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Bdelloid rotifers are a class of microscopic invertebrates that have existed for millions of years apparently without sex or meiosis. They inhabit a variety of temporary and permanent freshwater habitats globally, and many species are remarkably tolerant of desiccation. Bdelloids offer an opportunity to better understand the evolution of sex and recombination, but previous work has emphasised desiccation as the cause of several unusual genomic features in this group. Here, we present high-quality whole-genome sequences of 3 bdelloid species: Rotaria macrura and R. magnacalcarata, which are both desiccation intolerant, and Adineta ricciae, which is desiccation tolerant. In combination with the published assembly of A. vaga, which is also desiccation tolerant, we apply a comparative genomics approach to evaluate the potential effects of desiccation tolerance and asexuality on genome evolution in bdelloids. We find that ancestral tetraploidy is conserved among all 4 bdelloid species, but homologous divergence in obligately aquatic Rotaria genomes is unexpectedly low. This finding is contrary to current models regarding the role of desiccation in shaping bdelloid genomes. In addition, we find that homologous regions in A. ricciae are largely collinear and do not form palindromic repeats as observed in the published A. vaga assembly. Consequently, several features interpreted as genomic evidence for long-term ameiotic evolution are not general to all bdelloid species, even within the same genus. Finally, we substantiate previous findings of high levels of horizontally transferred nonmetazoan genes in both desiccating and nondesiccating bdelloid species and show that this unusual feature is not shared by other animal phyla, even those with desiccation-tolerant representatives. These comparisons call into question the proposed role of desiccation in mediating horizontal genetic transfer.

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.

Exceptional resistance to dehydration allows species of bdelloid rotifers to tolerate desiccation stress in unpredictable habitats. The roles of exogenous and endogenous factors in resistance to short-term dehydration were investigated in Rotaria rotatoria to better understand this capacity among bdelloid rotifers. As the dehydration period was increased and the relative humidity (RH) was reduced, the proportion of rotifers active after the dehydration period decreased. The RH and dehydration period substantially influenced the recovery rate of R. rotatoria under rehydration for 1 h but had less effect under 12-h or 24-h rehydration treatment. The recovery rate was lower in rotifers subjected to a long dehydration period and short rehydration time, but improved by shortening the dehydration period and extending the rehydration treatment. Strain ZJ27 showed higher survivability than strain HX8, appropriate to their habitat near the sea in a region where drought and tropical rainstorms are frequent. Accordingly, species or strains originating from different climate regions and habitats will have experienced different frequencies of desiccation over their adaptive evolutionary history, resulting in varying levels of resistance among species. In addition, RH is shown to be a key factor in resistance to desiccation.

UV-B radiation is an increasing threat to aquatic organisms and also a potential driving force for zooplankton population dynamics. To explore the ecological effects of UVR on rotifers and the responses of antioxidant system against UVR, the acute lethal effects, the life history strategies, population growth, and antioxidant enzyme activities were assessed in the Bdelloid Rotaria rotatoria after exposure to UV-B radiation. The results indicated that the persistence of tolerance in rotifer to stress was playing a more vital role than the radiation dose in survival. The larger the culture volume, the weaker the lethal effect. Rotifers prolonged their first reproductive time and shortened their reproductive period 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 the 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.

