POLYPHYLETIC ORIGINS OF ASEXUALITY IN DAPHNIA PULEX. I. BREEDING-SYSTEM VARIATION AND LEVELS OF CLONAL DIVERSITY

There is growing evidence that transitions from sexual to asexual reproduction are often provoked by internal genetic factors rather than extrinsic selection pressures. In the cladoceran crustacean Daphnia pulex, the shift to asexuality has been linked to sex-limited meiosis suppression. Most populations of this species reproduce by obligate parthenogenesis, but cyclically parthenogenetic populations persist in the southern portion of its range. The meiosis-suppressor model predicts that asexuality in D. pulex has polyphyletic origins and that the coexistence of cyclically parthenogenetic lines with male-producing obligately asexual clones should be unstable. For the present study, we examined the genotypic structure of D. pulex populations from a region in which there is an abrupt microgeographical shift in breeding system. Populations in Michigan largely reproduce by cyclic parthenogenesis, while those in Ontario are obligately asexual. Allozyme studies on 77 populations from this area revealed 50 obligately asexual clones, divisible into two groups: one derived from a single parent species and the other derived via interspecific hybridization. Although nearly 50% of the clones retained male production, there was, as predicted, no evidence of coexistence between cyclically parthenogenetic populations and male-producing obligately asexual clones. The survey did, however, reveal a low incidence of cyclically parthenogenetic populations in Ontario. The high genotypic diversity of these populations suggests that they are not only resistant to meiosis suppression, but able to rework genetic variation gained from asexual clones into a sexual breeding system.     

CLONAL-DIVERSITY PATTERNS AND BREEDING-SYSTEM VARIATION IN DAPHNIA PULEX,AN ASEXUAL-SEXUAL COMPLEX

Some individuals of the cladoceran crustacean, Daphnia pulex, reproduce by cyclic parthenogenesis, while others are obligate parthenogens. Cyclic parthenogenesis is the primitive breeding system; the transition to obligate parthenogenesis has been linked to sex-limited meiosis-suppression. Detailed study of patterns of breeding-system distribution and clonal diversity is justified because D. pulex is the first species in which the loss of sex has been related to this mechanism. The present study investigated the genotypic characteristics of 10 D. pulex populations from each of 22 sites in the Great Lakes watershed. This analysis revealed that populations reproducing by cyclic parthenogenesis were uncommon and restricted to southern sites. Most populations reproduced by obligate parthenogenesis, with the electrophoretic survey revealing an average of three clones per pond and 145 unique clones over the watershed. A combinatorial analysis was used to examine the relationships between clone discovery in the asexual populations and both sample size and genetic-sampling intensity. This analysis showed that the few clones found in individual ponds were readily discriminated, while diversity on a regional scale was underestimated. These methods provide a quantitative basis for assessing the level of clonal diversity in asexual populations and in asexually transmitted segments of the genome.     

POLYPHYLETIC ORIGINS OF ASEXUALITY IN DAPHNIA PULEX. II. MITOCHONDRIAL-DNA VARIATION

Allozyme studies of the cladoceran Daphnia pulex have shown that most populations reproduce by obligate parthenogenesis, although some cyclically parthenogenetic populations remain throughout the southern portion of its range. Clonal diversity within the obligate parthenogens is extremely high and has been attributed to the polyphyletic origin of asexuality. Specifically, it has been proposed that the clonal diversity in the obligate parthenogens was generated via the spread of a sex-limited meiosis suppressor through populations of a cyclically parthenogenetic ancestor. In this study, analysis of polymorphism of restriction-endonuclease sites in the mitochondrial genome, in conjunction with allozyme analysis, was used to determine whether obligate parthenogenesis has a monophyletic or polyphyletic origin in D. pulex. An allozyme survey of 77 populations from Ontario and Michigan was first conducted to determine breeding systems and levels of clonal diversity (Hebert et al., 1989). Mitochondrial-DNA variation was then surveyed in one isolate of each clone from each population reproducing by obligate parthenogenesis and in 2–4 isolates from each population reproducing by cyclic parthenogenesis. Seventeen restriction enzymes were used in this analysis. Thirty-five mitochondrial genotypes were found among the 36 obligate clones (as identified by allozyme analysis), while 17 mitochondrial genotypes were identified among 40 cyclic isolates from 14 populations. Five mitochondrial genotypes were found in both groups. Parsimony and phenetic-clustering methods were used to construct trees showing the genetic relationship among mitochondrial genotypes. The results clearly show that obligate parthenogenesis had a polyphyletic origin in this species. The close relationship between cyclic and obligate parthenogens in the Great Lakes region suggests that many obligate clones have recently been derived from cyclic populations and that the generation of clones is still occurring in this area. Patterns of clonal diversity based on the joint consideration of allozyme and mitochondrial-DNA data are discussed.     

