Bdelloid
Encyclopedia
Bdelloidea is a class of rotifer
s found in fresh water
and moist soil. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head. They may move by swimming or crawling. The latter commonly involves taking alternate steps with the head and tail, as do certain leech
es, which gives the group their name (Greek
βδελλα or bdella, meaning leech).
Bdelloids have been of interest to those interested in the evolutionary role of sexual reproduction
, because it has disappeared entirely from the group: males are not present within the species, and females reproduce exclusively by parthenogenesis
. Each individual has paired gonad
s. Despite the fact that they have been asexual for millions of years, they have diversified into more than 300 species and are fairly similar to other sexually-reproducting rotifer species.
Bdelloids respond to environmental stresses by entering a state of dormancy known as anhydrobiosis. This dormancy form enables the organism to rapidly dehydrate itself. The Bdelloid will remain in this cysted
state until optimal environmental conditions re-occur at which point they will rehydrate and become active within hours. Diapause
is the ability of the organism to produce offspring in a dormant and unhatched state. 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.
When these unusual creatures spring from hibernation, they undergo a fascinating and possibly unique genetic process. A study conducted by Matthew Meselson
of Harvard University
suggests that when bdelloid rotifers recover from suspended animation, they incorporate foreign DNA when patching up their own ruptured cell membranes. Any DNA in proximity to the organisms can be included in the new genome, including semi-digested food. This may be interpreted as an intermediate between true asexual and sexual reproduction.
Bdelloid rotifers have recently been shown to be extraordinarily resistant to damage from ionizing radiation. The same DNA-preserving adaptations used to survive dormancy are thought to work in this case, and may have also helped the organisms to thrive despite their totally asexual mode of reproduction.
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s found in fresh water
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...
and moist soil. Bdelloids typically have a well-developed corona, divided into two parts, on a retractable head. They may move by swimming or crawling. The latter commonly involves taking alternate steps with the head and tail, as do certain leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
es, which gives the group their name (Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
βδελλα or bdella, meaning leech).
Bdelloids have been of interest to those interested in the evolutionary role of sexual reproduction
Evolution of sex
The evolution of sexual reproduction is currently described by several competing scientific hypotheses. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi...
, because it has disappeared entirely from the group: males are not present within the species, and females reproduce exclusively by parthenogenesis
Parthenogenesis
Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...
. Each individual has paired gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s. Despite the fact that they have been asexual for millions of years, they have diversified into more than 300 species and are fairly similar to other sexually-reproducting rotifer species.
Bdelloids respond to environmental stresses by entering a state of dormancy known as anhydrobiosis. This dormancy form enables the organism to rapidly dehydrate itself. The Bdelloid will remain in this cysted
Microbial cyst
A microbial cyst is a resting or dormant stage of a microorganism, usually a bacterium or a protist, that helps the organism to survive in unfavorable environmental conditions. It can be thought of as a state of suspended animation in which the metabolic processes of the cell are slowed down and...
state until optimal environmental conditions re-occur at which point they will rehydrate and become active within hours. Diapause
Diapause
Diapause is the delay in development in response to regularly and recurring periods of adverse environmental conditions. It is considered to be a physiological state of dormancy with very specific initiating and inhibiting conditions...
is the ability of the organism to produce offspring in a dormant and unhatched state. Hatching of the young will only occur when conditions are at their most favourable. These forms of dormancy are also known as cryptobiosis
Cryptobiosis
Cryptobiosis is an ametabolic 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 metabolic procedures stop, preventing reproduction, development, and repair...
or quiescence.
When these unusual creatures spring from hibernation, they undergo a fascinating and possibly unique genetic process. A study conducted by Matthew Meselson
Matthew Meselson
Matthew Stanley Meselson is an American geneticist and molecular biologist whose research was important in showing how DNA replicates, recombines and is repaired in cells. In his mature years, he has been an active chemical and biological weapons activist and consultant...
of Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
suggests that when bdelloid rotifers recover from suspended animation, they incorporate foreign DNA when patching up their own ruptured cell membranes. Any DNA in proximity to the organisms can be included in the new genome, including semi-digested food. This may be interpreted as an intermediate between true asexual and sexual reproduction.
Bdelloid rotifers have recently been shown to be extraordinarily resistant to damage from ionizing radiation. The same DNA-preserving adaptations used to survive dormancy are thought to work in this case, and may have also helped the organisms to thrive despite their totally asexual mode of reproduction.
External links
- The Weird Sisters
- Bdelloids: No sex for over 40 million years
- Eighty million years without sex
- The benefits of 80 million years without sex
- An Evolutionary Scandal, from Harvard Magazine
- Who Needs Sex (or Males) Anyway?
- Organisms Gave up Sexual Reproduction for DNA Theft - Bdelloids able to incorporate foreign DNA