Repeat swimming performance and its implications for inferring the relative fitness of asexual hybrid dace (Pisces: Phoxinus) and their sexually reproducing parental species

While theories explaining the evolution and maintenance of sex are abundant, empirical data on the costs and benefits of asexual relative to sexual reproduction are less common. Asexually reproducing vertebrates, while few, provide a rare opportunity to measure differences in fitness between asexual and sexual species. All known asexually reproducing vertebrates are of hybrid origin, and hybrid disadvantage (i.e., reduced hybrid fitness) is thought to facilitate long-term coexistence between asexual and sexual species. We used repeat swimming performance as a proxy for fitness to compare the fitness of asexual hybrid dace (Pisces: Phoxinus) and their sexually reproducing parental species, finescale dace (Phoxinus neogaeus) and northern redbelly dace (Phoxinus eos). We tested the prediction that, given the widespread coexistence of these hybrid and parental dace, the parental species should show equivalent and perhaps superior repeat performance relative to hybrids. A repeat constant acceleration test (U(max)) was conducted at both acclimation temperature (16 °C) and at an elevated temperature (25 °C) to simulate the combined influence of a repeat swim and acute temperature change that fish might experience in the wild. The asexual hybrids performed more poorly than at least one of the parental species. There was a negative effect of temperature on repeat swimming performance in all fish, and the repeat performance of hybrids was more severely affected by temperature than that of finescale dace. No difference in the effect of temperature on repeat performance was detected between hybrids and northern redbelly dace. These results suggest that hybrids suffer physiological costs relative to the parentals or at least that the hybrids do not gain advantage from hybrid vigor, which probably contributes to the coexistence of asexual and sexual species in this system.     

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.     

How to go extinct by mating too much: population consequences of male mate choice and efficiency in a sexual–asexual species complex

Selection acting on individuals is not predicted to maximize population persistence, yet examples that explicitly quantify conflicts between individual and population level benefits are scarce. One such conflict occurs over sexual reproduction because of the cost of sex: sexual populations that suffer the cost of producing males have only half the growth rate compared to asexuals. Male behaviour can additionally impact population dynamics in a variety of ways, and here we study an example where the impact is unusually clear: the riddle of persistence of sperm‐dependent sexual–asexual species complexes. Here, a sexually reproducing host species coexists with an ameiotically reproducing all‐female sperm parasite. Sexual–asexual coexistence should not be stable because the proportion of asexually reproducing females will rapidly increase and the relative abundance of the sexually reproducing host species will decline. A severe shortage of males will lead to sperm limitation for sexual and asexual females and the system collapses. Male mate choice could reduce the reproductive potential of the asexual species and thus potentially prevent the collapse. In the gynogenetic (sperm‐dependent parthenogenetic) Amazon molly Poecilia formosa and its host (P. latipinna or P. mexicana), males discriminate against asexual females to some extent. Using a population‐dynamical model, we examine the population dynamics of this species complex with varying strengths of male discrimination ability and efficiency with which they locate females and produce sperm. The sexual species would benefit from stronger discrimination, thus preventing being displaced by the asexual females. However, males would be required to evolve preferences that are probably too strong to be purely based upon selection acting on individuals. We conclude that male behaviour does not fully prevent but delays extinction, yet this is highly relevant because low local extinction rates strongly promote coexistence as a metapopulation.     

Biogeography of sexual and asexual populations in Bostrychia moritziana (Rhodomelaceae, Rhodophyta)