Allelic and genotypic diversity in long-term asexual populations of the pea aphid, Acyrthosiphon pisum in comparison with sexual populations

Many aphid species exhibit geographical variation in the mode of reproduction that ranges from cyclical parthenogenesis with a sexual phase to obligate parthenogenesis (asexual reproduction). Theoretical studies predict that organisms reproducing asexually should maintain higher allelic diversity per locus but lower genotypic diversity than organisms reproducing sexually. To corroborate this hypothesis, we evaluated genotypic and allelic diversities in the sexual and asexual populations of the pea aphid, Acyrthosiphon pisum (Harris). Microsatellite analysis revealed that populations in central Japan are asexual, whereas populations in northern Japan are obligatorily sexual. No mixed populations were detected in our study sites. Phylogenetic analysis using microsatellite data and mitochondrial cytochrome oxidase subunit I (COI) gene sequences revealed a long history of asexuality in central Japan and negated the possibility of the recent origin of the asexual populations from the sexual populations. Asexual populations exhibited much lower genotypic diversity but higher allelic richness per locus than did sexual populations. Asexual populations consisted of a few predominant clones that were considerably differentiated from one another. Sexual populations on alfalfa, an exotic plant in Japan, were most closely related to asexual populations associated with Vicia sativa L. The alfalfa-associated sexual populations harboured one COI haplotype that was included in the haplotype clade of the asexual populations. Available evidence suggests that the sexuality of the alfalfa-associated populations has recently been restored through the northward migration and colonization of alfalfa by V. sativa- associated lineages. Therefore, our results support the theoretical predictions and provide a new perspective on the origin of sexual populations.     

Elimination of a specialised facultative symbiont does not affect the reproductive mode of its aphid host

Abstract.  1. Microorganisms that manipulate the reproduction of their hosts through diverse mechanisms including the induction of parthenogenesis are widespread among arthropods.
2. The pea aphid, Acyrthosiphon pisum , shows a variation in its reproductive mode, with lineages reproducing by cyclical parthenogenesis (obligate alternation of parthenogenetic and sexual generations each year) and others by obligate parthenogenesis (continuous asexual reproduction all year round). In addition, the pea aphid harbours, along with Buchnera the primary aphid endosymbiont, several facultative symbionts whose prevalence differs among host populations.
3. The possible influence of a Rickettsia facultative symbiont on the reproductive mode of its host was tested on two pea aphid clones by comparing the response of infected and uninfected individuals with the same genetic background to conditions that typically induce the production of sexual morphs.
4. No significant effect of the Rickettsia infection was found on the type of reproductive morphs produced (sexual vs. asexual) or on their quantities for the two clones.
5. However, the Rickettsia had a detrimental effect on the fitness of its aphid host, in apparent contradiction to the high prevalence of this symbiont in some host populations. It is suggested that this negative impact may disappear under specific environmental conditions, transforming a parasitic association into a mutualistic one.     

Does insecticide resistance alone account for the low genetic variability of asexually reproducing populations of the peach-potato aphid Myzus persicae?

