Molecular structure of the allelic variants of (AAT)n microsatellite locus Du47D in the parthenogenetic species Darevskia unisexualis and bisexual parental species D. valentini and D. raddei

Microsatellite repeats are one of the most widespread elements of the eukaryotic genome, but are poorly studied in species with clonal reproduction. PCR analysis and DNA sequencing were used to study the molecular structure of the allelic variants of microsatellite locus Du47D in the parthenogenetic species Darevskia unisexualis and its evolutionary ancestors, bisexual species D. raddei and D. valentini, of the genus Darevskia (Lacerta saxicola complex). Sequencing showed that the allelic variants of the D. unisexualis Du47D locus and the alleles of its D. raddei and D. valentini orthologs have a perfect microsatellite cluster structure, differ in number of ATT monomeric units, and have certain species-specific combinations of nucleotide substitutions, deletions, and insertions in the microsatellite-flanking DNA sequences. The Du47D alleles that the parthenogenetic species inherited from D. valentini or from D. raddei were identified.     

The origin and evolution of parthenogenesis in theHeteronotia binoei complex: Synthesis

TheHeteronotia binoei complex includes several cryptic species of sexually reproducing lizards and parthenogenetic lineages derived from them. This paper synthesizes analyses of distribution and variation of allozymes, chromosomes, mitochondrial DNA and ribosomal DNA genes in order to make inferences about the origins of the parthenogenetic lineages, the extent and source of their genetic diversity, their current and historical biogeography and their ecological properties. The parthenogens appear to have arisen recently (relative to geographic differentiation within the sexual taxa) via episodes of repetitive hybridization between two of the sexual taxa. These events probably occurred within one or two small geographic areas of western Australia, after which some of the parthenogenetic lineages rapidly expanded their ranges to central Australia. The parthenogenetic form has extraordinarily high genetic diversity, mostly derived from the repetitive origins, but with some contribution from mutation and biased gene conversion/recombination being apparent. The rapid and extensive range expansion of the parthenogenetic lineages from western to central Australia attests to the short-term success of this reproductive strategy, in this case perhaps reinforced by the parthenogenetic females having higher fecundity than their smaller sexual relatives. However, the parthenogens are orders of magnitude more susceptible to infection by ectoparasitic mites, suggesting that they could be at a long-term disadvantage. The detailed characterization of this system provides a basis for critical evaluation of hypotheses about the evolutionary advantage of sexual reproduction derived from broad comparative studies.     

Clonal diversity and polyphyletic origins of hybrid and spontaneous parthenogenetic Campeloma (Gastropoda: Viviparidae) from the south-eastern United States

Some theories for the maintenance of sexual reproduction indicate that parthenogens may persist if there is high clonal diversity and high dispersal rates. Using allozymic variation, we report on the origin, clonal diversity and population structure of hybrid and spontaneous parthenogens from south-eastern United States populations of the freshwater snail Campeloma. Independent origins of triploid hybrid parthenogens in the Florida panhandle occurred by hybridization between an Atlantic coastal species (C. limum or C. floridense) and the Florida sexual species (C. geniculum). Allozyme genotypic diversity is similar between these hybrid parthenogens and sexuals. Diploid spontaneous parthenogens originated multiple times from nonlocal C.limum sexual populations in Atlantic coastal rivers, and levels of genotypic diversity are significantly higher in sexual C. limum. How parthenogens originate, the degree of clonal diversity, and their subsequent dispersal influence whether basic assumptions of evolution-of-sex models are met.     

Variation of Mini- and Microsatellite DNA Markers in Populations of Parthenogenetic Rock Lizard Darevskia rostombekovi