Bostrychia moritziana (Sonder ex Kützing) J. Agardh is recorded from many regions around the world. Our laboratory culture investigations have verified a sexual life cycle in isolates from Australia, Venezuela, Colombia, South Africa, Fiji, New Zealand and Indonesia. By contrast, asexual isolates producing successive generations of tetrasporophytes in laboratory culture and, presumably, in the field, are known from Australia. New Caledonia and Japan. In Australia, asexual reproduction is absent only in Victoria. In Western Australia, Northern Territory and Queensland, 99% of the isolates have asexual reproduction. In New South Wales (NSW), asexual and sexual populations are often intermixed. Of the 176 worldwide field collections, 58% were vegetative, 39% were tetrasporic, 2% were female and 1% were male. After several years of observations on the asexual isolates in culture, at least 30 successive asexual tetrasporophytic generations have developed. Only two asexual isolates (3558 and 3575) from NSW have formed a single male and female gametophyte in culture. In a self-cross of 3568, the carpospores developed into tetrasporophytes that recycled asexually. All outcrosses done with normal sexual isolates produced normal carposporophytes and the carpospores developed into tetrasporophytes that also recycled asexually. Asexual populations may arise repeatedly by loss of meiosis in tetrasporangia of sexual populations. Asexual reproduction apparently does not diminish the overall dispersal and abundance in the field. Our present bio-geographic data show that sexually reproducing popuiations of B. moritziana occur worldwide, while asexuaily reproducing populations are confined to the western Pacific. Bostrychia bispora West et Zuccarello, initially described on the basis of its asexual reproduction to distinguish it from B, moritziana, is now reduced to synonymy with B. moritziana.     

Reproductive strategy, clonal structure and genetic diversity in populations of the aquatic macrophyte Sparganium emersum in river systems

Many aquatic and riparian plant species are characterized by the ability to reproduce both sexually and asexually. Yet, little is known about how spatial variation in sexual and asexual reproduction affects the genotypic diversity within populations of aquatic and riparian plants. We used six polymorphic microsatellites to examine the genetic diversity within and differentiation among 17 populations (606 individuals) of Sparganium emersum, in two Dutch-German rivers. Our study revealed a striking difference between rivers in the mode of reproduction (sexual vs. asexual) within S. emersum populations. The mode of reproduction was strongly related to locally reigning hydrodynamic conditions. Sexually reproducing populations exhibited a greater number of multilocus genotypes compared to asexual populations. The regional population structure suggested higher levels of gene flow among sexually reproducing populations compared to clonal populations. Gene flow was mainly mediated via hydrochoric dispersal of generative propagules (seeds), impeding genetic differentiation among populations even over river distances up to 50 km. Although evidence for hydrochoric dispersal of vegetative propagules (clonal plant fragments) was found, this mechanism appeared to be relatively less important. Bayesian-based assignment procedures revealed a number of immigrants, originating from outside our study area, suggesting intercatchment plant dispersal, possibly the result of waterfowl-mediated seed dispersal. This study demonstrates how variation in local environmental conditions in river systems, resulting in shifting balances of sexual vs. asexual reproduction within populations, will affect the genotypic diversity within populations. This study furthermore cautions against generalizations about dispersal of riparian plant species in river systems.     

LARGE POPULATION SIZE PREDICTS THE DISTRIBUTION OF ASEXUALITY IN SCALE INSECTS

Understanding why some organisms reproduce by sexual reproduction while others can reproduce asexually remains an important unsolved problem in evolutionary biology. Simple demography suggests that asexuals should outcompete sexually reproducing organisms, because of their higher intrinsic rate of increase. However, the majority of multicellular organisms have sexual reproduction. The widely accepted explanation for this apparent contradiction is that asexual lineages have a higher extinction rate. A number of models have indicated that population size might play a crucial role in the evolution of asexuality. The strength of processes that lead to extinction of asexual species is reduced when population sizes get very large, so that the long‐term advantage of sexual over asexual reproduction may become negligible. Here, we use a comparative approach using scale insects (Coccoidea, Hemiptera) to show that asexuality is indeed more common in species with larger population density and geographic distribution and we also show that asexual species tend to be more polyphagous. We discuss the implication of our findings for previously observed patterns of asexuality in agricultural pests.     