The typical life cycle of aphids includes several parthenogenetic generations and a single sexual generation (cyclical parthenogenesis), but some species or populations are totally asexual (obligate parthenogenesis). Genetic variability is generally low in these asexually reproducing populations, that is, few genotypes are spread over large geographic areas. Both genetic drift and natural selection are often invoked to account for this low genetic variability. The peach-potato aphid, Myzus persicae, which encompasses both cyclical and obligate parthenogens, has developed several insecticide resistance mechanisms as a consequence of intense insecticide use since the 1950s. We collected asexually reproducing M. persicae from oilseed rape and examined genetic variability at eight microsatellite loci and three insecticide resistance genes to determine whether their genetic structure was driven by drift and/or selection. We identified only 16 multilocus microsatellite genotypes among 255 individuals. One clone, which combined two insecticide resistance mechanisms, was frequently detected in all populations whatever their location over a large geographical area (the northern half of France). These unexpected findings suggest that drift is not the unique cause of this low variability. Instead, the intensification of both insecticide treatments and oilseed rape cultivation may have favored a few genotypes. Thus, we propose that selective pressures resulting from human activities have considerably modified the genetic structure of M. persicae populations in northern France in a relatively short period of time.     

Heritable genetic variation and potential for adaptive evolution in asexual aphids (Aphidoidea)

Aphid life cycles can encompass cyclical parthenogenesis, obligate parthenogenesis, obligate parthenogenesis with male production and an intermediate 'bet-hedging' strategy where an aphid genotype will over-winter by continuing to reproduce by parthenogenesis and by investment in sexually produced eggs. In this paper, we focus on aphid lineages that reproduce entirely parthenogenetically (asexual aphids), in contrast to those that have any sexual forms in the annual cycle. Using modern molecular techniques, aphid biologists have made many empirical observations showing that asexual lineages are widespread both geographically and temporally. Indeed, we are collectively beginning to gather data on the evolution and persistence of these lineages through time. Here we review aphid karyology and parthenogenesis, both essential for interpretation of the molecular and ecological evolution of aphid asexual lineages. We describe the growing list of studies that have identified aphid genotypes that are both temporally and geographically widespread. We then collate examples of molecular and chromosomal evolution in asexual aphids and review the literature pertaining to phenotypic evolution and ecological diversification of asexual aphid lineages. In addition, we briefly discuss the potential of bacterial endosymbionts and epigenetic effects to influence the evolution of asexual aphid lineages. Lastly we provide a list of aphid taxa believed to be obligately asexual. This will be a useful resource for those seeking parthenogenetic animals as study systems. In conclusion, we present guidelines for the use of the term clone in aphid biology and stress the need for well-designed and well-executed studies examining the potential of asexual aphid lineages for adaptive evolution.  © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 79 , 115–135.     

Phenotypic effects of an allele causing obligate parthenogenesis in a rotifer

Transitions to obligate asexuality have been documented in almost all metazoan taxa, yet the conditions favoring such transitions remained largely unexplored. We address this problem in the rotifer Brachionus calyciflorus. In this species, a polymorphism at a single locus, op, can result in transitions to obligate parthenogenesis. Homozygotes for the op allele reproduce strictly by asexual reproduction, whereas heterozygous clones (+/op) and wild-type clones (+/+) are cyclical parthenogens that undergo sexual reproduction at high population densities. Here, we examine dosage effects of the op allele by analyzing various life-history characteristics and population traits in 10 clones for each of the 3 possible genotypes (op/op, +/op, and +/+). For most traits, we found that op/op clones differed significantly (P < 0.05) from the 2 cyclical parthenogenetic genotypes (+/+ and +/op). By contrast, the 2 cyclical parthenogenetic genotypes were almost indistinguishable, except that heterozygote individuals were slightly but significantly smaller in body size compared with wild-type individuals. Overall, this indicates that the op allele is selectively neutral in the heterozygous state. Thus, selective sweeps of this allele in natural populations would first require conditions favoring the generation of homozygotes. This may be given by inbreeding in very small populations or by double mutants in very large populations.     

