| Adineta ricciae | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Rotifera |
| Class: | Bdelloidea |
| Order: | Bdelloida |
| Family: | Adinetidae |
| Genus: | Adineta |
| Species: | A. ricciae |
| Binomial name | |
| Adineta ricciae Segers & Shiel, 2005 | |
Adineta ricciae is a species of freshwater rotifers in the family Adinetidae. It was first described in 2005 after being discovered by chance in dry mud beside a billabong in Australia. It is used as a model organism as it is easy to keep in culture.
Adineta ricciae is a microscopic invertebrate that can grow to a length of about 0.2 millimetres (0.008 in). It has a retractable head with two pigmented, front-facing eyes, a mouth aperture and a corona of cilia. The bands of cilia are used in locomotion with two separate groups twirling in opposite directions. The body is slightly broader than the head and is cylindrical and contains the internal organs. It has muscles, a large stomach and short intestine, a cloaca, an excretory system, nerve ganglia, organs sensitive to touch and a pair of ovaries. At the posterior end is a foot with foot glands, short triangular spurs and three toes. The whole is enclosed in a flexible, transparent cuticle. [1] [2]
This rotifer was first described in 2005 from an individual found in rehydrated dry mud taken from Ryan's Billabong, Victoria, Australia by Hendrik Segers and Russell J. Shiel. The researchers set this specimen aside in a dish as they were concentrating on other aspects of rotifer research. When they came to re-examine it several weeks later, they were surprised to discover that a large population had developed. It transpired that this rotifer was new to science and, because it was exceptionally easy to grow in culture, it has since been used as a model organism. [2]
Bdelloid rotifers are aquatic organisms and when their habitat dries up, they have the capability of going into a dormant state known as cryptobiosis to survive desiccation. They can remain in this state for periods of several years and when circumstances improve, they can revitalise in a few hours and continue with their normal activities. [3] No male bdelloid rotifers have ever been found and it is believed that all bdelloid rotifers reproduce solely by means of asexual reproduction through the process of parthenogenesis. Molecular studies have shown that all bdelloid rotifers are descended from a common ancestor which lost its ability to reproduce sexually about 80 million years ago. [4] Adineta ricciae has been used in this research. [4]
In general, creatures that reproduce asexually are less able than other animals to adapt to environmental changes because they do not experience the interchange of genes that enables the "survival of the fittest". Research has been undertaken to investigate why bdelloid rotifers are not affected in this way and can cope with enormous environmental variations. The researchers studied the genome of Adineta ricciae. It seems that some genes, in particular a gene called LEA, have two quite different functions and code for two proteins. One of these is an LEA protein, which has a protective function that prevents other proteins from agglutinating during the desiccation process; the other one helps to maintain the cell membrane. This gives bdelloid rotifers a means of becoming genetically diverse even though not exchanging genes through sexual reproduction and suggests that asexual reproduction could even be an evolutionary means of creating diversity. [4] [5]
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the full set of genes of their single parent. Asexual reproduction is the primary form of reproduction for single-celled organisms such as archaea and bacteria. Many eukaryotic organisms including plants, animals, and fungi can also reproduce asexually. In vertebrates, the most common form of asexual reproduction is parthenogenesis, which is typically used as an alternative to sexual reproduction in times when reproductive opportunities are limited.
Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: asexual and sexual.
An echinoderm is any member of the phylum Echinodermata of marine animals. The adults are recognizable by their radial symmetry, and include starfish, sea urchins, sand dollars, and sea cucumbers, as well as the sea lilies or "stone lilies". Adult echinoderms are found on the sea bed at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7000 living species, making it the second-largest grouping of deuterostomes, after the chordates. Echinoderms are the largest phylum that has no freshwater or terrestrial members.
The rotifers, commonly called wheel animals or wheel animalcules, make up a phylum of microscopic and near-microscopic pseudocoelomate animals.
Tetrahymena, a unicellular eukaryote, is a genus of free-living ciliates that can also switch from commensalistic to pathogenic modes of survival. They are common in freshwater ponds. Tetrahymena species used as model organisms in biomedical research are T. thermophila and T. pyriformis.
Bdelloidea is a class of rotifers found in freshwater habitats all over the world. There are over 450 described species of bdelloid rotifers, 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 (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 permafrost.
In evolutionary genetics, Muller's ratchet is a process through which, absence of recombination, an accumulation of irreversible deleterious mutations results. This happens due to the fact that in the absence of recombination, and assuming reverse mutations are rare, offspring bear at least as much mutational load as their parents. Muller proposed this mechanism as one reason why sexual reproduction may be favored over asexual reproduction, as sexual organisms benefit from recombination and consequent elimination of deleterious mutations. The negative effect of accumulating irreversible deleterious mutations may not be prevalent in organisms which, while they reproduce asexually, also undergo other forms of recombination. This effect has also been observed in those regions of the genomes of sexual organisms that do not undergo recombination.
A planarian is one of many flatworms of the traditional class Turbellaria. It usually describes free-living flatworms of the order Tricladida (triclads), although this common name is also used for a wide number of free-living platyhelminthes. Planaria are common to many parts of the world, living in both saltwater and freshwater ponds and rivers. Some species are terrestrial and are found under logs, in or on the soil, and on plants in humid areas.
