DIFFERENTIAL SURVIVAL OF SEXUAL AND ASEXUAL POECILIOPSIS DURING ENVIRONMENTAL STRESS

According to the Frozen Niche-Variation model, coexisting clones of an asexual species can freeze and faithfully replicate ecologically relevant genetic variability that segregates in the sexual ancestors. The present experiments with fish of the genus Poeciliopsis provide further evidence in support of this model. Sexual and clonal forms of Poeciliopsis live in the desert streams of Sonora, Mexico, and are exposed to environmental extremes ranging from flash floods to hot, desiccating, residual pools. We examined coexisting members of the monacha complex to see whether the fish types differed with respect to survival during stress and swimming endurance in an artificial flume. The two coexisting clones of the triploid gynogenetic fish P. 2 monacha–lucida differed dramatically: clone MML/II had the best survival during heat and cold stress and the worst survival during hypoxic stress, whereas clone MML/I had the best survival during hypoxic stress and the worst during heat stress. Poeciliopsis monacha, the sexual species with which these clones coexist, had intermediate survival during heat and hypoxic stress and very poor swimming endurance in the flume. The physiological differences seen in this study are consistent with the Frozen Niche-Variation model and provide some insights into environmental factors that affect the distribution and abundance of these fish.     

A TEST OF THE SPONTANEOUS HETEROSIS HYPOTHESIS FOR UNISEXUAL VERTEBRATES

The coupling between clonal modes of reproduction and hybridization in unisexual vertebrates has led to the hypothesis that heterosis accounts for their ecological success (the “spontaneous heterosis” hypothesis). High levels of genic heterozygosity characteristic of unisexual-hybrid vertebrates are believed to result in enhanced growth, survivorship, and fertility relative to their sexual ancestors. To test this hypothesis, we synthesized 33 new unisexual-hybrid strains of fishes in the genus Poeciliopsis (Atheriniformes: Poeciliidae). On average, the synthetic unisexuals had lower survivorship and a higher incidence of birth defects than either of the sexual ancestors or two natural strains of unisexuals. However, a subset of these synthetic unisexuals exhibited characteristics within the range of the sexual ancestors and natural unisexual strains. These results support the alternative hypothesis that the ecological success of natural unisexuals results from selection of the most fit clones from a broad spectrum of genotypes that arose via multiple hybrid events. We propose that the coupling between unisexuality and hybridization in the vertebrates exists because hybridization is a dysgenic process that can disrupt normal gametogenesis and thus lead to clonal reproduction.     

ELECTROPHORETIC VARIATION IN THE PARTHENOGENETIC GRASSHOPPER WARRAMABA VIRGO AND ITS SEXUAL RELATIVES

Allozyme variation was examined within and between parthenogenetic clones of Warramabo virgo and the sexual ancestors, undescribed species P196 and P169. Both sexual species can be separated into northern and southern races using six loci, and the separate hybrid origin for the two major groups of parthenogenetic clones (the Standard Phylad and the Boulder-Zanthus Phylad) is substantiated by the racial variation in the sexual ancestors. Heterozygosity values in the parthenogenetic species are 6–9 times higher than those in the sexual species, and there is evidence for the accumulation of new variation subsequent to the hybrid origin of both phylads. The new variation is the result of either new mutations, recombination, or both. Three loci in the Standard Phylad clones reveal “orphan” alleles not found in the sexual ancestors; these alleles probably arose subsequent to hybridization but prior to the dispersal of the parthenogenetic clones. These data, in combination with those from other genetic studies, suggest that new variation may arise as a consequence of hybridization. Collectively, the allozyme, chromosome, molecular, and morphological data suggest that the Standard Phylad clones are of a more ancient but restricted origin, with clonal variation being the result of multiple hybridizations between individuals of P196 and P169.     

