A survey of introns in three genes of rotifers

Here we report on the occurrence and position of introns found in three genes of rotifers. A region of the gene for the TATA-box binding protein was examined in three species of Bdelloidea and one of Monogononta. There are two introns in both copies of this gene present in each of the three bdelloids examined – one at a position where introns occur in other eukaryotes and the other at a novel position; the monogonont has no introns in the region examined. A region of the gene encoding the 82 kD heat shock protein was examined in 10 species, with every rotifer class represented. Introns were found in only two species, both bdelloids: one of the species has an intron in all three copies of the gene; the other has an intron in only one of the three copies. Both introns occur at novel positions. The gene for triosephosphate isomerase was examined in one bdelloid. Both copies of the gene in this species contain introns, all at conserved positions: one copy contains five introns, the other copy three. These observations demonstrate the presence of introns in bdelloid rotifers, some in conserved positions, others apparently newly arisen during bdelloid evolution.     

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.     

Stress and fitness in parthenogens: is dormancy a key feature for bdelloid rotifers?

Background

Bdelloid rotifers are the most common and abundant group of animals that reproduce by ameiotic parthenogenesis, only. They are common in temporally ephemeral habitats, and it is unclear if they dwell in unstable habitats because are excluded from better conditions by stronger competitors, or because they need unstable conditions for their success. We tested the hypothesis that bdelloids 'require' stressful conditions for their persistence by comparing fitness-related traits of stressed (desiccated, D) and unstressed (hydrated, H) lines of two species, Adineta ricciae and Macrotrachela quadricornifera.

Results

For both bdelloid species, fecundity was significantly lower in H than in parallel D line. Fitness components decreased with time progressively in the H line but not in the D line. Recovery rates of D lines were recorded after every desiccation and did not reveal any trend in time, suggesting that no selection was operating.

Conclusion

Stress in the form of reiterated desiccations seemed to help both bdelloid species to keep fitness stable; in contrast under stable conditions, like permanent hydration, these bdelloid species had poorer performances. Bdelloids, although aquatic animals, are not only efficient in tolerating desiccation, but seem somehow dependent on anhydrobiosis, a circumstance that might represent a key event in their life cycle. If this is true, life in unpredictable habitats should not be seen as the result of competitive exclusion from 'easier' habitats, but a requirement for long-term survival of these parthenogenetic animals.

     

SEXUAL SPECIES ARE SEPARATED BY LARGER GENETIC GAPS THAN ASEXUAL SPECIES IN ROTIFERS

Why organisms diversify into discrete species instead of showing a continuum of genotypic and phenotypic forms is an important yet rarely studied question in speciation biology. Does species discreteness come from adaptation to fill discrete niches or from interspecific gaps generated by reproductive isolation? We investigate the importance of reproductive isolation by comparing genetic discreteness, in terms of intra‐ and interspecific variation, between facultatively sexual monogonont rotifers and obligately asexual bdelloid rotifers. We calculated the age (phylogenetic distance) and average pairwise genetic distance (raw distance) within and among evolutionarily significant units of diversity in six bdelloid clades and seven monogonont clades sampled for 4211 individuals in total. We find that monogonont species are more discrete than bdelloid species with respect to divergence between species but exhibit similar levels of intraspecific variation (species cohesiveness). This pattern arises because bdelloids have diversified into discrete genetic clusters at a faster net rate than monogononts. Although sampling biases or differences in ecology that are independent of sexuality might also affect these patterns, the results are consistent with the hypothesis that bdelloids diversified at a faster rate into less discrete species because their diversification does not depend on the evolution of reproductive isolation.     

Morphological response of a bdelloid rotifer to desiccation

We desiccated bdelloid rotifers (Macrotrachela quadricornifera), submitting the animals to four desiccation procedures (protocols A, B, C, D) that differed in the rate of water evaporation, in the time of desiccation, and in the substrates provided. We observed external morphological changes of the rotifer bodies during drying with scanning electron microscopy and, in parallel, assessed rates of recovery after a 7-day period of dormancy. Two protocols produced disorganized morphologies of the anhydrobiotic animals, with no (A) or very poor (B) recovery. Protocols C and D gave rather high rates of recovery and dry rotifers appeared unaltered and well organized. The different protocols affected rotifer morphology during the 7-day anhydrobiosis and rates of recovery after the 7-day anhydrobiosis; high recovery rates corresponded to well-organized morphologies of anhydrobiotic bdelloids, suggesting that a proper contraction of the body into a tun shape and probably a rigorous packing of internal structures are necessary for survival after anhydrobiosis. These features are affected by the time between water shortage and full desiccation, but also by the surrounding relative humidity and by the nature of the substrate. Possible adaptations of anhydrobiotic rotifers are discussed.     