The Behavior of a Daphnia pulex Transposable Element in Cyclically and Obligately Parthenogenetic Populations

Using Southern blot analysis, we have characterized restriction fragment patterns of a transposable element, Pokey, in obligately and cyclically parthenogenetic populations of the cladoceran crustacean Daphnia pulex. We show that the element is most likely active in cyclically parthenogenetic populations but is, for the most part, inactive in obligate parthenogens. This result is consistent with theory suggesting that transposable element dynamics are likely to change with a change in reproductive mode. Such changes could have important consequences for the long-term evolutionary potential of obligate parthenogens and may also be informative with regard to the underlying mechanisms that regulate transposable element frequencies in sexual organisms. Received: 29 August 2000 / Accepted: 1 March 2001     

The paradox of obligate sex: The roles of sexual conflict and mate scarcity in transitions to facultative and obligate asexuality

The maintenance of obligate sex in animals is a long‐standing evolutionary paradox. To solve this puzzle, evolutionary models need to explain why obligately sexual populations consistently resist invasion by facultative strategies that combine the benefits of both sexual and asexual reproduction. Sexual antagonism and mate availability are thought to shape the occurrence of reproductive modes in facultative systems. But it is unclear how such factors interact with each other to influence facultative invasions and transitions to obligate asexuality. Using individual‐based models, we clarify how sexually antagonistic coevolution and mate availability affect the likelihood that a mutant allele that gives virgin females the ability to reproduce parthenogenetically will invade an obligately sexual population. We show that male coercion cannot stop the allele from spreading because mutants generally benefit by producing at least some offspring asexually prior to encountering males. We find that effects of sexual conflict can lead to positive frequency‐dependent dynamics, where the spread of the allele is promoted by effective (no‐cost) resistance when males are common, and by mate limitation when sex ratios are female‐biased. However, once the mutant allele fixes, effective coercion prevents the complete loss of sex unless linkage disequilibrium can build up between the allele and alleles for effective resistance. Our findings clarify how limitations of female resistance imposed by the genetic architecture of sexual antagonism can promote the maintenance of sexual reproduction. At the same time, our finding of widespread obligate sex when costs of parthenogenesis are high suggests that developmental constraints could contribute to the rarity of facultative reproductive strategies in nature.     

Effects of competitors and Chaoborus predation on the cladocerans of a eutrophic lake: an enclosure study

The role of large laboratory grown food competitors ofthe genus Daphnia as well as the predationimpact of Chaoborus on the cladoceran communityof an eutrophic lake was assessed in five insitu enclosure experiments. The hypothesis tested wasthat the outcome of competition and gape-limitedpredation on cladocerans is size dependent. Accordingto the generally accepted assumptions on competitionand invertebrate predation, large-bodied cladocerantaxa were expected to be less affected by competingcongeners and by Chaoborus than weresmall-bodied taxa. Effects of the predator upon anassemblage of differently sized cladoceran taxa weremuch more pronounced than effects of competition.There was a tendency of predation and competitionimpact to decrease with cladoceran size, but predationpressure was also low for some small cladocerans andhigh for some large cladocerans. The general trendswere further obscured by factors not or indirectlylinked to body size.     

The Association Between Breeding System and Transposable Element Dynamics in <Emphasis Type="Italic">Daphnia Pulex</Emphasis>

Transposable elements (TEs) are major sources of genetic variation, and mating systems are believed to play a significant role in their dynamics. For example, insertion number is expected to be higher in sexual than in asexual organisms due to the inability of TEs to colonize new genomes in the absence of sex. The goal of this study was to determine the impact of the loss of sexual reproduction on TE load. Daphnia pulex has two reproductive modes, obligate and cyclical parthenogenesis, which differ with respect to the production of diapausing eggs. Cyclical parthenogens produce them meiotically, while obligate parthenogens produce them clonally. Pokey is a TTAA-specific DNA transposon, and is a stable component of Daphnia genomes. We used a PCR-based approach, TE-Display, to estimate the number of Pokey insertions in 22 cyclic and 22 obligate isolates of D. pulex. As expected, the copy number of Pokey insertions is significantly higher in cyclic than in obligate isolates. However, the distribution of elements among isolates within each breeding system is similar, which is congruent with the recent establishment of obligate lineages from a cyclic ancestor. We also assayed 46 isolates from eight cyclic populations and found that very few Pokey insertions were observed in more than one isolate, suggesting that Pokey has been active recently. Sequencing of PCR products from the TE-Display analysis shows that Pokey inserts into both coding and noncoding regions of the genome. However, there is no obvious similarity among sequences downstream of the TTAA Pokey insertion site.     