Variation and clonal diversity in populations of the parthenogenetic rock lizard Darevskia rostombekovi was examined by means of multilocus DNA fingerprinting using mini- and microsatellite DNA markers M13, (GATA)₄, and (TCC)₅₀). The animals examined were shown to exhibit a clonally inherited, species-specific pattern of DNA markers (fingerprint profile) that is different from the species-specific patterns of parthenogenetic species D. dahli, D. armenica, and D. unisexualis. The mean intraspecific similarity indexS was 0.950 (0.003) for a sample of 19 animals from three isolated populations of North Armenia. This significantly differed from the estimate of this parameter for a sample of 21 animals including two individuals from mountainous, relict population from the vicinity of the Sevan Lake, which was equal to 0.875 (0.001). A comparison of DNA fingerprints showed differences between 21 individuals attaining 79 DNA fragments of 1801 mini- and microsatellite markers included in the analysis. The results obtained show that intraspecific variation in D. rostombekovi is higher than that in the previously studied parthenogenetic species D. dahli (S = 0.962) andD. unisexualis (S= 0.950) (P< 0.001). Taking into account that D. rostombekovi is considered monoclonal on the basis of allozyme data, the problem of clonal variability is discussed with regard to the evidence on nuclear DNA markers. It is suggested that the hybrid karyotype of D. rostombekovi, which is more unstable than that of D. dahli and D. unisexualis, generates a series of chromosomal rearrangements (mutations). This may lead to the appearance of a geographically isolated chromosomal race (clone) in the population inhabiting the southeastern coast of the Sevan Lake.     

Allozyme Variation Patterns and Multiple Hybridization Origins: Clonal Variation Among Four Sibling Parthenogenetic Caucasian Rock Lizards

Allozyme electrophoresis of four sibling parthenogenetic Caucasian rock lizards Darevskia unisexualis, D.uzzelli, D.sapphirina, and D.bendimahiensis found seven clones and five variable loci. The data supported the hypothesis that D.raddei and D.valentini are the parental species of all four parthenogens. Variation patterns in Darevskia were summarized. Species that originated from a single F₁ typically consisted of one widespread clone with a few rare clones. Species with multiple origins displayed variation only slightly higher than species with a single origin. This is contrary to other genera of parthenogenetic lizards, in which cases massive clonal variations were observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Geographic ranges, population structure, and ages of sexual and parthenogenetic snail lineages

Abstract A sexual reproduction is thought to doom organisms to extinction due to mutation accumulation and parasite exploitation. Theoretical models suggest that parthenogens may escape the negative effects of conspecific and biological enemiecs through escape in space. Through intensive sequencing of a mitochondrial DNA (mtDNA) and a nuclear intron locus in sexual and pathenogenetic freshwater snails (Campelom), I examine three questionss: (1) Are sexual mtDNA lineage more restricted geographically than parthenogenetic mtDNA lineages? (2) Are independent pathenogenetic lineages shorter lived than sexual lineages? (3) Do pathenogens have higher intraindividual nuclear sequence diversity and form well‐differentiated monophyletic groups as expected under the Meselson effect? Geographic ranges of parthenogenetic lineages are significantly larger than geographic ranges of sexual lineages. Based on coalescence times under different deographic assumptions, asexual lineages are short lived, but there is variation in clonal ages. Although alternative explanations exit, these results suggest that asexual lineages may persist in the short term through dispersal, and that various constraints may cause geographic restriction of sexual lineagess. Both allotriploid and diploid Campleloma parthenogens have significantly higher allelic divergence within individuals, but show limited nuclear sequence divergence from sexual ancestors. In contrast to previous allozyme evidence for nonhybrid origins of diploid Campeloma parthenogens, cryptic hybridization may account for elevated heterozygosity.     

REDUCED MALE ALLOCATION IN THE PARTHENOGENETIC HERMAPHRODITE DUGESIA POLYCHROA

Parthenogenetic lineages that arise in a hermaphroditic, sexual population will inherit the male function from their sexual progenitors. Natural selection then acts to reduce male allocation of the parthenogens, freeing resources presumably for the female function. Depending on age and the available genetic variation, one therefore expects to find reduced male allocation in naturally occurring parthenogenetic lineages. We investigated the allocation to sperm production in the hermaphroditic flatworm Dugesia polychroa in three lakes containing a sexual (S), a (pseudogamous) parthenogenetic (P), and a mixed sexual-parthenogenetic population (M). Parthenogenetic lineages from M were assumed to be relatively young due to recurrent origins from the coexisting sexuals, whereas those from P were assumed to be older on biogeographical grounds. As predicted, we found drastically reduced sperm production in parthenogens compared to sexuals, even in the parthenogenetic lineages from M, which may be younger. M parthenogens did not have more testes, but produced more sperm than individuals from the purely parthenogenetic population (P). However, the latter result could not be reproduced with laboratory-raised animals and therefore may be a consequence of different ecological conditions in the different lakes, for example, differences in mating rates. To study the behavioral component of male allocation, copulation frequencies were recorded for sexuals from M and for parthenogens from P. Compared to the drastic reduction in sperm production, copulation frequency was less reduced in parthenogens. This may be a consequence of allosperm limitation in pseudogamous parthenogenetic populations.     

