Genomic Signature of Sexual Reproduction in the Bdelloid Rotifer Macrotrachella quadricornifera

Bdelloid rotifers, common freshwater invertebrates of ancient origin and worldwide distribution have long been thought to be entirely asexual, being the principal exception to the view that in eukaryotes the loss of sex leads to early extinction. That bdelloids are facultatively sexual is shown by a study of allele sharing within a group of closely related bdelloids of the species Macrotrachella quadricornifera, supporting the view that sexual reproduction is essential for long-term success in all eukaryotes.

An extraordinary new fluvial bdelloid rotifer, Coronistomus impossibilis gen. nov., sp. nov., with adaptations for turbulent flow (Rotifera: Bdelloidea: Coronistomidae fam. nov.)

A new bdelloid rotifer, Coronistomus impossibilis gen. nov., sp. nov. is described from the Hawlings River in Maryland, USA. The new species is a non-swimming sediment dweller that exceeds 550 µm in body length. Its corona is a weakly bilobed ciliated field on the ventral side of the head with lateral borders continuing into mouth. The species has ramate trophi that have on each half three widely spaced major teeth with two interproximal teeth. The foot lacks spurs and ends with a pair of long, thick ventral toes and a medially positioned caudal appendage. The new species lives in microhabitats exposed to turbulent flow and its morphological idiosyncrasies are interpreted as adaptations to reduce the likelihood of getting dislodged by the water. Although the corona morphology and the caudal appendage of the new species are similar to the corresponding traits of some of the species in the family Philodinavidae, the structure of its nonprotrusible trophi and its possession of only two toes (as opposed to four) separate it from all other members of the Philodinavidae. Therefore, a new family, Coronistomidae fam. nov., is erected for the new genus Coronistomus.

The influence of environmental variables on bdelloid rotifers of the genus Rotaria in Thailand

This research investigates the influence of environmental parameters that are known to affect bdelloid rotifer species richness and composition in temperate zones on the genus Rotaria in tropical zone. Our study analysed species richness and composition of the genus Rotaria from 390 samples collected from several types of aquatic habitats in Thailand. Coordinates, elevation, limnological parameters such as water temperature, conductivity, total dissolved solids, salinity, dissolved oxygen and pH were measured. A total of nine species was recorded. Of these, one species, Rotaria macrura (Ehrenberg, 1832), was a new record for Thailand and new to the oriental region, and was a yet undescribed species, Rotaria sp. The species diversity of this genus increased from eight to 10 species. The presence or absence of the genus Rotaria was significantly influenced by dissolved oxygen and habitat type. For the samples where the genus occurred, species richness was not affected by any of the limnological or bioclimatic variables. Differences in species composition were affected only by habitat type. The results support former suggestions that common abiotic parameters do not seem to strongly influence diversity in bdelloids, whereas major ecological differences between habitats influence bdelloid occurrence.

Genomic signatures of recombination in a natural population of the bdelloid rotifer Adineta vaga

Sexual reproduction is almost ubiquitous among extant eukaryotes. As most asexual lineages are short-lived, abandoning sex is commonly regarded as an evolutionary dead end. Still, putative anciently asexual lineages challenge this view. One of the most striking examples are bdelloid rotifers, microscopic freshwater invertebrates believed to have completely abandoned sexual reproduction tens of Myr ago. Here, we compare whole genomes of 11 wild-caught individuals of the bdelloid rotifer Adineta vaga and present evidence that some patterns in its genetic variation are incompatible with strict clonality and lack of genetic exchange. These patterns include genotype proportions close to Hardy-Weinberg expectations within loci, lack of linkage disequilibrium between distant loci, incongruent haplotype phylogenies across the genome, and evidence for hybridization between divergent lineages. Analysis of triallelic sites independently corroborates these findings. Our results provide evidence for interindividual genetic exchange and recombination in A. vaga, a species previously thought to be anciently asexual.

Responses of the Ecological Characteristics and Antioxidant Enzyme Activities in Rotaria Rotatoria to UV-B Radiation

Background: UVB radiation is an increasing threat to aquatic organisms, and also a potential driving force for zooplankton population dynamics. The bdelloid rotifer, for its parthenogenesis and high resistance to stress, can be used as an excellent model in ecology and evolutionary biology. Given the strong tolerance of bdelloid rotifer to stressors, we hypothesized that it could hold the ability to tolerate significant UV radiation, and this capacity may be associated with the trade-off strategies among life historic components. With this study, to estimate the ecological effects and responses of antioxidant system against UV radiation, the acute lethal, life history strategies, population growth, and antioxidant enzyme activities were assessed in the bdelloid Rotaria rotatoria after exposure to UV-B radiation. Results: The persistence of tolerance (evaluating by LT50) in rotifer to stress was playing a more vital role than the radiation dose (LD50) in survival, with a significant negative correlation between the radiation intensity and LT50. The larger the culture volume, the weaker the lethal effect. Rotifers prolonged their first reproductive time (JP), and shortened their reproductive period (RP) and longevity with the increasing of radiation dose, and the fecundity was significant inhibited by UV-B radiation. These responses can be taken as energy trade-off to retard their mortality. The population density of rotifers increased at the lowest dose of radiation, and then descended with the increasing of UVR dose, and this pattern was also corroborated by detecting the content of SOD and CAT, which suggested that hormesis also applies to R. rotatoria under UV-B radiation stress. The enzyme SOD has higher level of content and more sensitive to low UVR than CAT.Conclusions: The lethal effects of UV-B radiation to organisms depends not only on the conditions they exposed to, like as intensity and time of exposure, but also combination of factors namely, the type and effectiveness of the strategy they employ to cope with the changed environment. The species specificity, ages and physiological status also could be the regulating factors under the stresses, also in this case there was the strategy of energy trade-off between reproduction and survival in rotifers.

Weak swimming response of a bdelloid rotifer to chemical cues of a native copepod predator

Whether bdelloid rotifers can detect and respond to the chemical cues of predators has not been studied, and research on their behaviour is scarce in general. To test for such response, we observed the behaviour of Philodina megalotrocha rotifers in either control or copepod-conditioned water. We then introduced a mechanical disturbance to test if predator odour sensitises rotifers to subsequent cues. We found intensified swimming in P. megalotrocha exposed to Macrocyclops fuscus-conditioned water, i.e. the larger and more predatory of the tested copepods. Such response may be adaptive, depending on the predominant hunting mode of the predator in the particular habitat.