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Rhysenn
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Endosymbiosis in progress?
« on: 2005-05-13 11:57:06 » |
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Undesirable Sex Partners: Bacteria manipulate reproduction of insects and other species
http://www.sciencenews.org/pages/sn_arch/11_16_96/bob1.htm
For many insect species and other arthropods, the truth can be as strange as fiction when bacteria known as Wolbachia are around.
These microorganisms populate cells in the testes and ovaries of arthropods, often profoundly altering the reproduction of their hosts. In some species, infected males can generate offspring only if they mate with infected females. In others, infected females give birth without the need for the opposite sex. In one arthropod species, Wolbachia even transform embryos that would normally be males into females.
"These traits have all evolved because they increase the transmission of the microorganisms," says John H. Werren of the University of Rochester (N.Y.), who has documented the diversity of animals infected by Wolbachia.
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Cytoplasmic incompatibility, the researchers found, occurs when males infected with Wolbachia mate with uninfected females. In such unions, no offspring, or just a few in some host species, result. This reproductive barrier can be eliminated with antibiotics that rid the mosquitoes of the bacteria.
Why does Wolbachia generate cytoplasmic incompatibility? To favor reproduction by infected females, says O'Neill. That helps the bacteria, which dwell in the cytoplasm of egg cells, pass on to future generations.
In species affected by cytoplasmic incompatibility, infected females have no trouble reproducing with infected males. Infected females also breed easily with uninfected males. Both kinds of unions transfer Wolbachia to offspring. Consequently, cytoplasmic incompatibility can spread Wolbachia rapidly through an uninfected population, says O'Neill, who organized a session on Wolbachia at the recent Symbiosis 96! Meeting in Bar Harbor, Maine.
Researchers are finding that Wolbachia infects a surprisingly large variety of species. Werren and Donald Windsor of the Smithsonian Tropical Research Institute in Panama reported last year that 16 percent of Panamanian insect species, including some in all of the major insect orders, harbor Wolbachia. Since the estimated number of insect species ranges from 10 million to 30 million, that means roughly 2 million to 5 million insect species play host to the bacteria.
Wasps are among the favored hosts of Wolbachia. Take the jewel wasp, Werren's favorite research subject. Wolbachia infections in these insects produce an odd variation on cytoplasmic incompatibility: Uninfected female wasps mating with infected males can produce offspring, but their progeny are all male.
An explanation rests in the fact that wasps, like bees and ants, have an unusual mechanism for determining sex. In wasps, eggs fertilized by sperm contain a maternal and a paternal set of chromosomes and develop into females. Unfertilized eggs, with only a maternal chromosome set, develop into males.
When an infected male jewel wasp mates with an uninfected female, the paternal chromosomes from the sperm seem to fragment and fail to join the maternal set, says Werren. Consequently, only males result from such a mating. This indirectly aids the spread of Wolbachia by reducing the number of uninfected daughters produced by uninfected females, explains Werren.
Wolbachia sometimes takes a more feminist approach. In many parasitic wasps, which lay their eggs in developing insects that they have killed, Wolbachia infections eliminate the need for males. An infected female reproduces via an asexual process known as parthenogenesis. The unfertilized eggs simply duplicate their one set of chromosomes and develop into females.
These parthenogenetic wasps had long been a biological curiosity until a few years ago, when Richard Stouthamer, working with Werren, showed that the phenomenon stemmed from Wolbachia infection. With antibiotics, "you can cure a line of its parthenogenesis and make it sexual," says Stouthamer, now at the Wageningen Agricultural University in the Netherlands.
The clear preference for females isn't limited to Wolbachia strains that infect wasps. At the Bar Harbor meeting, Thierry Rigaud of the University of Poitiers in France, reported finding the bacteria in the wood louse Armadillidium vulgare.
In these lice, Wolbachia frequently overrides genetic inheritance. The bacteria, says Rigaud, "feminize" an embryonic wood louse that is genetically male by disrupting the production or effects of masculinizing hormones during its development. The increased number of daughters allows Wolbachia to spread quickly.
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The most provocative question surrounding Wolbachia may be whether the bacteria have played a role in the development of new species.
A central concept in many theories of speciation is reproductive isolation. This idea holds that if two populations of a species cannot breed together, then the genes of each population will evolve independently and diverge (SN: 11/2/96, p. 284). "Reproductive isolation is a key component of speciation because without it, genomes would mix and you can't get divergence," says Werren.