GENOTYPIC AND ENVIRONMENTAL COMPONENTS OF VARIATION IN GROWTH AND REPRODUCTION OF FISH HEMICLONES (POECILIOPSIS: POECILIIDAE)

The frozen-niche-variation model was proposed to account for the coexistence of genetically related clones in naturally occurring unisexual populations. This model is based on two assumptions: 1) ecologically different clones have multiple independent origins from sexual ancestors; and 2) the population of sexual ancestors contains genetic variability for ecologically relevant traits. To test these assumptions, we produced 14 new “hemiclones” (nonrecombining haploid genotypes) of fish (Poeciliopsis: Poeciliidae). Our ability to synthesize many new hemiclones demonstrates the feasibility of multiple independent origins of nonrecombining genotypes. A substantial proportion (10–50%) of the phenotypic variation among hemiclones in size at birth, juvenile growth rate, and fecundity had a genetic basis. Thus, we conclude that multiple origins can give rise to an assemblage of genetically distinct hemiclones, each with a unique combination of life-history traits. Additionally, a comparative analysis of two natural hemiclones revealed that the synthetic strains represent a broad field of variation from which natural hemiclones can be selected.     

Evolution of sexual mimicry in sperm-dependent all-female forms of Poeciliopsis (Pisces: Poeciliidae)

The twofold advantage of all-female reproduction is limited in many asexual lineages because females are sperm-dependent. Males of a related sexual-host species typically prefer conspecific females as mates. According to the “sexual mimicry” hypothesis, an all-female lineage that closely resembles females of the sexual-host species should have enhanced mating success. Examination of mating success in all-female fish of the genus Poeciliopsis supported this hypothesis. The excellent sexual mimicry of some all-female strains could have evolved through mutations within clonal lineages subsequent to their origins as interspecific hybrids. Alternatively, this mimicry may have been “frozen” from variation in the sexual gene pool when new unisexual lineages first arose. To test the latter hypothesis, we examined laboratory synthesized strains of the hybridogenetic fish P. monacha-lucida. The frozen variation hypothesis was supported by the present results.     

Sex Ratios and Resource Allocation among Sexually Reproducing Plants of Rubus chamaemorus

Sex ratios and patterns of size variation and resource allocationwere investigated in the dioecious species Rubus chamaemorus.Sex ratios among flowering ramets varied from 6% to 40% of females.Female ramets were slightly, although not significantly, tallerthan males. It appeared that population effects (including bothgenetic population and environmental site effects) on plantsize and allocation patterns at flowering are considerably greaterthan sex effects. If both flowering and fruit production areconsidered, then female allocation to reproduction clearly exceedsmale allocation. In females, no significant relationship wasdetected between the mass of reproductive and vegetative tissues,while males did exhibit such a relationship. Reproductive effortwas less for tall males than for small males. Despite the occurrence of sexual reproduction, the main modeof reproduction in R. chamaemorus is vegetative propagation,which is the best strategy for reproduction in the unpredictableclimate of high latitudes but which leads to skewed sex ratios.As a consequence of vigorous vegetative reproduction, individualclones can grow to be large. The results of electrophoreticstudies show that the numbers of clones per population are low.Copyright1994, 1999 Academic Press Rubus chamaemorus, cloudberry, sex ratios, resource allocation, clonal structure, electrophoresis     

Variation in life history responses of Daphnia to toxic Microcystis aeruginosa

Three clones of Daphnia pulex and two clones of Daphnia longispinawere exposed to toxic Microcystis aeruginosa for 21 days ina lifetable experiment. The growth and reproduction of individualdaphnids were followed daily to study the long-term effectsof toxic Microcystis. Exposure to Microcystis increased mortality,decreased growth, delayed maturation and decreased offspringproduction, indicating nutritional deficiency and toxic effects.We found variation in life history responses between speciesand among clones. Our results suggest that toxic cyanobacteriamay act as a modifying agent in zooplankton communities at boththe species and clonal level.     