Sexual reproduction is an adaptive feature which is common to almost all multi-cellular organisms with many being incapable of reproducing asexually. Prior to the advent of sexual reproduction, the adaptation process whereby genes would change from one generation to the next happened very slowly and randomly. Sex evolved as an extremely efficient mechanism for producing variation, and this had the major advantage of enabling organisms to adapt to changing environments. Sex did, however, come with a cost. In reproducing asexually, no time nor energy needs to be expended in choosing a mate. And if the environment has not changed, then there may be little reason for variation, as the organism may already be well adapted. Sex, however, has evolved as the most prolific means of species branching into the tree of life. Diversification into the phylogenetic tree happens much more rapidly via sexual reproduction than it does by way of asexual reproduction.
Trichoplax adhaerens is one of the three named species in the phylum Placozoa. The others are Hoilungia hongkongensis and Polyplacotoma mediterranea. The Placozoa is a basal group of multicellular animals (metazoa). Trichoplax are very flat organisms around a millimetre in diameter, lacking any organs or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated "cover cells" flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess cilia used in locomotion, and gland cells that lack cilia. Between these layers is the fibre syncytium, a liquid-filled cavity strutted open by star-like fibres.
Matthew Stanley Meselson is a geneticist and molecular biologist currently at Harvard University, known for his demonstration, with Franklin Stahl, of semi-conservative DNA replication. After completing his Ph.D under Linus Pauling at the California Institute of Technology, Meselson became a Professor at Harvard University in 1960, where he has remained, today, as Thomas Dudley Cabot Professor of the Natural Sciences.
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, preventing reproduction, development, and repair. When environmental conditions return to being hospitable, the organism will return to its metabolic state of life as it was prior to the cryptobiosis.
The starlet sea anemone is a species of small sea anemone in the family Edwardsiidae native to the east coast of the United States, with introduced populations along the coast of southeast England and the west coast of the United States. Populations have also been located in Nova Scotia, Canada. This sea anemone is found in the shallow brackish water of coastal lagoons and salt marshes where its slender column is usually buried in the mud and its tentacles exposed. Its genome has been sequenced and it is cultivated in the laboratory as a model organism, but the IUCN has listed it as being a "Vulnerable species" in the wild.
Parthenogenesis is a natural form of asexual reproduction in which growth and development of embryos occur without fertilization by sperm. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell. In plants parthenogenesis is a component process of apomixis.
The Red Queen hypothesis, also referred to as Red Queen's, the Red Queen effect, the Red Queen model, Red Queen's race, and Red Queen dynamics, is a hypothesis in evolutionary biology which proposes that species must constantly adapt, evolve, and proliferate in order to survive while pitted against ever-evolving opposing species. The hypothesis was intended to explain the constant (age-independent) extinction probability as observed in the paleontological record caused by co-evolution between competing species; however, it has also been suggested that the Red Queen hypothesis explains the advantage of sexual reproduction at the level of individuals, and the positive correlation between speciation and extinction rates in most higher taxa.
Brachionus is a genus of planktonic rotifers occurring in freshwater, alkaline and brackish water.
The "Vicar of Bray" hypothesis attempts to explain why sexual reproduction might have advantages over asexual reproduction. Reproduction is the process by which organisms give rise to offspring. Asexual reproduction involves a single parent and results in offspring that are genetically identical to each other and to the parent. In contrast to asexual reproduction, sexual reproduction involves two parents. Both the parents produce gametes through meiosis, a special type of cell division that reduces the chromosome number by half. During an early stage of meiosis, before the chromosomes are separated in the two daughter cells, the chromosomes undergo genetic recombination. This allows them to exchange some of their genetic information. Therefore, the gametes from a single organism are all genetically different from each other. The process in which the two gametes from the two parents unite is called fertilization. Half of the genetic information from both parents is combined. This results in offspring that are genetically different from each other and from the parents. In short, sexual reproduction allows a continuous rearrangement of genes. Therefore, the offspring of a population of sexually reproducing individuals will show a more varied selection of phenotypes. Due to faster attainment of favorable genetic combinations, sexually reproducing populations evolve more rapidly in response to environmental changes. Under the Vicar of Bray hypothesis, sex benefits a population as a whole, but not individuals within it, making it a case of group selection.
Late embryogenesis abundant proteins are proteins in plants, and some bacteria and invertebrates that protect against protein aggregation due to desiccation or osmotic stresses associated with low temperature. LEA proteins were initially discovered accumulating late in embryogenesis of cotton seeds. Although abundant in seeds and pollens, LEA proteins have been found to protect against desiccation, cold, or high salinity in a variety of organisms, including the bacterium Deinococcus radiodurans, nematode Caenorhabditis elegans, Artemia, and rotifers.
In biology, Gonozooids are any of the reproductive individuals of Tunicates, Bryozoan, or Hydrozoan colonies that produce gametes. Gonozooids may play a role in labour division or in alternation of generations. A gonozooid typically has hardly any other function than reproduction, amounting to little more than a motile gonad.
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