Sophisticated Symbiosis

Specialized giant clam cells distribute photosynthetically productive light to symbiotic microalgae in an energy-efficient manner.

ByTracy Vence|October 8, 2014

Iridescent cells in the mantle tissue of giant clams spread light of a wavelength that drives photosynthesis to microalgae that provide nutrition for the animals, the University of Pennsylvania’s Alison Sweeney and colleagues reported inJournal of the Royal Society Interfacethis month (October 1). These so-called iridocytes not only distribute photosynthetically productive light to the algae, they also reflect nonproductive light, the researchers showed. “At incident light levels found on shallow coral reefs, this arrangement may allow algae within the clam system to both efficiently use all incident solar energy and avoid the photodamage and efficiency losses,” the researchers wrote in their paper.

“What makes this system in the clam special is that the design can extract every last photon from sunlight,” Sweeney toldNew Scientist.

“While earlier work speculated on the role of these iridescent cells, this paper clearly shows how clams use iridocytes to control and redistribute the light that reaches their algal symbionts,” added Ryan Kerney of Gettysburg College in Pennsylvania who was not involved in the work.

In their paper, the researchers likened the symbiotic system to an electric transformer, “which changes energy flux per area in a system while conserving total energy.” Given this parallel, the authors proposed that the clam system might inspire the development of more efficient and resilient photovoltaic materials.

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The Telltale Tail

A symbiotic relationship between squid and bacteria provides an alternative explanation for bacterial sheathed flagella.

ByRina Shaikh-Lesko|May 1, 2014

EDITOR'S CHOICE IN MICROBIOLOGY

The paperC. Brennan et al., “A model symbiosis reveals a role for sheathed-flagellum rotation in the release of immunogenic lipopolysaccharide,”eLife, doi:10.7554/eLife01579, 2014.The backgroundThe bioluminescent bacteriumVibrio fischeri,whose flagellum is encased in a membrane-derived sheath, colonizes newborn Hawaiian bobtail squid (Euprymnascolopes), contributing a light source that the squid use for camouflage. Researchers have assumed that the sheath prevented the host’s immune system from reacting to proteins in the flagellum. But the sheath itself sheds an immunostimulatory protein called lipopolysaccharide (LPS), a feature that has puzzled scientists.The findingTo better understand the role of the sheath, Edward Ruby of the University of Wisconsin–Madison and colleagues colonized squid with mutant bacteria that had either no flagellum or one that wouldn’t spin. They found that flagellar rotation releases LPS and is a crucial trigger for normal development of the squid’s light-emitting organ. Bacteria with nonrotating flagella elicited as little response from the squid as bacteria without any flagellum at all. “Even though [the bacteria are] sitting right next to the [appropriate] tissue, they’re not able to cause the host to pull the trigger” on forming a proper light organ, says Ruby.The partnershipThe authors suspect that in the squid-vibrio symbiosis, the host immune response, rather than being defensive, may take part in alerting the squid to the presence of the beneficial bacteria.The impactThe finding “reveals a previously unknown role of flagellar function and a previously unappreciated role of flagella-mediated LPS release,” says developmental biologist John Rawls of Duke University. Still, he notes, other questions remain, such as the sheath’s advantage for other bacteria and its broader biological function.

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Anadaptationis amutation, orgeneticchange, that helps an organism, such as a plant or animal, survive in itsenvironment. Due to the helpful nature of the mutation, it is passed down from onegenerationto the next. As more and more organismsinheritthe mutation, the mutation becomes a typical part of thespecies. The mutation has become an adaptation.
Structural and Behavioral Adaptations
An adaptation can be structural, meaning it is a physical part of the organism. An adaptation can also be behavioral, affecting the way an organism acts.
An example of astructural adaptationis the way some plants have adapted to life in thedesert. Deserts are dry, hot places. Plants calledsucculents have adapted to thisclimateby storing water in their thick stems and leaves.
Animalmigrationis an example of abehavioral adaptation. Grey whalesmigratethousands of miles every year as they swim from the cold Arctic Ocean to the warm waters off thecoastof Mexico. Grey whale calves are born in the warm water, and then travel in groups called pods to thenutrient-rich waters of theArctic.
Some adaptations are calledexaptation. An exaptation is an adaptation developed for one purpose, but used for another. Feathers were probably adaptations for keeping the animal warm that were later used for flight, making feathers an exaptation for flying.
Some adaptations, on the other hand, become useless. These adaptations arevestigial: remaining but functionless. Whales and dolphins have vestigial leg bones, the remains of an adaptation (legs) that their ancestors used to walk.
Habitat

Adaptations usually develop in response to a change in the organism’shabitat.
A famous example of an animal adapting to a change in its environment is the English peppered moth. Prior to the 19th century, the most common type of thismoth was cream-colored with darker spots. Few peppered moths displayed a mutation of being grey or black.