Special Adaptive Problems Associated with Unisexual Fishes

SYNOPSIS. Two genera of fishes, Poecilia and Poeciliopsis, bothin the family Poeciliidae, have established unisexual populations.One "species," Poecilia formosa, is a diploid form that reliesupon sperm from Poecilia latipinna or P. mexicana to initiateparthenogenetic development (gynogenesis). Triploid individualsof P. formosa have also been discovered but the mode of reproductionand the significance of such fish to natural populations haveyet to be established. In the genus Poeciliopsis three gynogenetic triploids are known.Each is superimposed on a different bisexual host: Poeciliopsislucida, P. monacha or P. viriosa. Five diploid hybridogeneticformsalso have been identified. These are fertilized by either P.lucida, P. latidens, or P. occidentalis. In hybridogenetic forms,sperm actually feitilizes the egg. A hybrid with maternal andpaternal characters is produced, but only maternal genes aretransmitted through the egg to the next generation. Because of their sperm dependency, all unisexual fishes mustreside with at least one of the parental species. They areforcedto compete with them for sperm, space, and other limited resources.The ability to produce two females for every one that bisexualsproduce suggests that the unisexuals have the potential to winthis competition; but in so doing, they would lose their sexualsupport and eliminate themselves. A density-dependent mate selectionsystem plays a role in establishing a workable balance betweenthe competing elements. Unisexual forms of both Poecilia and Poeciliopsis are of hybridorigin. For most of them, the parental precursors and the proportionof genetic material that each contributed to the unisexual isknown. The various genomes that make up the unisexuals are adaptedto a variety of habitats. Once these genomes are combined ina unisexual hybrid, potential for adaptation to multiple nichesexists; this coupled with heterosis contributes greatly to thesuccess of gynogenetic and hybridogenetic fishes.     

Ecological Implications of Clonal Diversity in Parthenogenetic Morphospecies

Most parthenogenetic animal taxa which have been investigatedelectrophoretically, cytologically, or with tissue graftingtechniques are clonally diverse. I have examined data on multiclonalparthenogenetic populations using ecological diversity measuresto elucidate patterns of clonal coexistence. Analysis of a discretepopulation cage experiment on clones of Drosophila mercatorumrevealed monotonic decay of clonal diversity and evenness; however,in a continuous generation cage, clonal diversity appeared tostabilize. Clonal diversity and evenness fluctuated widely overtime in several multiclonal populations of Daphnia magna althoughno clonal extinction was observed. There were few spatial trendsin clonal diversity and evenness within parthenogenetic taxa.It is suggested that the degree of clonal differentiation, determinedby the mode of clonal origin, is important in determining whetheror not selection occurs among sympatric clones     

Environmental gradients structure Daphnia pulex × pulicaria clonal distribution

The rarity of eukaryotic asexual reproduction is frequently attributed to the disadvantage of reduced genetic variation relative to sexual reproduction. However, parthenogenetic lineages that evolved repeatedly from sexual ancestors can generate regional pools of phenotypically diverse clones. Various theories to explain the maintenance of this genetic diversity as a result of environmental and spatial heterogeneity [frozen niche variation (FNV), general-purpose genotype] are conceptually similar to community ecological explanations for the maintenance of regional species diversity. We employed multivariate statistics common in community ecological research to study population genetic structure in the freshwater crustacean, Daphnia pulex × pulicaria. This parthenogenetic hybrid arose repeatedly from sexual ancestors. Daphnia pulex × pulicaria populations harboured substantial genetic variation among populations and the clonal composition at each pond corresponded to nutrient levels and invertebrate predator densities. The interclonal selection process described by the FNV hypothesis likely structured our D. pulex × pulicaria populations.     

LET US NOT BE UNFAIR TO ASEXUALS: THEIR EPHEMERALITY MAY BE EXPLAINED BY NEUTRAL MODELS WITHOUT INVOKING ANY EVOLUTIONARY CONSTRAINTS OF ASEXUALITY

Phylogenetic studies typically demonstrate lower evolutionary ages of clones, relative to their sexual ancestors. This has often been attributed to heightened extinction risk of asexual organisms. We previously criticized such interpretations and demonstrated that the life span of clones is ultimately limited by neutral drift depending on the rate at which new clones are spawned into an asexual community of a finite size. Therefore, it is important to investigate whether the natural rates of such influxes are sufficiently high to account for the relative ephemerality of clones without assuming their increased extinction rate. I applied the neutral clonal turnover model to phylogenies of polyploid asexual ferns and simulated the coalescent trees over a wide range of demographic structures and sampling schemes. On parameterizing the model with biologically relevant estimates of population sizes and plant polyploidization rates, simulated clonal assemblages appeared younger than their sexual counterparts even in the absence of selection against clones. Therefore, differences observed between the ages of sexual and clonal lineages may be explained by the neutral clonal turnover. Researchers should consider the possibility that natural clones may get lost by neutral drift before their fate could eventually be affected by any long‐term constraints of asexuality.     

