Bdelloidea ( Greek ''βδέλλα'', ''bdella'', "leech") is a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

rotifer 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 ...

s found in

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

habitats all over the world. There are over 450 described species of bdelloid rotifers (or 'bdelloids'), distinguished from each other mainly on the basis of morphology. 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 Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps an organism to conserve energy. Dormancy tends to be clo ...

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

Siberian Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive region, geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a ...

permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...

Evolutionary relationships

The

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Rotifera 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, w ...

and

Seisonidea Seisonidae is a family of rotifers, found on the gills of ''Nebalia'', a marine crustacean. Peculiar among rotifers, males and females are both present and equal in size. Males and females are similar with paired gonads. It is considered to have ...

. Prior to 1990, phylogenetic studies based on morphology seemed to indicate that the sister group to bdelloid rotifers was Monogononta, with seisonid rotifers as an early-diverging

outgroup Outgroup may refer to: * Outgroup (cladistics), an evolutionary-history concept * Outgroup (sociology), a social group {{disambig ...

. Modern

molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

studies demonstrate that this classic understanding of 'Rotifera' is incomplete (

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

), because it omits a fourth

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of closely related organisms: the Acanthocephala, or thorny-headed worms. Originally classified as a separate

, molecular and morphological evidence accumulated between 1994 and 2014 to indicate that Acanthocephala forms a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

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 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 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 Outgroup may refer to: * Outgroup (cladistics), an evolutionary-history concept * Outgroup (sociology), a social group {{disambig ...

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

of the

Rotifera, consisting of three orders: Philodinavida, Philodinida and Adinetida. These orders are divided into four families and about 450 species. Since these organisms are asexual the usual definition of a species as a group of organisms capable of creating fertile offspring is inapplicable, therefore the species concept 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 *Strong teeth (labelled by a tooth index) *Many

cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...

*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 certain

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

, (''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 Feeding is the process by which organisms, typically animals, obtain food. Terminology often uses either the suffixes -vore, -vory, or -vorous from Latin ''vorare'', meaning "to devour", or -phage, -phagy, or -phagous from Greek φαγ ...

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

leech Leeches are segmented parasitic or predatory worms that comprise the subclass Hirudinea within the phylum Annelida. They are closely related to the oligochaetes, which include the earthworm, and like them have soft, muscular segmented bodie ...

es, which gives the group their name ( Greek ''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 because a male has never been observed, and females reproduce exclusively by parthenogenesis, a form of asexual reproduction where embryos grow and develop without the need for fertilization; this is akin to the

apomixis In botany, apomixis is asexual reproduction without fertilization. Its etymology is Greek for "away from" + "mixing". This definition notably does not mention meiosis. Thus "normal asexual reproduction" of plants, such as propagation from cuttin ...

seen in some plants. Each individual has paired

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...

s. 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 parthenogenesis 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 of the gene ''op.''

Stress-induced behaviour

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

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. 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 of co-evolution, 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 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 Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...

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