The Origin and Evolution of Parthenogenesis in Heteronotia Binoei (Gekkonidae): Evidence for Recent and Localized Origins of Widespread Clones

The parthenogenetic form of the gecko lizard species Heteronotia binoei has an unusually broad geographic range and high genetic diversity. Restriction enzyme analysis revealed two basic types of mitochondrial DNA (mtDNA) among the parthenogens. One type is restricted to western populations. The other type, analyzed in detail here, was widespread, being found in populations from central to western Australia. The diversity within this widespread type was low. The variation among parthenogens from central to western Australia was similar to that found within local populations of the sexual species that provided the mtDNA, and was an order of magnitude less than the differentiation shown between sexual populations across the same geographic distance. Phylogenetic analysis revealed that the widespread type of mtDNA in the parthenogens is most closely related to mtDNAs from western populations of the "CA6" sexual parent. These data suggest that these parthenogenetic clones arose recently within a small geographic area, most probably in Western Australia. The parthenogens must have spread rapidly to occupy much of the central and western Australian deserts. This rapid and extensive range expansion provides strong evidence that parthenogenesis can be a successful strategy for lizards in an environment with low and unpredictable rainfall.     

Origin and Genetic Diversity of Diploid Parthenogenetic Artemia in Eurasia

There is wide interest in understanding how genetic diversity is generated and maintained in parthenogenetic lineages, as it will help clarify the debate of the evolution and maintenance of sexual reproduction. There are three mechanisms that can be responsible for the generation of genetic diversity of parthenogenetic lineages: contagious parthenogenesis, repeated hybridization and microorganism infections (e.g. Wolbachia). Brine shrimps of the genus Artemia (Crustacea, Branchiopoda, Anostraca) are a good model system to investigate evolutionary transitions between reproductive systems as they include sexual species and lineages of obligate parthenogenetic populations of different ploidy level, which often co-occur. Diploid parthenogenetic lineages produce occasional fully functional rare males, interspecific hybridization is known to occur, but the mechanisms of origin of asexual lineages are not completely understood. Here we sequenced and analysed fragments of one mitochondrial and two nuclear genes from an extensive set of populations of diploid parthenogenetic Artemia and sexual species from Central and East Asia to investigate the evolutionary origin of diploid parthenogenetic Artemia, and geographic origin of the parental taxa. Our results indicate that there are at least two, possibly three independent and recent maternal origins of parthenogenetic lineages, related to A. urmiana and Artemia sp. from Kazakhstan, but that the nuclear genes are very closely related in all the sexual species and parthenogegetic lineages except for A. sinica, who presumable took no part on the origin of diploid parthenogenetic strains. Our data cannot rule out either hybridization between any of the very closely related Asiatic sexual species or rare events of contagious parthenogenesis via rare males as the contributing mechanisms to the generation of genetic diversity in diploid parthenogenetic Artemia lineages.     

Variation of Mini- and Microsatellite DNA Repeats in Parthenogenetic Lizard Darevskia armeniaca as Revealed by DNA Fingerprinting Analysis

Population and family samples of two morphological forms (mutant and normal with respect to dorsal color) of parthenogenetic lizard species Darevskia armeniaca were examined by means of DNA fingerprinting using M13 mini- and (GACA) _n and (TCC) _n microsatellite DNA markers. The morphological forms examined were characterized by clonally inherited, species-specific patterns of the DNA markers, which were different from the species-specific DNA fingerprints of the other parthenogenetic species of the genus Darevskia (D. dahli, D. unisexualis, and D. rostombekovi). The mean index of similarity (S) obtained for a sample of 36 individuals from three isolated populations using three types of DNA markers was 0.966. This was similar to those observed in D. dahli (0.962) (P > 0.05), but higher than that in D. unisexualis (0.950) (P < 0.05) and D. rostombekovi(0.875) (P < 0.01). Inheritance of M13 minisatellite and (TCC) _n microsatellite DNA markers in the F1 offspring of parthenogenetic lizards was examined. It was shown that variability and clonal diversity of the fingerprint phenotypes observed in the populations and families of D. armeniaca could be at least partly explained by RFLP mutations in microsatellite repeats.     