Understanding the driving factors of the biogeography of microscopic animals is crucial for global biodiversity research. However, their geographical distribution patterns remain poorly resolved, with ongoing debate regarding the primary roles of climate, geography, and human activities. In this study, we investigated the genetic diversity and lineage distribution of two understudied bdelloid rotifer species complexes— Adineta beysunae Örstan, 2018 and Rotaria sordida (Western, 1893)—across 15 and 35 geographic regions (primarily in China and Europe), respectively. Phylogenetic analyses based on mitochondrial COI markers identified 10 distinct clades in A. beysunae and 58 in R. sordida , revealing high cryptic diversity. Analysis of Molecular Variance (AMOVA) indicated that genetic variation in both species occurred predominantly within populations ( A. beysunae : 75.26%; R. sordida : 75.55%) rather than among populations. These results demonstrate a heterogeneous phylogeographic structure characterized by widespread lineages and fragmented endemic clades (e.g., island-restricted lineages). Hierarchical partitioning analysis identified climatic factors—particularly seasonal temperature variation, annual precipitation, and mean wind speed—as the dominant drivers of phylogeographic patterns in Chinese populations, surpassing the effects of spatial variables, including longitude and elevation. Collectively, this study uncovers high cryptic diversity in A. beysunae and R. sordida , quantifies the relative contributions of key driving factors, and supplements existing knowledge of microfaunal biogeographic patterns, providing foundational data to elucidate global mechanisms of micro-metazoan diversification and adaptive evolution.

Bdelloids are commonly found in freshwaters or limno-terrestrial habitats. No formal attempt has yet been performed to define bdelloid niche and to identify whether species respond in a clear, consistent, and quantifiable fashion to environmental parameters. Therefore, we analysed the correlation between the occurrence of common Rotaria species in Thailand and environmental variables, including limnological, climatic, and biotic features. We followed two approaches to determine the niche of the investigated species: performing regression models for each species and reconstructing the niche spaces occupied by each species using n-dimensional hypervolumes. The effect of local-scale limnological and large-scale climatic variables was almost negligible at explaining the occurrence and distribution of Rotaria species. Surprisingly, primary productivity, known in temperate areas to be a major positive correlate of the occurrence of R. neptunia, appeared to have no effect on this species when measured as chlorophyll a, and a negative effect when measured as cyanobacterial productivity. Biotic variables revealed that different Rotaria species have a similar response to environmental variables. Two main messages are supported: (i) no clear environmental features unambiguously affected bdelloids species; (ii) features that are relevant for limnological processes at temperate latitudes may not be applicable for explaining processes in tropical latitudes.

Global warming has raised concerns regarding the potential impact on aquatic biosafety and health. To illuminate the adaptive mechanisms of bdelloid rotifers in response to global warming, the ecological and transcriptomic characteristics of two strains (HX and ZJ) of Rotaria rotatoria were investigated at 25°C and 35°C. Our results showed an obvious genetic divergence between the two geographic populations. Thermal stress significantly reduced the average lifespan of R. rotatoria in both strains, but increased the offspring production in the ZJ strain. Furthermore, the expression levels of genes Hsp70 were significantly upregulated in the HX strain, while GSTo1 and Cu/Zn‐SOD were on the contrary. In the ZJ strain, the expression levels of genes Hsp70, CAT2, and GSTo1 were upregulated under thermal stress. Conversely, a significant decrease in the expression level of the Mn‐SOD gene was observed in the ZJ strain under thermal stress. Transcriptomic profiling analysis revealed a total of 105 and 5288 differentially expressed genes (DEGs) in the HX and ZJ strains under thermal stress, respectively. The PCA results showed clear differences in gene expression pattern between HX and ZJ strains under thermal stress. Interestingly, compared to the HX strain, numerous downregulated DEGs in the ZJ strain were enriched into pathways related to metabolism under thermal stress, suggesting that rotifers from the ZJ strain prioritize resource allocation to reproduction by suppressing costly metabolic processes. This finding is consistent with the life table results. This study provides new insights into the adaptive evolution of aquatic animals in the context of global climate change. Under global warming, all organisms are inevitably exposed to temperature stress. To gain a comprehensive understanding of how bdelloid rotifers respond to thermal stress, we investigated both the ecological and molecular responses of stress‐tolerant Rotaria rotatoria, and explored whether rotifers with different geographic origins exhibit differential responses to thermal stress.

