Bdelloidea (

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

''βδέλλα'', ''bdella'', "leech") is a class of rotifers found in freshwater habitats all over the world. There are over 450 described

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly on the basis of

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

. The main characteristics that distinguish bdelloids from related groups of rotifers are exclusively parthenogenetic reproduction and the ability to survive in dry, harsh environments by entering a state of desiccation-induced dormancy (

anhydrobiosis Cryptobiosis or anabiosis is a metabolic state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all measurable metabolic processes sto ...

) at any life stage. They are often referred to as "ancient asexuals" due to their unique asexual history that spans back to over 25 million years ago through fossil evidence. Bdelloid rotifers are microscopic organisms, typically between 150 and 700 µm in length. Most are slightly too small to be seen with the naked eye, but appear as tiny white dots through even a weak

hand lens A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to crea ...

, especially in bright light. In June 2021, biologists reported the restoration of bdelloid rotifers after being frozen for 24,000 years in the Siberian permafrost.

Evolutionary relationships

The phylum

Rotifera The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

traditionally included three classes: Bdelloidea,

Monogononta Monogononta is a class of rotifers, found mostly in freshwater but also in soil and marine environments. They include both free-swimming and sessile forms. Monogononts generally have a reduced corona, and each individual has a single gonad, ...

and Seisonidea. Prior to 1990,

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

studies based on morphology seemed to indicate that the sister group to bdelloid rotifers was Monogononta, with seisonid rotifers as an early-diverging outgroup. Modern molecular phylogenetic studies demonstrate that this classic understanding of 'Rotifera' is incomplete ( paraphyletic), because it omits a fourth clade of closely related organisms: the

Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to p ...

, or thorny-headed worms. Originally classified as a separate phylum, molecular and morphological evidence accumulated between 1994 and 2014 to indicate that Acanthocephala forms a monophyletic group with Bdelloidea, Monogononta and Seisonidea. To accommodate this finding, some authors extend the term 'Rotifera' to include the highly modified, parasitic 'acanthocephalan rotifers' alongside bdelloid, monogonont and seisonid rotifers. Others refer to the grouping of the four taxa as Syndermata, a term derived from their shared

syncytial A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...

epidermis. The position of Bdelloidea within Syndermata (or Rotifera) is not entirely clear. Alternative possible phylogenetic relationships within the clade are illustrated by the accompanying cladograms. As of 2014, the "most comprehensive phylogenomic analysis of syndermatan relationships" to date was based on

transcriptome The transcriptome is the set of all RNA transcripts, including coding and non-coding, in an individual or a population of cells. The term can also sometimes be used to refer to all RNAs, or just mRNA, depending on the particular experiment. The t ...

data from all four groups, and provided "strong support" for the hypothesis illustrated in the bottom left of the figure, in which Seisonidea and Acanthocephala are sister taxa. The study further indicated that the sister group to this taxon is Bdelloidea, whereas Monogononta is the outgroup to all three. This would mean that the closest living relatives of bdelloid rotifers are not monogonont rotifers, as previously believed, but seisonid rotifers and acanthocephalans, despite their highly modified morphology.

Classification and identification

Bdelloidea is a class of the phylum

, consisting of three orders: Philodinavida, Philodinida and Adinetida. These orders are divided into four

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

and about 450

. Since these organisms are asexual the usual definition of a

as a group of organisms capable of creating fertile offspring is inapplicable, therefore the

species concept The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for se ...

in these organisms is based on a mixture of morphological and molecular data instead. DNA studies suggest that the diversity is much greater than the original morphological classifications suggest. Bdelloids can only be identified by eye while they are alive because many of the characteristics significant to classification are related to feeding and crawling; however, genetic identification of bdelloids is possible on dead individuals. Once preserved, the individuals contract into "blobs" which restricts analysis. There are currently three morphological identification methodologies, two of which are considered dated: Bartoš (1951) and Donner (1965). The third method is a diagnostic key developed in 1995 by Shiel.

Morphology

There are three main regions of the body of bdelloids: head, trunk and foot. The adjacent image depicts each area to show how body parts can be very different although they are named the same depending on the species involved. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head. Some identifiable features of the bdelloids include : *Well-developed foot glands *A mouth opening with a long

oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to th ...

*Strong teeth (labelled by a tooth index) *Many cilia *Species-specific upper lip shape *Order-specific corona type **Philodinida consist of two ciliated discs **Adinetida consist of a ventral ciliated field **Philodinavida have a small corona The bdelloid digestive and reproductive systems can be found within the trunk sections of their bodies, with the stomach being the most visible of the

organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

. In certain genera, (''Habrotrocha'', ''Otostephanos'' and ''Scepanotrocha'') the bdelloid can actually be identified by the appearance of distinct spherical pellets within the stomach, which will be released as faeces. These pellets are a distinguishing characteristic since all the other genera release faeces as loose material. Most bdelloids retract the foot while they eat, but there are four genera that lack a foot: ''Adineta'', ''Bradyscela'', ''Henoceros'' and ''Philodinavus''. This affects not only how they feed but also how they crawl; for instance ''Adineta'' and ''Bradyscela'' slide whereas the other genera loop.

Behaviour

The behaviour of bdelloids can be split into four categories: feeding, locomotion, reproduction and stress-induced behaviours.

