the plant emphasized vegetative growth under the shaded conditions.     

The relative importance of sexual and clonal reproduction for population growth in the perennial herb Fragaria vesca

The relative importance of sexual and clonal reproduction for population growth in clonal plants is highly variable. Clonal reproduction is often more important than sexual reproduction but there is considerable interspecific variation and the importance of the two reproductive modes can change with environmental conditions. We carried out a demographic study on the woodland strawberry (Fragaria vesca), a widespread clonal herb, at 12 sites in Switzerland during 2 years. Study sites were selected in two different habitats, i.e., forest and forest edge. We used periodic matrix models to estimate annual population growth rates and carried out prospective analyses to identify life cycle components that influence population growth rates most. Retrospective analyses were applied to study how the two different habitats affected population dynamics. Furthermore, we tested whether trade-offs between sexual and clonal reproduction occurred. There were large differences in annual population growth rates between sites and large within-site differences between years. Results of the prospective analyses clearly indicate that clonal reproduction is the dominant reproductive pathway whereas sexual reproduction is rather insignificant for population growth. Compared to forest habitats, forest edge habitats had higher population growth rates in the first year but smaller growth rates in the second year. We attribute these differing habitat effects to different water availabilities during consecutive years. No trade-offs between sexual and clonal reproduction were found. In conclusion, population growth of F. vesca relies heavily on clonal reproduction. Furthermore, reproduction and survival rates of F. vesca depend highly on spatio-temporal variation of environmental conditions.