A carnivorous bdelloid rotifer, Abrochtha carnivora n.sp.

Abstract. Here we describe a new bdelloid rotifer, Abrochtha carnivora n.sp., that preys on other bdelloids and monogonont rotifers. This is the first report of predatory behavior in bdelloids. Despite this extraordinary behavior, A. carnivora displays no major structural deviation from the standard body plan for members of the bdelloid family Philodinavidae: body with head (with ciliated corona), trunk, and foot, mastax with ramate trophi. The finding of such a carnivorous species, unique for the class, appears to contradict the common assumption that the wholly parthenogenetic bdelloids are evolutionary dead-ends.     

A Putative LEA Protein,but no Trehalose,is Present in Anhydrobiotic Bdelloid Rotifers

Some eukaryotes, including bdelloid rotifer species, are able to withstand desiccation by entering a state of suspended animation. In this ametabolic condition, known as anhydrobiosis, they can remain viable for extended periods, perhaps decades, but resume normal activities on rehydration. Anhydrobiosis is thought to require accumulation of the non-reducing disaccharides trehalose (in animals and fungi) or sucrose (in plant seeds and resurrection plants), which may protect proteins and membranes by acting as water replacement molecules and vitrifying agents. However, in clone cultures of bdelloid rotifers Philodina roseola and Adineta vaga, we were unable to detect trehalose or other disaccharides in either control or dehydrating animals, as determined by gas chromatography. Indeed, trehalose synthase genes (tps) were not detected in these rotifer genomes, suggesting that bdelloids might not have the capacity to produce trehalose under any circumstances. This is in sharp contrast to other anhydrobiotic animals such as nematodes and brine shrimp cysts, where trehalose is present during desiccation. Instead, we suggest that adaptations involving proteins might be more important than those involving small biochemicals in rotifer anhydrobiosis: on dehydration, P. roseola upregulates a hydrophilic protein related to the late embryogenesis abundant (LEA) proteins associated with desiccation tolerance in plants. Since LEA-like proteins have also been implicated in the desiccation tolerance of nematodes and micro-organisms, it seems that hydrophilic protein biosynthesis represents a common element of anhydrobiosis across several biological kingdoms.     

Anhydrobiosis in bdelloid species, populations and individuals

Bdelloid rotifers are aquatic microinvertebrates common in waterbodies and in unstable "terrestrial" habitats, such as mossesand lichens. The key to the adaptability to live in unstablehabitats is their capacity to tolerate habitat desiccation throughanhydrobiosis, that is assumed apomorphic to the taxon. Thelife history traits of some "moss" and "water" species of bdelloidare compared, showing that the water species have shorter lifespan, higher fecundity and earlier age at first reproductionthan the moss species. These traits are discussed in the lightof current life history theories. Contrary to the assumptionsof the models, anhydrobiosis of bdelloids does not appear toimply energy demand. Past research on bdelloid anhydrobiosisis briefly reviewed, focusing on the factors that affect anhydrobiosissuccess, like morphological and physiological adjustments, andon the effect of events of anhydrobiosis during life time. Desiccationproduces a time shift on the age of the bdelloid, which disregardsthe time spent as anhydrobiotic, following the so-called "SleepingBeauty" model. Average fecundity is never found to decreaseas a consequence of anhydrobiosis, but is either equal or evenhigher than that of a hydrated rotifer. Bdelloid populationsseem to benefit from anhydrobiosis; fitness of a bdelloid isfound to decline, if populations are maintained hydrated forseveral generations, but not if populations are cyclically desiccated.We hypothesize that anhydrobiosis can be an essential eventfor long-term survival of bdelloid populations.     