Genetic evidence of variation in the contributions of sexual and asexual reproduction to populations of the freshwater ostracod Candonocypris novaezelandiae

SUMMARY. 1 Genetic (electrophoretic) and sex ratio data were used to assess the contributions of sexual and asexual reproduction to recruitment to populations of the freshwater ostracod Candonocypris novaezelandiae in temporary and permanent water bodies of varying size.
2. Two distinct types of population structure were found. Populations from eight permanent ponds, a reservoir and a temporary pond, apparently comprised only females and were dominated by a few highly replicated genotypes. Significant departures from Hardy-Weinberg equilibria were observed for at least one locus in all populations, and multi-locus genotypic diversity ranged between 16% and 48% of that expected in a population with the same underlying gene frequencies reproducing solely by sexual means. These results were consistent with the predicted consequences of predominantly asexually derived recruitment.
3. In contrast, sexual reproduction was probably most important in a population inhabiting a large temporary swamp. This population displayed 79% of the genotypic diversity expected for a sexually reproducing population, and contained both males and females.
4. Most theoretical models predict that sexually reproducing individuals should have a selective advantage in unstable environments. The results of this study do not provide a perfect association of sexually derived recruitment with unstable habitats.     

The role of pleiotropy in the maintenance of sex in yeast

In facultatively sexual species, lineages that reproduce asexually for a period of time can accumulate mutations that reduce their ability to undergo sexual reproduction when sex is favorable. We propagated Saccharomyces cerevisiae asexually for approximately 800 generations, after which we measured the change in sexual fitness, measured as the proportion of asci observed in sporulation medium. The sporulation rate in cultures propagated asexually at small population size declined by 8%, on average, over this time period, indicating that the majority of mutations that affect sporulation rate are deleterious. Interestingly, the sporulation rate in cultures propagated asexually at large population size improved by 11%, on average, indicating that selection on asexual function effectively eliminated most of the mutations deleterious to sporulation ability. These results suggest that pleiotropy between mutations' effects on asexual fitness and sexual fitness was predominantly positive, at least for the mutations accumulated in this experimental evolution study. A positive correlation between growth rate and sporulation rate among lines also provided evidence for positive pleiotropy. These results demonstrate that, at least under certain circumstances, selection acting on asexual fitness can help to maintain sexual function.     

Modeling the impact of reproductive mode on masting

Masting is defined as the intermittent highly variable production of seed in a plant population. According to reproductive modes, that is, sexual and asexual reproduction, masting species can be separated into three groups, that is, (1) species, for example, bamboo, flower only once before they die; (2) species, for example, Fagus, reproduce sexually; and (3) species, for example, Stipa tenacissima, reproduce both sexually and asexually. Several theories have been proposed to explore the underlying mechanisms of masting. However, to our knowledge, no theory has been found to explain the mechanism of masting species that reproduce both sexually and asexually. Here we refine the Resource Budget Model by considering a trade‐off between sexual and asexual reproduction. Besides the depletion efficient (i.e., the ratio of the cost of seed setting and the cost of flowering), other factors, such as the annual remaining resource (i.e., the rest of the resource from the photosynthetic activity after allocating to growth and maintenance), the trade‐off between sexual and asexual reproduction, and the reproductive thresholds, also affect masting. Moreover, two potential reproductive strategies are found to explain the mechanisms: (1) When the annual remaining resource is relatively low, plants reproduce asexually and a part of the resource is accumulated as the cost of asexual reproduction is less than the annual remaining resource. Plants flower and set fruits once the accumulated resource exceeds the threshold of sexual reproduction; (2) when the annual remaining resource is relatively high, and the accumulated resource surpasses the threshold of sexual reproduction, masting occurs. Remarkably, under certain depletion efficient, more investigation in sexual reproduction will lead plants to reproduce periodically. Additionally, plants investigate less resource to reproduce periodically when depletion efficient keeps increasing as plants can reproduce efficiently. Overall, our study provides new insights into the interpretation of masting, especially for species that reproduce both sexually and asexually.     