Spatial and temporal patterns of sexual reproduction in a hybrid Daphnia species complex

Evidence for extensive interspecific hybridization among speciesof the genus Daphnia has been accumulating on a global scale.Although there is evidence for limited gene flow between taxavia hybridization, many species still maintain discrete morphologicaland molecular characteristics. We studied temporal and spatialpatterns of sexual reproduction within the Daphnia galeata–hyalina–cucullatahybrid species complex in a lake (Plußsee), located innorthern Germany. Allozyme electrophoresis allowed us to trackseasonal changes in taxon composition as well as the quantificationof back-crosses. Sexually-reproducing animals (ephippial femalesand males) were mainly found in autumn. The simultaneous presenceof sexual morphs of D. galeata and D. galeata x hyalina withthe dominant D. hyalina taxa makes recent hybridization, aswell as back-crossing, plausible. Males and ephippial femalesof D. hyalina were not back-crossed as were the parthenogeneticfemales. The low number of sexual clones of the hybrid D. galeatax hyalina might reflect its reduced fertility, although thesefew clones were detected in high densities. Only hybrid-clonesthat had a back-cross genotype (towards D. hyalina) exhibitedephippial females and males. This indicates that male and ephippialfemale production within the Daphnia taxa is not random, whichmight increase the chance for the parental Daphnia species toremain distinct.     

EVOLUTIONARY INTERACTIONS BETWEEN SEXUAL AND ALL-FEMALE TAXA OF CYPRINOTUS (OSTRACODA: CYPRIDIDAE)

Freshwater ostracodes show both an exceptionally high incidence of transitions to unisexuality and, in some cases, an extraordinary level of clonal diversity. There is no understanding of the agents promoting these transitions to thelytoky, although it has been suggested that their frequency may set the stage for sexual taxa to infuse clonal diversity into unisexuals. This study examines the nature and origins of clonal diversity in the unisexual ostracode Cyprinotus incongruens. A combination of allozyme and cytogenetic studies revealed the presence of two diploid clones of this species at three temperate sites and ten clones at one arctic site including three diploids, five triploids, and two tetraploids. The low heterozygosity (0%–20%) of its diploid clones suggests that parthenogenesis has arisen spontaneously in C. incongruens rather than through hybridization, as in vertebrate asexuals. Polyploid clones appear to owe their origin to genome additions from sexual taxa, although subsequent mutational divergence has played a role in further enhancing diversity. Two triploid clones have apparently originated from the incorporation of a haploid genome from the sexually reproducing C. glaucus, as evidenced by their high heterozygosity and possession of alleles otherwise found only in that species. Other polyploid clones have likely arisen as a result of interbreeding between bisexual and unisexual C. incongruens. These results suggest that both the incidence of spontaneous transitions to clonality and the frequency of interbreeding with relatives may be the key processes that govern clonal diversity in unisexual ostracodes.     

Dynamic Formation of Asexual Diploid and Polyploid Lineages: Multilocus Analysis of Cobitis Reveals the Mechanisms Maintaining the Diversity of Clones

Given the hybrid genomic constitutions and increased ploidy of many asexual animals, the identification of processes governing the origin and maintenance of clonal diversity provides useful information about the evolutionary consequences of interspecific hybridization, asexuality and polyploidy. In order to understand the processes driving observed diversity of biotypes and clones in the Cobitis taenia hybrid complex, we performed fine-scale genetic analysis of Central European hybrid zone between two sexual species using microsatellite genotyping and mtDNA sequencing. We found that the hybrid zone is populated by an assemblage of clonally (gynogenetically) reproducing di-, tri- and tetraploid hybrid lineages and that successful clones, which are able of spatial expansion, recruit from two ploidy levels, i.e. diploid and triploid. We further compared the distribution of observed estimates of clonal ages to theoretical distributions simulated under various assumptions and showed that new clones are most likely continuously recruited from ancestral populations. This suggests that the clonal diversity is maintained by dynamic equilibrium between origination and extinction of clonal lineages. On the other hand, an interclonal selection is implied by nonrandom spatial distribution of individual clones with respect to the coexisting sexual species. Importantly, there was no evidence for sexually reproducing hybrids or clonally reproducing non-hybrid forms. Together with previous successful laboratory synthesis of clonal Cobitis hybrids, our data thus provide the most compelling evidence that 1) the origin of asexuality is causally linked to interspecific hybridization; 2) successful establishment of clones is not restricted to one specific ploidy level and 3) the initiation of clonality and polyploidy may be dynamic and continuous in asexual complexes.     