THE ORIGIN AND DISTRIBUTION OF CLONAL DIVERSITY IN ALSOPHILA POMETARIA (LEPIDOPTERA: GEOMETRIDAE)

A survey of spatial and temporal variation in the frequency of electrophoretically defined genotypes in the geometrid moth Alsophila pometaria revealed a high diversity of uncommon or rare asexual genotypes and clinal distributions of two of the more common clones. There was substantial year-to-year variation in genotype frequencies in seven of eleven sites. Progeny tests have revealed that sexual reproduction is uncommon in two populations and that new asexual genotypes arise from the sexual population. The recurrent origin of asexual genotypes is likely to account for the high genetic and ecological diversity of the asexual contingent of this species' populations, in contrast to the lower genetic diversity in some obligately asexual species in which such recruitment does not occur.     

THE ORIGIN AND GENETIC BASIS OF OBLIGATE PARTHENOGENESIS IN DAPHNIA PULEX

Sex in Daphnia is environmentally determined, and some obligately parthenogenetic clones of D. pulex have retained the ability to produce males. In the present study, males from 13 such clones were crossed to sexual females from closely related cyclical parthenogens both to determine whether the males were capable of producing viable hybrids and to determine the mode of reproduction of the hybrids. A total of 178 genetically confirmed hybrids were produced, with each of the 19 attempted crosses resulting in some viable hybrids. On average, only 34% of the hybrid eggs that initiated development survived to the reproductive stage, suggesting some incompatibility between the parents. The absence of any association between survivorship and parental or hybrid genotype indicated, however, that there is no specific genetic incompatibility associated with the marker loci used. The inability of most hybrids to produce normal resting eggs is further evidence of a general genomic incompatibility between the parents. Ten of the hybrids produced viable resting eggs, permitting tests to determine their mode of reproduction. Six of the 10 hybrids reproduced by cyclical parthenogenesis, like their maternal parent. The remaining four hybrids reproduced by obligate parthenogenesis, like their paternal parent, demonstrating that the genes suppressing meiosis can be transmitted by the male parent. These results support a model for the generation of new clones that involves the spread of genes suppressing meiosis and provide evidence that the high genotypic diversity observed in obligately parthenogenetic populations of D. pulex is a result of the multiple origin of new clones from the cyclical parthenogens. Evidence was also obtained suggesting that the obligately parthenogenetic clones carry a load of recessive deleterious genes.     

GENOTYPE DIVERSITY AND SELECTION IN ASEXUAL BRINE SHRIMP (ARTEMIA)

An obligately asexual population of Artemia parthenogenetica at Salin de Giraud, France consists of numerous clones that are genetically and ecologically divergent. The clones are distributed in a nonrandom fashion among eight ponds of highly variable environments. A total of 63 allozymically unique genotypes were identified, with significant differences in numerous fitness traits found among a sample subset of clones. The frequencies of four alleles, as well as the percentage of loci polymorphic, were significantly correlated with salinity. Genotypes face rapid selection along salinity gradients in the field and this selection is reproducible in the lab. mtDNA analysis suggests that obligate asexuality in this population is of monophyletic origin.     

Avoiding the cost of males in obligately asexual Daphnia pulex (Leydig)

Asexual organisms are thought to gain an advantage by avoiding the cost of producing males. In the cladoceran Daphnia pulex (Leydig), male production is determined by the environment and is independent of the origin of the asexual obligate parthenogens from the sexual cyclical parthenogens. If there is a cost to producing males, successful obligate parthenogens should have reduced or eliminated male production. Field and laboratory observations showed that obligate parthenogens have much-reduced male production compared to cyclical parthenogens. Although the reduction or elimination of males in the obligate parthenogens suggests that the cost of males is avoided, the coexistence of sexual and asexual forms of D. pulex may be partially explained by cyclical parthenogens compensating for the cost of males by having greater fecundity. In addition, the absence of a mating constraint for the obligate parthenogens may favour an increased allocation to asexual diapausing eggs earlier in the season compared to the cyclical parthenogens which require mating with males to produce sexual diapausing eggs. No difference in the production of diapausing eggs was observed, probably because males were abundant in populations of cyclical parthenogens and do not appear to limit the production of sexual diapausing eggs. D. pulex is a useful system for determining the ecological consequences of abandoning sexual reproduction and explaining the coexistence of sexual and asexual forms of a species.     