Evolutionary and functional insights into reproductive strategies of aphids

Aphids are among the few organisms capable of reproducing either sexually or asexually. This plasticity in reproductive mode is viewed as an adaptive response to cope with seasonal changes. Clonal reproduction occurs during the growing season allowing rapid population increase, while sexual reproduction occurs during late summer and leads to frost-resistant eggs that can survive winter conditions. This shift between these two extreme reproductive modes is achieved by using the same genotype, i.e. within the same genetic clone, and is triggered by photoperiodic changes perceived by the aphid brain or visual system. Advances have been made recently to depict genetic programs that relate to the regulation of reproductive modes in aphids. These studies have benefited from the rapid development of genomic and post-genomic resources obtained through the International Aphid Genomics Consortium. Here, we underline the importance of several candidate genes in the switch from clonal to sexual reproduction in aphids and whose roles await full validation. Besides reproductive mode variation expressed at the genotypic level, aphid species also frequently encompass lineages which have lost the sexual phase and hence the alternating clonal and sexual reproductive phases of the life cycle. This coexistence of sex and asexual reproduction within the same species raises questions on its evolutionary and ecological significance. We summarize the knowledge accumulated to date on the maintenance of sex as well as on the origin and evolution of asexuality in aphids. By combining functional genomics, genetic and ecological approaches on reproductive plasticity and polymorphism, we hope to obtain an integrative view of the evolutionary forces shaping aphid reproductive strategies, from gene to population and species levels.     

Dispersal and life history strategies in epiphyte metacommunities: alternative solutions to survival in patchy,dynamic landscapes

Host trees for obligate epiphytes are dynamic patches that emerge, grow and fall, and metacommunity diversity critically depends on efficient dispersal. Even though species that disperse by large asexual diaspores are strongly dispersal limited, asexual dispersal is common. The stronger dispersal limitation of asexually reproducing species compared to species reproducing sexually via small spores may be compensated by higher growth rates, lower sensitivity to habitat conditions, higher competitive ability or younger reproductive age. We compared growth and reproduction of different groups of epiphytic bryophytes with contrasting dispersal (asexual vs. sexual) and life history strategies (colonists, short- and long-lived shuttle species, perennial stayers) in an old-growth forest stand in the boreo-nemoral region in eastern Sweden. No differences were seen in relative growth rates between asexual and sexual species. Long-lived shuttles had lower growth rates than colonists and perennial stayers. Most groups grew best at intermediate bark pH. Interactions with other epiphytes had a small, often positive effect on growth. Neither differences in sensitivity of growth to habitat conditions nor differences in competitive abilities among species groups were found. Habitat conditions, however, influenced the production of sporophytes, but not of asexual diaspores. Presence of sporophytes negatively affected growth, whereas presence of asexual diaspores did not. Sexual species had to reach a certain colony size before starting to reproduce, whereas no such threshold existed for asexual reproduction. The results indicate that the epiphyte metacommunity is structured by two main trade-offs: dispersal distance vs. reproductive age, and dispersal distance vs. sensitivity to habitat quality. There seems to be a trade-off between growth and sexual reproduction, but not asexual. Trade-offs in species traits may be shaped by conflicting selection pressures imposed by habitat turnover and connectivity rather than by species interactions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

LOWER MITE INFESTATIONS IN AN ASEXUAL GECKO COMPARED WITH ITS SEXUAL ANCESTORS

What advantage do sexually reproducing organisms gain from their mode of reproduction that compensates for their twofold loss in reproductive rate relative to their asexual counterparts? One version of the Red Queen hypothesis suggests that selective pressure from parasites is strongest on the most common genotype in a population, and thus genetically identical clonal lineages are more vulnerable to parasitism over time than genetically diverse sexual lineages. Our surveys of the ectoparasites of an asexual gecko and its two sexual ancestral species show that the sexuals have a higher prevalence, abundance, and mean intensity of mites than asexuals sharing the same habitat. Our experimental data indicate that in one sexual/asexual pair this pattern is at least partly attributable to higher attachment rates of mites to sexuals. Such a difference may occur as a result of exceptionally high susceptibility of the sexuals to mites because of their low genetic diversity (relative to other more-outbred sexual species) and their potentially high stress levels, or as a result of exceptionally low susceptibility of the asexuals to mites because of their high levels of heterozygosity.     