Feeding

The specific feeding behaviour of bdelloids is varied but most use rings of cilia in the corona organ to create currents of water which blow food through the mouth to the mastax organ which has been adapted specifically for grinding food. Food includes suspended bacteria, algae, detritus, and other things.

Locomotion

There appear to be three main methods of movement: free swimming, inch-worming along a substrate, or sessility. Inch-worming (or crawling) involves taking alternate steps with the head and tail, as do certain leeches, which gives the group their name (

''βδέλλα'' or ''bdella'', meaning leech). This video demonstrates how bdelloids move in three different situations
locomotion and feeding of bdelloid rotifers

Reproduction

Bdelloids are of interest in the study of the

evolution of sex Sexual reproduction is an adaptive feature which is common to almost all multicellular organisms and various unicellular organisms, with some organisms being incapable of asexual reproduction. Currently the adaptive advantage of sexual repro ...

because a male has never been observed, and females reproduce exclusively by

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

, a form of asexual reproduction where embryos grow and develop without the need for fertilization; this is akin to the apomixis seen in some plants. Each individual has paired gonads. Despite having been asexual for millions of years, they have diversified into more than 450 species and are fairly similar to other sexually reproducing rotifer species. However, a new study provided evidence for interindividual genetic exchange and recombination in ''Adineta vaga'', a species previously thought to be anciently asexual.

Evolution of obligate parthenogenetic reproduction

In 2003, the mode of asexual reproduction in the bdelloid rotifers was wholly unknown. One theory of how obligate

arose in bdelloid rotifers was that parthenogenic lineages lost the ability to respond to sex-inducing signal, which is why these lineages retained their asexuality. The obligate parthenogenetic strains of bdelloid rotifers produce a sex-inducing signal but have lost the ability to respond to that signal. It was later discovered that the inability to respond to sex-inducing signals in obligate parthenogens was caused by simple

Mendelian inheritance Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularize ...

of the gene ''op.''

Stress-induced behaviour

Bdelloids are able to survive environmental stresses by entering a state of dormancy known as

which enables the organism to rapidly dehydrate and thus resist desiccation. While preparing for this dormant state many metabolic processes are adjusted to equate for the change in state; e.g. the production of protective chemicals. The bdelloid can remain in this state, which is known as a 'xerosome' until the return of a sufficient amount of water, at which point they will rehydrate and become active within hours. Hatching of the young will only occur when conditions are at their most favourable. These forms of dormancy are also known as cryptobiosis or quiescence. Bdelloids have been known to survive in this state for up to 9 years while waiting for favourable conditions to return. In addition to surviving desiccation through anhydrobiosis, desiccation stress on two bdelloid species actually helped to maintain fitness and even improved their species

fecundity Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to pr ...

. The rotifers that were consistently kept hydrated fared worse than those desiccated and rehydrated. Bdelloidea have evolved a unique mechanism to help overcome one of the major perils of asexual reproduction. According to the

Red Queen hypothesis The Red Queen hypothesis is a hypothesis in evolutionary biology proposed in 1973, that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended ...

co-evolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...

, obligate asexuals will be driven extinct by rapidly changing parasites and pathogens, because they cannot change their genotypes quickly enough to keep up in this never-ending race. In populations of bdelloid rotifers, however, many parasites are destroyed during periods of extended desiccation. Moreover, desiccated bdelloid rotifers are easily blown away from parasite-infested habitats by wind, and establish new, healthy populations elsewhere, which allows them to escape the Red Queen by moving in time and space instead of using sex to change their genotype. When these creatures recover from desiccation, it has been shown that they undergo a potentially unique genetic process where

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...

occurs, resulting in a significant proportion of the bdelloid genome, up to 10%, having been obtained through horizontal gene transfer from bacteria, fungi and plants. How and why horizontal gene transfer occur in bdelloids is under much debate at present; particularly with regards to possible connections between the foreign genes and the desiccation process as well as possible connections to bdelloids' ancient asexuality. Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. This repair mechanism was studied in two Bdelloidea species, ''Adineta vaga'', and ''Philodina roseola''. and appears to involve mitotic recombination between homologous DNA regions within each species.

Horizontal gene transfer

Large-scale horizontal transfer of bacterial, plant and fungal genes into ''bdelloid rotifers'' has been documented, and may represent an important factor in bdelloid evolution.

Gallery

File:Bdelloidea1 w.jpg, Lateral view of a bdelloid. File:Bdelloidea2 w.jpg, Frontal view of a bdelloid's corona. File:Bdelloid rotifer (2009 04 11).jpg, Lateral view of a bdelloid. File:Bdelloid Rotifer.jpg, Lateral view of a bdelloid. File:Bdelloid Rotifier - Danelle Vivier.jpg, Lateral view of a bdelloid in algae-rich water File:Philodina sp. (Classː Bdelloidea) - Rotifera.jpg, Specimen of the genus ''Philodina''

References

External links

Introduction to rotifers

Rotifer World Catalog, by Jersabek C.D. & Leitner M.F.

The Weird Sisters

Bdelloids: No sex for over 40 million years

Who Needs Sex (or Males) Anyway?

Tiny Creature Comes Back To Life After 24,000 Years In Siberian Deep Freeze
{{Taxonbar, from=Q430301 Eurotatoria Radiodurants Protostome classes