Bdelloid rotifers: ‘sleeping beauties’ and ‘evolutionary scandals’, but not only

Bdelloid rotifers are mostly known for two peculiarities, continuous parthenogenetic reproduction and dormancy in response to habitat desiccation, a phenomenon named anhydrobiosis. These uncommon traits earned them the names of ‘evolutionary scandals’ and ‘sleeping beauties’, respectively. Relevant aspects of bdelloid biology have recently been described that connect parthenogenesis to anhydrobiosis and that might account for their evolutionary survival in spite of the conservative reproduction. In the present study, I explore recent literature, in the attempt to disentangle the apparent incongruency between the apomictic reproduction and the presumed long-term evolutionary survival of bdelloid species. Recent results remarkably improved our knowledge of bdelloid population biology, genetics, and molecular biology. The most relevant findings concern (i) acquisition of foreign genes through horizontal transfer, (ii) presence of divergent sequences possibly corresponding to ancient gene duplications and (iii) capacity to escape parasites: events that appear to be connected with dormancy. I also address the results of recent studies on the relationships between bdelloids and other rotifers, including acanthocephalans, in an attempt to highlight similarities and differences that should be clarified to better understand phylogenetic relationship among the Rotifera sensu lato.

     

Recipes for successful anhydrobiosis in bdelloid rotifers

We tested the effect of several environmental variables on the ability of three bdelloid rotifers (Macrotrachela quadricornifera, Philodina roseola and Adineta oculata) to recover from the anhydrobiotic state. The variables we examined were (1) rate of water evaporation, (2) relative humidity during anhydrobiosis, (3) temperature during anhydrobiosis, (4) duration of anhydrobiosis, and (5) rehydration rate. Our results indicate that bdelloids can regulate to some degree net water balance during onset and termination of anhydrobiosis.     

Immediate and heritable costs of desiccation on the life history of the bdelloid rotifer <Emphasis Type="Italic">Philodina roseola</Emphasis>

The long-term persistence of the ‘ancient asexual’ bdelloid rotifers, a clade of small aquatic invertebrates, is often tied to their ability to enter anhydrobiosis. This ability has both clear benefits (e.g. survival of desiccating conditions), but offers considerable costs (e.g. subsequent repair of the genome as well as physiological and metabolic costs to re-establish the phenotype). Despite these costs, several studies show that the time spent dry is effectively ignored with respect to life expectancy (the Sleeping Beauty hypothesis) and that reconstruction of the genome after a desiccation event might even be necessary to repair mistakes accumulated in it from obligate parthenogenesis while the animals were active. We propose that this genomic repair might not derive exclusively because desiccation per se, but could also result from genetic exchange that appears to occur between individuals during this time. By comparing individuals of Philodina roseola Ehrenberg, 1832 desiccated in groups versus individually, we document costs to desiccation in the isolated treatment group that impact negatively on lifespan and reproduction. In addition, comparing both groups with continuously active individuals reveals no strong evidence for the Sleeping Beauty hypothesis in this species nor any decline in fitness over a six-month period for the latter group. Finally, many treatment effects are at least partly heritable and were found in the untreated F1 generations. In particular, individuals desiccated in groups and their offspring could both reproduce faster than the offspring of continuously active individuals. Thus, our results offer additional support for the hypothesis of genetic exchange occurring during desiccation events in P. roseola and highlight the importance of considering this factor, and desiccation in general, in explaining bdelloid fitness. Moreover, our results provide additional context for understanding how the genetic information of bdelloids is ultimately shaped.     

Extreme levels of hidden diversity in microscopic animals (Rotifera) revealed by DNA taxonomy

Knowledge and estimates of species richness at all scales are biased both by our understanding of the evolutionary processes shaping diversity and by the methods used to delineate the basic units of diversity. DNA taxonomy shows that diversity may be underestimated by traditional taxonomy, especially for microscopic animals. The effects of such hidden diversity are usually overlooked in ecological studies. Here, we estimate hidden diversity in bdelloid rotifers, a group of microscopic animals. We analyse cryptic diversity using a coalescent approach to infer taxonomical units from phylogenetic trees. Cryptic diversity was measured for eight traditional species of bdelloid rotifers and the results compared to that of the monogonont rotifer Brachionus plicatilis species complex, which is well studied and for which cross-mating experiments have been performed to explicitly define some of the species boundaries. A taxonomic inflation of up to 34 potential cryptic taxa was found in bdelloids. Cryptic taxa within each traditional species may be spatially isolated, but do not have narrower ecological niches. The species deemed as generalists exhibit the highest cryptic diversity. Cryptic diversity based on molecular characterization is commonly found in animals; nevertheless, the amount of cryptic diversity in bdelloids is much higher than in other groups analysed so far, maybe because of their peculiar parthenogenetic reproduction, other than microscopic size. We discuss this hypothesis in the light of the available empirical evidence from other groups of microscopic invertebrates, such as tardigrades and mites, which share size, habitat heterogeneity, potential for dispersal, and/or parthenogenetic reproduction.     