THE ORIGIN AND EVOLUTION OF PARTHENOGENESIS IN HETERONOTIA BINOEI (GEKKONIDAE): EXTENSIVE GENOTYPIC DIVERSITY AMONG PARTHENOGENS

Variation at 18 allozyme loci was assayed among representatives of the geographically widespread, triploid parthenogenetic form of Heteronotia binoei. A minimum of 52 different genotypes were observed among 143 individuals. Virtually all localities sampled had multiple genotypes among the unisexuals. This represents unusually high genotypic diversity for a unisexual vertebrate. Heterozygosity in the triploids was higher than in diploid bisexual populations of H. binoei. Comparison with the alleles present in the diploid bisexuals confirms that the parthenogens are hybrids and indicates that most of the genotypic diversity stems from repetitive hybrid origins. However, the presence of some alleles unique to the parthenogens suggests that mutation adds to their genetic diversity. The genetic structure of this geographically widespread parthenogen suggests the hypothesis that the persistence and spread of the unisexual lineages is facilitated by genotypic diversity.     

Quantitative Assessment of Gene Diversity and Between-Population Differentiation of Parthenogenetic Lizard of the Genus DarevskiaUsing Mini- and Microsatellite DNA Markers

Methods of estimating within- and between-population gene diversity in parthenogenetic species using mini- and microsatellite DNA markers and modified Wright's F _ST statistic are presented with special reference to model populations of lizards of the genus Darevskia(D. dahli, D. armeniaca, D. unisexualis). We used DNA fingerprinting data for several populations of these species examined earlier. The effects of variation in M13 minisatellite, (GACA) _n - and (TCC) _n -microsatellite loci on the formation of within-population gene diversity in parthenogenetic species D. dahli and D. armeniaca were shown to be different. The equality of the realized gene diversity Hand its maximum possible value H _max in two populations of D. dahli (H _max = 0.032, H = 0.031, P 0.0431; H _max = 0.024, H = 0.027, P = 0.09) and D. armeniaca (H _max = 0.05, H = 0.053, P = 0.03; H _max= 0.054, H = 0.055, P= 0.02) suggests that variation in (GACA) _n loci substantially contributes to the maintenance of within-population genetic diversity. Analysis of between-population genetic diversity using loci M13, (GACA) _n , and (TCC) _n showed differentiation of D. dahli populations from northeastern and northwestern Armenia (F _ST = 0.0272, P = 3 × 10^–13) and genetic homogeneity of the Armenian and introduced to the Ukraine populations of D. armeniaca characteristic of one clone (F _ST = 0, P = 1).     

Physiological implications of genomic state in parthenogenetic lizards of reciprocal hybrid origin

Parthenogenesis often evolves in association with hybridization, but the associated ecological consequences are poorly understood. The Australian gecko Heteronotia binoei is unusual because triploid parthenogenesis evolved through reciprocal crosses between two sexual lineages, resulting in four possible cytonuclear genotypes. In this species complex, we compared the performance of these parthenogenetic genotypes with their sexual progenitors for a suite of physiological traits (metabolic rate, thermal tolerance, locomotor performance, and in vitro activity and gene sequence divergence of a cytonuclear metabolic pathway, cytochrome C oxidase). Mass‐specific metabolic rate scaled differently with body mass for parthenogens and sexuals, while heat tolerance provided the only evidence for cytonuclear incompatibility in hybrid parthenogens. The most prominent phenotypic effects were attributable to nuclear genome dosage. Overall, our results suggest that the hybrid/polyploidy origin of parthenogenetic H. binoei has had surprisingly few negative fitness consequences and may have produced a broader overall niche for the species.     