Differential susceptibility to food stress in neonates of sexual and asexual mollies (<Emphasis Type="Italic">Poecilia</Emphasis>, Poeciliidae)

The maintenance of sex is still an evolutionary puzzle given its immediate costs. Stably coexisting complexes of asexually and sexually reproducing forms allow to study mechanisms that balance the costs and benefits of both asexual and sexual reproduction. Here, we tested whether coexisting asexual and sexual fish of the genus Poecilia differed in neonate mortality when exposed to environmental stress in the form of fluctuating temperatures and food deprivation. We find that asexual Amazon mollies, Poecilia formosa, are significantly more sensitive to food stress than their sexual relative Poecilia latipinna, but both are equally unaffected by variable temperatures. Differences in the susceptibility to environmental stress may contribute to diminishing the asexuals’ benefits of a higher intrinsic population growth rate and thus mediate stable coexistence of the two reproductive forms.     

Parasites and sexual reproduction in psychid moths

Persistence of sexual reproduction among coexisting asexual competitors has been a major paradox in evolutionary biology. The number of empirical studies is still very limited, as few systems with coexisting sexual and strictly asexual lineages have been found. We studied the ecological mechanisms behind the simultaneous coexistence of a sexually and an asexually reproducing closely related species of psychid moth in Central Finland between 1999 and 2001. The two species compete for the same resources and are often infected by the same hymenopteran parasitoids. They are extremely morphologically and behaviorally similar and can be separated only by their reproductive strategy (sexual vs. asexual) or by genetic markers. We compared the life-history traits of these species in two locations where they coexist to test predictions of the cost-of-sex hypothesis. We did not find any difference in female size, number of larvae, or offspring survival between the sexuals and asexuals, indicating that sexuals are subject to cost of sex. We also used genetic markers to check and exclude the possibility of Wolbachia bacteria infection inducing parthenogenesis. None of the samples was infected by Wolbachia and, thus, it is unlikely that these bacteria could affect our results. We sampled 38 locations to study the prevalence of parasitoids and the moths' reproductive strategy. We found a strong positive correlation between prevalence of sexual reproduction and prevalence of parasitoids. In locations where parasitoids are rare asexuals exist in high densities, whereas in locations with a high parasitoid load the sexual species was dominant. Spatial distribution alone does not explain the results. We suggest that the parasite hypothesis for sex may offer an explanation for the persistence of sexual moths in this system.     

Sexual reproduction reshapes the genetic architecture of digital organisms

Modularity and epistasis, as well as other aspects of genetic architecture, have emerged as central themes in evolutionary biology. Theory suggests that modularity promotes evolvability, and that aggravating (synergistic) epistasis among deleterious mutations facilitates the evolution of sex. Here, by contrast, we investigate the evolution of different genetic architectures using digital organisms, which are computer programs that self-replicate, mutate, compete and evolve. Specifically, we investigate how genetic architecture is shaped by reproductive mode. We allowed 200 populations of digital organisms to evolve for over 10 000 generations while reproducing either asexually or sexually. For 10 randomly chosen organisms from each population, we constructed and analysed all possible single mutants as well as one million mutants at each mutational distance from 2 to 10. The genomes of sexual organisms were more modular than asexual ones; sites encoding different functional traits had less overlap and sites encoding a particular trait were more tightly clustered. Net directional epistasis was alleviating (antagonistic) in both groups, although the overall strength of this epistasis was weaker in sexual than in asexual organisms. Our results show that sexual reproduction profoundly influences the evolution of the genetic architecture.     

Virus epidemics can lead to a population‐wide spread of intragenomic parasites in a previously parasite‐free asexual population

Sexual reproduction is problematic to explain due to its costs, most notably the twofold cost of sex. Yet, sex has been suggested to be favourable in the presence of proliferating intragenomic parasites given that sexual recombination provides a mechanism to confine the accumulation of deleterious mutations. Kraaijeveld et al. compared recently the accumulation of transposons in sexually and asexually reproducing lines of the same species, the parasitoid wasp Leptopilina clavipes. They discovered that within asexually reproducing wasps, the number of gypsy‐like retrotransposons was increased fourfold, whereas other retrotransposons were not. Interestingly, gypsy‐like retrotransposons are closely related to retroviruses. Endogenous retroviruses are retroviruses that have integrated to the germ line cells and are inherited thereafter vertically. They can also replicate within the genome similarly to retrotransposons as well as form virus particles and infect previously uninfected cells. This highlights the possibility that endogenous retroviruses could play a role in the evolution of sexual reproduction. Here, we show with an individual‐based computational model that a virus epidemic within a previously parasite‐free asexual population may establish a new intragenomic parasite to the population. Moreover and in contrast to other transposons, the possibility of endogenous viruses to maintain a virus epidemic and simultaneously provide resistance to individuals carrying active endogenous viruses selects for the presence of active intragenomic parasites in the population despite their deleterious effects. Our results suggest that the viral nature of certain intragenomic parasites should be taken into account when sex and its benefits are being considered.     