Eventually, he explains, the genes of two populations would diverge so much that they become genetically incompatible for reproduction. At that point, most evolutionary biologists would argue, the single original species has given way to two species.
Wolbachia may serve as an excellent mechanism to engender reproductive isolation, argues Werren. He and other researchers have found that such isolation can arise in an insect species infected by different Wolbachia strains. Members of the species infected by one strain cannot reproduce with members infected by the other strain.
While theories about evolution are notoriously difficult to prove, Werren suggests that mapping the diversity of insect species infected and not infected by Wolbachia may bolster his theory. Species infected with Wolbachia should have many more closely related species than uninfected species do.
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my commentary:
Wolbachia also infect some species of filarial parasitic nematodes, and in these species the worms cannot reproduce without the Wolbachia bacteria. They are found only to live in the lateral cords of the worms. It's unclear why Wolbachia is required for reproduction, but it seems like endosymbiosis in progress.
A primary example of endosymbiosis is the mitochondrion. The mitochondrion, the organelle necessary for cell aerobic respiration, is thought to have originated as a bacterium which was then taken up by some species of protist. Eventually it ceased to be a separate organism and is now present and essential in all animal, plant, and at least some fungal and protist species. The evidence for this comes from the fact that the mitochondrion has a double membrane rather than a single membrane and it has its own set of DNA.
But its interesting to think that the discovery of this bacterium allows scientists to view evolution and endosymbiosis in progress.
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rhinoceros
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Re:Endosymbiosis in progress?
« Reply #1 on: 2005-06-28 08:21:44 » |
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Very interesting. In the recent "World Summit on Evolution" in Galapagos, some speakers took the extra step from endosymbiosis to symbiogenesis:
http://www.skeptic.com/eskeptic/eskeptic05-06-24.html
<snip>
A.F.W. Schimper noted that chloroplasts in plant cells very much resembled cyanobacteria, but the ultimate theoretical model was provided by Lynn Margulis: the key step was the endosymbiosis of cyanobacteria within a phagotrophic eukaryotic host, a process she calls symbiogenesis. In primary endosymbiosis, 1,000 genes were acquired by the nucleus from the incorporated cyanobacteria. In secondary endosymbiosis, there was a second round of gene transfer in which the eukaryote cell engulfs another plastid-containing eukaryote.
<snip>
(Lynn Margulis) : “I think we are missing important information about the origins of variation. I differ from the neo-Darwinian bullies on this point.” She then outlined the basis of her theory of the origin of the cell nucleus as a fusion between archaebacteria (thermoplasma) and Eubacteria (Spirochaeta). “We live on a bacterial planet,” she reflected.
“The cell is the fundamental unit of life. A minimal cell has DNA, mRNA, tRNA, rRNA, amino acylating enzymes, polymerases, sources of energy and electrons, lipoprotein membranes, and ion channels, all contained within a cell wall, and is an autopoietic (self-regulating feedback) system.”
The biggest break in life, she explained, was between the prokaryotes (cells with nucleoids: monera, prokaryota; archaebacteria, eubacteria) and eukaryotes (cells with nuclei: protoctista, fungi, plantae, animalia).
In this framework, Margulis continued, all of life’s history can be divided into three major eons: Archean (3,500–2,500 million years ago), Proterozoic (2,500–540 Ma), and Phanerozoic (540–0 Ma). “Most evolutionary biologists deal with the Phanerozoic, which is like saying that history began in 1909 when the Ford Motor Company opened shop in Dearborn, MI,” Margulis quipped. The major steps in evolution involved symbiogenesis, which Margulis described succinctly as “the inheritance of acquired genomes” and more formally in its relationship to symbiosis, “the long-term physical association between members of different types (species).” The problem with neo-Darwinism, Margulis concluded, is that
“Random changes in DNA alone do not lead to speciation. Symbiogenesis — the appearance of new behaviors, tissues, organs, organ systems, physiologies, or species as a result of symbiont interaction — is the major source of evolutionary novelty in eukaryotes — animals, plants, and fungi.”
There were no direct challenges to Margulis in the discussion period that followed, so I once again queried a number of the experts in this area after the lecture. The overall impression I received was that Margulis goes too far in her rejection of neo-Darwinism, but because she was right about the role of symbiogenesis in the origin of the first eukaryote cells, they are taking a wait-and-see approach.
<snip>
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