Study of Allelic Polymorphism of (GATA) n -Containing Loci in Parthenogenetic Lizards Darevskia unisexualis (Lacertidae)

The genesis of mini- and microsatellite loci, which is under extensive study in humans and some other bisexual species, has been virtually overlooked in species with clonal mode of reproduction. Earlier, using multilocus DNA fingerprinting, we have examined variability of some mini- and microsatellite DNA markers in parthenogenetic lizards from the genus Darevskia. In particular, mutant (GATA)n-restriction DNA fragments were found in Darevskia unisexualis. In the present study, we examined intraspecific polymorphism of three cloned loci of D. unisexualis—Du323, Du215, and Du281—containing (GATA)₇GAT(GATA)₂, GAT(GATA)₉, and (GATA)₁₀TA(GATA) microsatellite clusters, respectively. Different levels of intrapopulation and interpopulation variability of these loci were found. Locus Du281 showed the highest polymorphism—(six allelic variants in the sample of 68 DNA specimens). Three alleles were found for locus Du215. The Du323 locus was electrophoretically invariant. The primers chosen for loci Du323, Du215, and Du281 were also used for PCR analysis of homologous loci in two presumptive parental bisexual species, D. valentini and D. nairensis. The PCR products of the corresponding loci of the parental species had approximately the same size (200 bp) as their counterparts in D. unisexualis, but the polymorphism levels of the paternal, maternal, and hybrid species were shown to be somewhat different. These data on the structure of the D. unisexualis loci provide a possibility to study genetic diversity in the parthenogenetic species D. unisexualis and other related unisexual and bisexual species of this genus, which can provide new information on the origin of parthenogenetic species and on the phylogenetic relationships in the genus Darevskia. These data can also be used for resolving problems of marking the lizard genome, which is still poorly studied.     

Speciation through androgenesis in the stick insect genus Clonopsis (Insecta Phasmatodea)

In Morocco, Clonopsis stick insects showed tangled reproductive interactions actually resulting into a network of phylogenetic relationships known as ‘reticulate evolution’. Peculiar to parthenogenetic C. gallica and C. soumiae (54 and 72 chromosomes, respectively) – closely related to the bisexual C. felicitatis (2n = 36) – is the finding of numerically polyploid karyotypes with a diploid structure. Androgenesis appeared to be the most parsimonious explanation accounting for both the low mitochondrial differentiation and the quick onset of those polyploids with structurally diploid karyotypes, paired with neat nuclear differentiations. According to a proposed model, hybrid triploid females would segregate balanced haploid and diploid 2nd oocytes immediately producing all kinds of parthenogens and androgens. Owing to these peculiar reproductive issues, we felt useful searching for stronger evidence by deeply analysing the mitochondrial genome. This new analysis showed a neat separation of sexual Tetouan haplotypes from the parthenogenetic and androgenetic ones, which are grouped in two slightly overlapping groups by network analysis: Moroccan parthenogens and androgens vs European C. gallica. It could be also envisaged that C. gallica has multiple origins, being a complex of parthenogenetic strains originated through independent hybridizations. The straightforward mechanism originating both triploid and tetraploid parthenogens well fits with both their widely ascertained low mitochondrial differentiation and the geographical closeness of the most similar samples, independently from their specific karyotype. Combining the outcomes of the hybridization events and androgenesis, which completely substitutes hybrid genomes with those of a related paternal species, would conceivably realize the observed picture of species structure and distribution. Owing to the reinforced data set, it now appears much more sensible to support androgenesis as a quick pathway to originate polyploids with numerically and genetically sharply differing chromosome sets, while maintaining, at the same time, high mitochondrial similarity.     