Feeding and anhydrobiosis in bdelloid rotifers: a preparatory study for an experiment aboard the International Space Station

Abstract. Here we report the effect of food concentration on the recovery from anhydrobiosis of a bdelloid rotifer, Macrotrachela quadricornifera . Cohorts were either starved, or fed high or low concentrations of food, before being dried and their subsequent recovery rates determined. The rotifers starved for 3 d before anhydrobiosis recovered in significantly higher proportion, and those fed lower food concentration recovered better than those fed higher food concentration. In addition, starvation did not decrease the recovery of other bdelloid species ( Philodina roseola and Adineta sp. 1) which were either fed or starved before anhydrobiosis. These results suggest that a successful recovery from anhydrobiosis is not dependent on prior resource level supplied to the bdelloids. However, the lack of resources might not be the only factor in a successful recovery from anhydrobiosis. Observations using scanning electron microscopy of fed individuals of M. quadricornifera entering anhydrobiosis showed that some food remained in the digestive tract. Thus, we propose that the negative effect of rich food may be due to a purely mechanical effect and may be interfering with a proper folding of the rotifer body at the onset of anhydrobiosis. This contribution results from studies carried out in preparation for biological experiments scheduled on the International Space Station (ISS).     

Resurrecting Van Leeuwenhoek's rotifers: a reappraisal of the role of disaccharides in anhydrobiosis

In 1702, Van Leeuwenhoek was the first to describe the phenomenon of anhydrobiosis in a species of bdelloid rotifer, Philodina roseola. It is the purpose of this review to examine what has been learned since then about the extreme desiccation tolerance in rotifers and how this compares with our understanding of anhydrobiosis in other organisms. Remarkably, much of what is known today about the requirements for successful anhydrobiosis, and the degree of biostability conferred by the dry state, was already determined in principle by the time of Spallanzani in the late 18th century. Most modern research on anhydrobiosis has emphasized the importance of the non-reducing disaccharides trehalose and sucrose, one or other sugar being present at high concentrations during desiccation of anhydrobiotic nematodes, brine shrimp cysts, bakers' yeast, resurrection plants and plant seeds. These sugars are proposed to act as water replacement molecules, and as thermodynamic and kinetic stabilizers of biomolecules and membranes. In apparent contradiction of the prevailing models, recent experiments from our laboratory show that bdelloid rotifers undergo anhydrobiosis without producing trehalose or any analogous molecule. This has prompted us to critically re-examine the association of disaccharides with anhydrobiosis in the literature. Surprisingly, current hypotheses are based almost entirely on in vitro data: there is very limited information which is more than simply correlative in the literature on living systems. In many species, disaccharide accumulation occurs at approximately the same time as desiccation tolerance is acquired. However, several studies indicate that these sugars are not sufficient for anhydrobiosis; furthermore, there is no conclusive evidence, through mutagenesis or functional knockout experiments, for example, that sugars are necessary for anhydrobiosis. Indeed, some plant seeds and micro-organisms, like the rotifer, exhibit excellent desiccation tolerance in the absence of high intracellular sugar concentrations. Accordingly, it seems appropriate to call for a re-evaluation of our understanding of anhydrobiosis and to embark on new experimental programmes to determine the key molecular mechanisms involved.     

Diversity of Bacillus-like organisms isolated from deep-sea hypersaline anoxic sediments

Background

In spite of considerable efforts over past decades we still know relatively little regarding the biogeography of rotifers of inland waters in México. To help rectify this we undertook an extensive survey of the rotifer fauna of 48 water bodies in the Chihuahuan Desert of México.

Results

Of the sites surveyed, 21 had salinities ≥ 2000 μS cm^-1 and in these we found 57 species of monogonont rotifers and several bdelloids. Species richness in the saline sites varied widely, with a range in species richness of 1 to 27 and a mean (± 1SD) = 8.8 (± 6.2). Collectively all sites possess relatively high percent single- and doubletons, 33.3 and 21.7%, respectively. Simpson's Asymmetric Index indicated that similarity in rotifer species composition varied widely among a set of 10 sites. These were selected because they were sampled more frequently or represent unusual habitats. These SAI values ranged from 0.00 (complete dissimilarity) to 1.00 (complete similarity). The Jaccard Index varied between 0.00 and 0.35. This observation probably reflects similarities and differences in water chemistry among these sites. Inland saline systems differed in their chemical composition by region. Conductivity was related to hardness and alkalinity. In addition, hardness was positively associated with chloride and sulfate. RDA showed that several species were positively associated with chloride concentration. Other factors that were significantly associated with rotifer species included the presence of macrophytes, nitrate content, oxygen concentration, TDS, latitude and whether the habitat was a large lake or reservoir.