CLONAL DIVERSITY IN HIGH-ARCTIC POPULATIONS OF DAPHNIA PULEX,A POLYPLOID APOMICTIC COMPLEX

Based largely on analogy with latitudinal trends in species diversity, it has been proposed that levels of genotypic (clonal) diversity in parthenogenetic populations from high latitudes should be lower than those in populations from the temperate zone or the tropics. Prior studies have shown that low-arctic populations of obligately asexual Daphnia pulex are less clonally diverse than temperate-zone populations. To test for the existence of a latitudinal trend, an allozymic survey of obligately parthenogenetic populations of D. pulex was conducted at a site in the Canadian high-arctic. The study revealed the presence of 75 clones in 179 tundra ponds that were surveyed. On average, 4.5 clones coexisted in single ponds with a range of 1–14 clones. These diversity values are as great (or greater) than those observed in more southerly populations and conflict with the notion of reduced levels of genetic variation in arctic populations. Mechanisms that may influence genetic (clonal) diversity in apomictic complexes are discussed.     

Temporal dynamics of genotypic diversity reveal strong clonal selection in the aphid Myzus persicae

Parthenogenetic organisms often harbour substantial genotypic diversity. This diversity may be the result of recurrent formations of new clones, or it may be maintained by environmental heterogeneity acting on ecological differences among clones. In aphids, both processes may be important because obligate and cyclical parthenogens can form mixed populations. Using microsatellites, I analysed the temporal dynamics of clonal diversity in such a population of the aphid Myzus persicae over a 1-year period. The frequency distribution of clonal genotypes was very skewed, with many rare and few common clones. The relative frequencies of common clones underwent strong and rapid changes indicative of intense clonal selection. Differences in their host associations suggest that these shifts may partly be caused by changes in the abundance of annual host plants. Other selective factors of potential importance are also discussed. New, sexually produced genotypes made a minor contribution to clonal diversity, consistent with the observed heterozygote excess characteristic of predominantly asexual populations in M. persicae.     

Ecological differences among clones of Daphnia pulex Leydig

Summary Natural populations of Daphnia pulex that reproduce by obligate parthenogenesis include a number of clones. Studies on two common and two rare clones from southwestern Ontario revealed significant differences in their intrinsic rates of increase, competitive abilities, rates of ephippial egg production, and lifespans. Environmental factors such as temperature and food type had large inluences on the rate of increase of each clone. Differences in rates of increase among clones were most pronounced at temperatures higher than those encountered in nature. In general, the covariance of life history traits among clones was high. The outcome of competitive encounters between clones was deterministic and in most cases was unaffected by temperature. Clones with high rates of increase tended to be better competitors than those with low rates of increase.     

Genetic characterization of an arctic zooplankter: insights into geographic polyploidy

Species of Bosmina from the temperate regions of North America and Europe are diploid and reproduce by cyclical parthenogenesis. By contrast, this study provides evidence that the dominant bosminid taxon in High Arctic lakes reproduces by obligate parthenogenesis and is a polyploid derived from interspecific hybridization. Sinobosmina liederi, a species common in temperate North America, is likely to have been one parent of these hybrids, but the other parent is unknown. As neither parent was detected in the Arctic, it seems unlikely that the hybrid clones that now occupy arctic lakes were synthesized locally. Most habitats contained only one or two clones, despite a total of 38 clones in the region, suggesting that priority effects have been important in restricting diversity within single lakes. The high regional diversity of arctic bosminids could reflect either repeated hybridization between the parent taxa or the genetic instability of newly formed polyploid lineages. These processes would produce hybrid polyploids that are considerably more diverse than their sexual parent taxa, and this difference in genetic diversity may confer an advantage to the polyploid biotype. As many zooplankton taxa from the arctic possess genetic characteristics similar to those of bosminids, these processes may provide a general explanation for the widespread occurrence of polyploids in the Arctic.