Molecular genetic characteristic of dinucleotide microsatellite loci in parthenogenetic lizards <Emphasis Type="Italic">Darevskia unisexualis</Emphasis>

In the present study, the first molecular genetic investigation of dinucleotide (GT) _n microsatellite loci in parthenogenetic lizards Darevskia unisexualis was performed. New polymorphic locus, Du214, (GenBank Ac. No. EU252542) was identified and characterized in detail. It was demonstrated that allele of this locus differed in the size and structure of microsatellite locus, as well as in point mutations, the combinations of which enabled the isolation of stabile fixed double nucleotide substitutions A-A (alleles 2 and 4) and G-T (alleles 1, 3, 5, and 6). Double nucleotide substitutions described were also identified in the orthlogous loci of the parental species genomes, D. raddei (G-T) and D. valentine (A-A). Based on the analysis of allele distribution pattern at this locus in all populations of parthenospecies D. unisexualis, mathematic model was elaborated and realized. Using this model, frequencies of allelic variants for all populations of the species of interest were calculated and population genetic structure of D. unisexualis was characterized. Genetic contribution of each population to the species gene pool was determined. The data obtained demonstrated that microsatellite variation was one of the factors of clonal and genetic diversity of a parthenospecies.     

Home ranges of parthenogenetic and bisexual species in a community of Darevskia lizards (Reptilia: Lacertidae)

We analysed the home ranges of a community of Darevskia rock lizards composed of a bisexual species (D. valentini), two parthenogens (D. armeniaca and D. unisexualis), and two backcross forms between bisexual and unisexual forms. We estimated home range areas of ink-marked, GPS-located lizards using Minimum Convex Polygon (MCP) and 95% of the locations for those individuals with five or more sightings. The bisexual D. valentini was the species with the largest home ranges, distances travelled, and the most intersections. No differences between unisexual species and backcrosses were recorded for any comparison. In males, home range size and perimeter were related to morphological characteristics. Contrary to what has been described in allopatry, unisexual species showed smaller home ranges and fewer overlaps than sympatric bisexual species. We tentatively suggest that the presence of potential bisexual partners might increase sexual competition among parthenogenetic females while differences in habitat use should also be considered.     

Multiple direct transitions from sexual reproduction to apomictic parthenogenesis in Timema stick insects

Transitions from sexual reproduction to parthenogenesis may occur along multiple evolutionary pathways and involve various cytological mechanisms to produce diploid eggs. Here, we investigate routes to parthenogenesis in Timema stick insects, a genus comprising five obligate parthenogens. By combining information from microsatellites and karyotypes with a previously published mitochondrial phylogeny, we show that all five parthenogens likely evolved spontaneously from sexually reproducing species, and that the sexual ancestor of one of the five parthenogens was probably of hybrid origin. The complete maintenance of heterozygosity between generations in the five parthenogens strongly suggests that eggs are produced by apomixis. Virgin females of the sexual species were also able to produce parthenogenetic offspring, but these females produced eggs by automixis. High heterozygosity levels stemming from conserved ancestral alleles in the parthenogens suggest, however, that automixis has not generated the current parthenogenetic Timema lineages but that apomixis appeared abruptly in several sexual species. A direct transition from sexual reproduction to (at least functional) apomixis results in a relatively high level of allelic diversity and high efficiency for parthenogenesis. Because both of these traits should positively affect the demographic success of asexual lineages, spontaneous apomixis may have contributed to the origin and maintenance of asexuality in Timema .     

The cost of sex and competition between cyclical and obligate parthenogenetic rotifers

The ubiquity of sexual reproduction is an evolutionary puzzle because asexuality should have major reproductive advantages. Theoretically, transitions to asexuality should confer substantial benefits in population growth and lead to rapid displacement of all sexual ancestors. So far, there have been few rigorous tests of one of the most basic assumptions of the paradox of sex: that asexuals are competitively superior to sexuals immediately after their origin. Here I examine the fitness consequences of very recent transitions to obligate parthenogenesis in the cyclical parthenogenetic rotifer Brachionus calyciflorus. This experimental system differs from previous animal models, since obligate parthenogens were derived from the same maternal genotype as cyclical parthenogens. Obligate parthenogens had similar fitness compared with cyclical parthenogens in terms of the intrinsic rate of increase (calculated from life tables). However, population growth of cyclical parthenogens was predicted to be much lower: sexual female offspring do not contribute to immediate population growth in Brachionus, since they produce either males or diapausing eggs. Hence, if cyclical parthenogens constantly produce a high proportion of sexual offspring, there is a cost of sex, and obligate parthenogens can invade. This prediction was confirmed in laboratory competition experiments.