A quasispecies approach to the evolution of sexual replication in unicellular organisms

This study develops a simplified model describing the evolutionary dynamics of a population composed of obligate sexually and asexually reproducing, unicellular organisms. The model assumes that the organisms have diploid genomes consisting of two chromosomes, and that the sexual organisms replicate by first dividing into haploid intermediates, which then combine with other haploids, followed by the normal mitotic division of the resulting diploid into two new daughter cells. We assume that the fitness landscape of the diploids is analogous to the single-fitness-peak approach often used in single-chromosome studies. That is, we assume a master chromosome that becomes defective with just one point mutation. The diploid fitness then depends on whether the genome has zero, one, or two copies of the master chromosome. We also assume that only pairs of haploids with a master chromosome are capable of combining so as to produce sexual diploid cells, and that this process is described by second-order kinetics. We find that, in a range of intermediate values of the replication fidelity, sexually reproducing cells can outcompete asexual ones, provided the initial abundance of sexual cells is above some threshold value. The range of values where sexual reproduction outcompetes asexual reproduction increases with decreasing replication rate and increasing population density. We critically evaluate a common approach, based on a group selection perspective, used to study the competition between populations and show its flaws in addressing the evolution of sex problem.     

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.     

Asexually produced Cape honeybee queens (Apis mellifera capensis) reproduce sexually

Unmated workers of the Cape honeybee Apis mellifera capensis can produce female offspring including daughter queens. As worker-laid queens are produced asexually, we wondered whether these asexually produced individuals reproduce asexually or sexually. We sampled 11 colonies headed by queens known to be the clonal offspring of workers and genotyped 23 worker offspring from each queen at 5 microsatellite loci. Without exception, asexually produced queens produced female worker offspring sexually. In addition, we report the replacement of a queen by her asexually produced granddaughter, with this asexually produced queen also producing offspring sexually. Hence, once a female larva is raised as a queen, mating and sexual reproduction appears to be obligatory in this subspecies, despite the fact that worker-laid queens are derived from asexual lineages.     

On the reality and recognisability of asexual organisms: morphological analysis of the masticatory apparatus of bdelloid rotifers

Species concepts and definitions have been a long-standing debate in evolutionary biology since before Darwin, and almost all proposed solutions are based upon grouping and clustering, with species conceived as somehow biological distinct entities, originated and maintained mainly by reproductive isolation. Lacking reproductive exchange, asexual organisms such as bdelloid rotifers, the best-supported clade of so-called 'ancient asexuals', pose an interesting challenge to debates over the reality of species. However, few data are available on bdelloid rotifers. The only evidence has been that bdelloid species have been more consistently recognised than in their sister sexual group, the monogonont rotifers, across successive taxonomic treatments, but this is confounded by the much lesser degree of taxonomic interest in bdelloids. We applied geometric morphometrics analyses on shape and size of hard masticatory pieces, named trophi, of 1420 bdelloids, belonging to 48 populations of eight traditional species, to test the hypothesis of recognisability of bdelloids. Our morphological analysis confirms that traditional bdelloid species are separated distinct entities, possessing trophi morphologies divided by gaps between taxa, similar to patterns of morphological features in sexually reproducing organisms. In common with most microscopic understudied organisms, bdelloid rotifers harbour much previously undescribed diversity: we found significant differences in trophi morphology within traditional species, revealing the existence of cryptic taxa, similar to those also found in facultatively sexual monogonont rotifers. We confirm that recognisability in bdelloids is not qualitatively different from other small understudied animals such as monogononts, and that sexual versus asexual reproduction does not lead to differences in morphological diversity patterns, as previously suggested based on interpretation of taxonomic revisions.