Bdelloidea (from

Greek Greek may refer to: 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 of all kno ...

βδέλλα, ''bdella'' 'leech') is a

class Class, Classes, 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 d ...

rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...

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. The term excludes seawater and brackish water, but it does include non-salty mi ...

habitats all over the world. There are over 450 described

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 Dormancy is a period in an organism's Biological life cycle, life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolism, metabolic activity and therefore helps an organism to conserv ...

( 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 A magnifying glass is a convex lens—usually mounted in a frame with a handle—that is used to produce a magnification, magnified image of an object. A magnifying glass can also be used to focus light, such as to concentrate the Sun's radiati ...

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

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

permafrost Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...

Evolutionary relationships

The

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

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 Sessility (zoology), sessile forms. Monogononts generally have a reduced Rotifer#Anatomy, corona, and e ...

and Seisonidea. Prior to 1990,

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

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 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 Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

), because it omits a fourth

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

of closely related organisms: the

Acanthocephala Acanthocephala ( Greek , ' 'thorn' + , ' 'head') is a group 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 t ...

, 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 biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

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

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 ...

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

A 2016

analysis of the gene order in the mitochondria suggests that Seisonidea and Acanthocephala are sister clades and that the Eurotatoria are the sister clade to this group, producing the cladogram below. Bdelloidea is a

of the

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

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

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 A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification and ...

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), oesophagus (British English), or œsophagus ( archaic spelling) ( see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), colloquially known also as the food pipe, food tube, or gullet, ...

*Strong teeth (labelled by a tooth index) *Many

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

*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 a multicellular organism, 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 a ...

. In certain

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

, (''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 φαγε ...

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 parasitism, parasitic or Predation, predatory worms that comprise the Class (biology), subclass Hirudinea within the phylum Annelida. They are closely related to the Oligochaeta, oligochaetes, which include the earthwor ...

es, which gives the group their name (

''βδέλλα'' or ''bdella'', meaning leech).

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 Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

, 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 development of seed or embryo without fertilization. However, other definitions include replacement of the seed by a plantlet or replacement of the flower by bulbils. Apomictically produced offspring are geneti ...

seen in some plants. Each individual has paired

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

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. ''Adineta vaga'' is capable of carrying out

DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...

by a nonreductional

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

Germline In biology and genetics, the germline is the population of a multicellular organism's cells that develop into germ cells. In other words, they are the cells that form gametes ( eggs and sperm), which can come together to form a zygote. They dif ...

DNA repair occurs in a specific period of

oogenesis Oogenesis () or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated before birth during embryonic devel ...

during which

homologous chromosome Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...

s take on a meiotic-like juxtaposed configuration. This germline DNA repair results in accurate reconstitution of the genetic material transmitted to offspring.

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 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 in extremophilic organisms in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all measurable metabolic processes sto ...

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 capability to produc ...

. 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's 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 intende ...

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 a ...

, 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 organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). HGT is an important factor in the e ...

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. When they desiccate completely, their DNA breaks up into many pieces. And when they come back to life after being rehydrated, it creates an opportunity for alien DNA fragments to enter their genome. This process was improved 60 million years ago when they captured a bacterial gene this way, which gave them a new gene regulatory system. The new system was used to keep

transposons A transposable element (TE), also transposon, or jumping gene, is a type of mobile genetic element, a nucleic acid sequence in DNA that can change its position within a genome. The discovery of mobile genetic elements earned Barbara McClinto ...

in check. Bdelloid rotifers are extraordinarily resistant to damage from

ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

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