Conclusion

This study illustrates the diversity of the rotiferan fauna of inland saline systems and the uniqueness among waterbodies. Conservation of these systems is needed to preserve these unique sources of biodiversity that include rotifers and the other endemic species found in association with them.     

Demographic processes underlying fitness restoration in bdelloid rotifers emerging from dehydration

相似文献   

Anhydrobiosis without trehalose in bdelloid rotifers

Eukaryotes able to withstand desiccation enter a state of suspended animation known as anhydrobiosis, which is thought to require accumulation of the non-reducing disaccharides trehalose (animals, fungi) and sucrose (plants), acting as water replacement molecules and vitrifying agents. We now show that clonal populations of bdelloid rotifers Philodina roseola and Adineta vaga exhibit excellent desiccation tolerance, but that trehalose and other disaccharides are absent from carbohydrate extracts of dried animals. Furthermore, trehalose synthase genes (tps) were not found in rotifer genomes. This first observation of animal anhydrobiosis without trehalose challenges our current understanding of the phenomenon and calls for a re-evaluation of existing models.     

Evidence for inefficient selection against deleterious mutations in cytochrome oxidase I of asexual bdelloid rotifers

Evolutionary theory predicts that natural selection should be less efficient in asexually than in sexually reproducing organisms. Obligate asexuals are expected to adapt slowly to changing environments and to accumulate mildly deleterious mutations to their genomes, potentially explaining their typically short evolutionary lifespans. One group of animals that appear to challenge these ideas is the bdelloid rotifers, a large and ancient clade of obligate asexuals. Previous work has found no evidence for inefficient selection against deleterious mutations in protein-coding genes of bdelloids. However, these studies relied mostly on between-species comparisons and were therefore unable to detect mildly deleterious mutations that persist within populations but are removed by selection over longer time periods. Here, we test for inefficient purifying selection acting on the cytochrome oxidase I (cox1) mitochondrial gene in 3 clades of bdelloids. Patterns of variation are compared to those of two facultatively sexual clades: a monogonont rotifer (Brachionus) and a branchiopod crustacean (Daphnia). As predicted due to the strict linkage between mitochondrial and nuclear genomes, bdelloids exhibit higher frequencies of putatively deleterious amino acid polymorphism within populations than the two facultatively sexual clades. While the monophyly and age of bdelloids makes it hard to rule out other explanations for the observed differences, several possible confounding factors, such as differences in effective population size or patterns of codon usage, are shown not to explain the observed differences. We therefore conclude that bdelloid mitochondrial DNA variation does display the signature of inefficient selection expected of obligate asexuals.     

Evolutionary Dynamics of ‘the’ Bdelloid and Monogonont Rotifer Life-history Patterns

Substantial differences in both life-table characteristics and reproductive patterns distinguish bdelloid from monogonont rotifers. Bdelloids reproduce only asexually, whereas most monogononts are cyclical parthenogens. We explore some of the adaptive consequences of these life-history differences using a computer model to simulate the evolutionary acquisition of new beneficial mutations. A one-locus mutation-selection regime based on the life-history characteristics of bdelloids indicates that asexuals can maintain higher levels of both allelic and genotypic diversity over a longer time period than obligate sexuals. These results are produced by differences in the magnitude of random genetic drift (RGD) associated with the different types of reproduction. Cyclical parthenogens have significantly higher evolutionary rates than sexual forms in a single-locus model, but incorporate beneficial mutations more slowly than sexuals in a two-locus simulation. Our results are therefore strongly influenced by the number of loci being evaluated as well as the pattern of reproduction. The asexual life history was found to maintain higher levels of allelic diversity than any pattern including sexual reproduction. This intriguing finding is amplified as the number of loci undergoing selection is increased. We end by considering the adaptive consequences of the remarkably divergent life histories found in typical bdelloid and monogonont rotifers.