Asexuals are an important test case for theories of why species exist. If asexual clades displayed the same pattern of discrete variation as sexual clades, this would challenge the traditional view that sex is necessary for diversification into species. However, critical evidence has been lacking: all putative examples have involved organisms with recent or ongoing histories of recombination and have relied on visual interpretation of patterns of genetic and phenotypic variation rather than on formal tests of alternative evolutionary scenarios. Here we show that a classic asexual clade, the bdelloid rotifers, has diversified into distinct evolutionary species. Intensive sampling of the genus Rotaria reveals the presence of well-separated genetic clusters indicative of independent evolution. Moreover, combined genetic and morphological analyses reveal divergent selection in feeding morphology, indicative of niche divergence. Some of the morphologically coherent groups experiencing divergent selection contain several genetic clusters, in common with findings of cryptic species in sexual organisms. Our results show that the main causes of speciation in sexual organisms, population isolation and divergent selection, have the same qualitative effects in an asexual clade. The study also demonstrates how combined molecular and morphological analyses can shed new light on the evolutionary nature of species.

Background The Syndermata (Rotifera+Acanthocephala) is one of the best model systems for studying the evolutionary origins and persistence of different life styles because it contains a series of lineage-specific life histories: Monogononta (cyclic parthenogenetic and free-living), Bdelloidea (entirely parthenogenetic and mostly benthic dweller), Seisonidea (exclusively bisexual and epizoic or ectoparasitic), and Acanthocephala (sexual and obligatory endoparasitic). Providing phylogenetic resolution to the question of Eurotatoria (Monogononta and Bdelloidea) monophyly versus paraphyly is a key factor for better understanding the evolution of different life styles, yet this matter is not clearly resolved. In this study, we revisited this issue based on comparative analysis of complete mitochondrial genome information for major groups of the Syndermata. Results We determined the first complete mitochondrial genome sequences (15,319 bp) of a bdelloid rotifer, Rotaria rotatoria . In order to examine the validity of Eurotatoria (Monogononta and Bdelloidea) monophyly/paraphyly, we performed phylogenetic analysis of amino acid sequences for eleven protein-coding genes sampled from a wide variety of bilaterian representatives. The resulting mitochondrial genome trees, inferred using different algorithms, consistently failed to recover Monogononta and Bdelloidea as monophyletic, but instead identified them as a paraphyletic assemblage. Bdelloidea (as represented by R. rotatoria ) shares most common ancestry with Acanthocephala (as represented by L. thecatus ) rather than with monogonont B. plicatilis , the other representative of Eurotatoria. Conclusion Comparisons of inferred amino acid sequence and gene arrangement patterns with those of other metazoan mtDNAs (including those of acanthocephalan L. thecatus and monogonont B. plicatilis ) support the hypothesis that Bdelloidea shares most common ancestry with Acanthocephala rather than with Monogononta. From this finding, we suggest that the obligatory asexuality of bdelloideans may have secondarily derived from some other preexisting condition in earlier lineage of rotifers. Providing a more complete assessment of phylogenetic relationships and inferring patterns of evolution of different types of life styles among Syndermata awaits comparisons requiring mitochondrial genome sequencing of Seisonidea.

The hard articulated jaws of some pseudocoelomate metazoans were recently used in reconstructing their phylogenetic relationships, but we still do not know if these structures could change in size and shape during the life of individuals, and experimental data are lacking on their post-embryonic development. Rotifers are one of the groups in which hard articulated jaws, called trophi, are well known, and are widely used taxonomically. Here we report on SEM study of trophi of rotifers of different ages, to determine if the trophi structures change in shape and/or in size during post-embryonic development. We used linear measurements and geometric morphometrics analyses from scanning electron microscopic pictures of trophi of Cupelopagis vorax, Dicranophorus forcipatus, Macrotrachela quadricornifera, Notommata glyphura, Rotaria macrura, R. neptunoida, and R. tardigrada. Results for these species show that trophi do not change after hatching, either in size or in shape. In contrast, data on Asplanchna priodonta reveal trophi growth after hatching.[PUBLICATION ABSTRACT]