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.     

Neutral processes forming large clones during colonization of new areas

In species reproducing both sexually and asexually clones are often more common in recently established populations. Earlier studies have suggested that this pattern arises due to natural selection favouring generally or locally successful genotypes in new environments. Alternatively, as we show here, this pattern may result from neutral processes during species’ range expansions. We model a dioecious species expanding into a new area in which all individuals are capable of both sexual and asexual reproduction, and all individuals have equal survival rates and dispersal distances. Even under conditions that favour sexual recruitment in the long run, colonization starts with an asexual wave. After colonization is completed, a sexual wave erodes clonal dominance. If individuals reproduce more than one season, and with only local dispersal, a few large clones typically dominate for thousands of reproductive seasons. Adding occasional long‐distance dispersal, more dominant clones emerge, but they persist for a shorter period of time. The general mechanism involved is simple: edge effects at the expansion front favour asexual (uniparental) recruitment where potential mates are rare. Specifically, our model shows that neutral processes (with respect to genotype fitness) during the population expansion, such as random dispersal and demographic stochasticity, produce genotype patterns that differ from the patterns arising in a selection model. The comparison with empirical data from a post‐glacially established seaweed species (Fucus radicans) shows that in this case, a neutral mechanism is strongly supported.     

Genetic variation in sexual and clonal lineages of a freshwater snail

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

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.     

SPATIAL AND TEMPORAL FACTORS AFFECTING COEXISTENCE AMONG SEXUAL AND CLONAL FORMS OF POECILIOPSIS

We examined spatial and seasonal factors affecting coexistence of closely related sexual and asexual fishes of the genus Poeciliopsis living in two ecologically distinct stream habitats. Stream order and productivity influenced frequencies of these fish, and local habitat patchiness contributed strongly to spatial segregation. The magnitude and direction of local segregation was unaffected by striking seasonal variation in one stream. For the second stream, the relative contributions of local, geographical, and seasonal components of spatial variation were assessed through estimation of variance components. Spatial segregation at the smallest scale accounted for the largest proportion of variance in the distribution of species and clones.     

Causes and consequences of large clonal assemblies in a poplar hybrid zone

Asexual reproduction is a common and fundamental mode of reproduction in plants. Although persistence in adverse conditions underlies most known cases of clonal dominance, proximal genetic drivers remain unclear, in particular for populations dominated by a few large clones. In this study, we studied a clonal population of the riparian tree Populus alba in the Douro river basin (northwestern Iberian Peninsula) where it hybridizes with Populus tremula, a species that grows in highly contrasted ecological conditions. We used 73 nuclear microsatellites to test whether genomic background (species ancestry) is a relevant cause of clonal success, and to assess the evolutionary consequences of clonal dominance by a few genets. Additional genotyping‐by‐sequencing data were produced to estimate the age of the largest clones. We found that a few ancient (over a few thousand years old) and widespread genets dominate the population, both in terms of clone size and number of sexual offspring produced. Interestingly, large clones possessed two genomic regions introgressed from P. tremula, which may have favoured their spread under stressful environmental conditions. At the population level, the spread of large genets was accompanied by an overall ancient (>0.1 Myr) but soft decline of effective population size. Despite this decrease, and the high clonality and dominance of sexual reproduction by large clones, the Douro hybrid zone still displays considerable genetic diversity and low inbreeding. This suggests that even in extreme cases as in the Douro, asexual and sexual dominance of a few large, geographically extended individuals does not threaten population survival.