As theIndustrial Revolutionchanged the environment, the appearance of the peppered moth changed. The darker-colored moths, which were rare, began tothrive in theurbanatmosphere. Theirsooty color blended in with the trees stained byindustrialpollution. Birds couldn’t see the dark moths, so they ate the cream-colored moths instead. The cream-colored moths began to make a comeback after the United Kingdom passed laws that limitedair pollution.
Speciation
Sometimes, an organism develops an adaptation or set of adaptations that create an entirely new species. This process is known asspeciation.
The physicalisolationor specialization of a species can lead to speciation.
The wide variety ofmarsupials in Oceania is an example of how organisms adapt to an isolated habitat. Marsupials,mammals that carry theiryoungin pouches, arrived in Oceania before the land split with Asia. Placental mammals, animals that carry their young in the mother’swomb, came todominateevery other continent, but not Oceania. There, marsupials faced no competition.
Koalas, for instance, adapted to feed oneucalyptus trees, which are native to Australia. Theextinct Tasmanian tiger was acarnivorousmarsupial and adapted to thenichefilled bybig cats like tigers on other continents. Marsupials in Oceania are an example of adaptive radiation, a type of speciation in which species develop to fill a variety of empty ecological niches.
Thecichlidfish found in Africa’s Lake Malawi exhibit another type of speciation,sympatric speciation. Sympatric speciation is the opposite of physical isolation. It happens when species share the same habitat. Adaptations have allowed hundreds of varieties of cichlids to live in Lake Malawi. Each species of cichlid has aunique, specializeddiet: One type of cichlid may eat only insects, another may eat onlyalgae, another may feed only on other fish.
Coadaptation
Organisms sometimes adapt to and with other organisms. This is calledcoadaptation. Certain flowers have adapted theirpollento appeal to the hummingbirds nutritional needs.Hummingbirds have adapted long, thin beaks toextractthe pollen from certain flowers. In this relationship, the hummingbird gets food, while the plants pollen isdistributed. The coadaptation is beneficial to both organisms.
Mimicry is another type of coadaptation. With mimicry, one organism has adapted toresembleanother. The harmless king snake (sometimes called a milk snake) has adapted a color pattern that resembles the deadly coral snake. This mimicry keeps predators away from the king snake.
The mimic octopus has behavioral as well as structural adaptations. This species of octopus can mimic the look and movements of animals such as sea stars, crabs, jellyfish, and shrimp.
Coadaptation can also limit an organisms ability to adapt to new changes in their habitat. This can lead toco-extinction. In Southern England, the large blue butterfly adapted to eat red ants. When humandevelopment reduced the red ants habitat, the local extinction of the red ant led to the local extinction of the large blue butterfly.

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Self-Improvement Through the Ages

A 50,000-generation-long experiment shows that bacteria keep getting fitter.

ByKerry Grens|February 1, 2014

ANDRZEJ KRAUZEIn 1988, when evolutionary biologist Richard Lenski was an assistant professor at the University of California, Irvine, he started a simple experiment: tossE. coliinto a new environment and watch what happens. He wanted to know how reproducible evolution would be, so he put the same strain of the bacteria into 12 flasks with the same simple medium and waited to see how they would evolve.E. colinormally lives in the guts of animals, so the experiment would allow for a way to observe adaptations to a new environment.

After about a year and 2,000E. coligenerations, Lenski and his colleagues published the first results of what they then considered to be a long-term experiment in evolution. Little did they know that 25 years and 50,000 generations later, the experiment would still be chugging along—those 12 flasks representing alternate universes of bacterial existence. “I guess I didn’t view it as [being as] open-ended as it clearly has become, not only as an experiment but in terms of the ability of the organisms to keep improving,” says Lenski.

These experiments provide us with clear evidence that adaptive evolution truly is relentless.—John Thompson,
University of California, Santa Cruz

In his latest publication on the experiment, Lenski reported that the bacteria continually become more fit. His team pitted bacteria from various evolutionary time points (from each flask, a sample is frozen every 500 generations) against one another to see which would grow better when combined in the same container. “I like to think of this project as time travel because we’re comparing organisms that lived at different points in the past, resurrecting them, and comparing them head to head,” says Lenski.

Lenski and his collaborators can distinguish the competitor populations from different flasks because of color-coded genetic markers. For instance, they would pit a sample taken from one flask of red bacteria at 50,000 generations against an ancestral sample from another flask housing white bacteria. To make sure the resurrected bacteria weren’t at a dis-advantage, they would give the organisms time to acclimate after being thawed. Lenski’s team found that bacteria that had evolved for a greater length of time—those from later generations—appeared more fit than those resurrected from earlier generations; fitness never peaked (Science, 342:1364-67, 2013). Their data suggest that, at least in this situation, evolutionary fitness is ever increasing.

Rees Kassen of the University of Ottawa says the most interesting finding is that most adaptations happened early on in the experiment. “That means [initially] there are lots of opportunities for [the bacteria] to get better,” he says. “Even though beneficial mutations are still very rare events, there are still different ways they could get better, and they also are likely to improve fitness by a large amount.”

The results came as a surprise to Lenski, who expected fitness to plateau. It’s not the first time his bacterial cells have proven unpredictable, such as when they began to utilize a new food source. In 2008, one of the strains evolved to metabolize citrate, which is ordinarily just a buffer in the medium. “It was a quantum leap in the evolution of this species, and it was totally unexpected,” says Tadeusz Kawecki of the University of Lausanne.

John Thompson, an evolutionary biologist at the University of California, Santa Cruz, says that the results show there are many adaptive solutions, even in a simple environment. “It is, then, no wonder that life has evolved to be so diverse,” Thompson writes in an e-mail. “That does not mean, though, that all populations in nature will always continually evolve increases in adaptation.” In cases where the environment is changing rapidly, for instance, slow increases in fitness will not be able to continue.

The finding contradicts the “naive” view that an organism will cease getting fitter once it’s well adapted to an environment, says Kassen. Without Lenski’s experiment, there wouldn’t be much empirical data to show that. “The fact of the matter is, it’s the only experiment we can test,” he says. “No other experiments have gone on as long.”

Although there are other experimental evolution projects, and Lenski’s was not the first, it is arguably the most important. “The great thing about these bacteria—and I have a great envy—is that [the researchers] can freeze the samples at any time and revive them so they can compete the evolved strain with the ancestors,” says Kawecki, who studies evolution inDrosophila.

The long-term evolution project is extremely simple, but it isn’t without careful planning and a huge effort on the part of Lenski’s lab. Over the years, three lab technicians in succession have served as stewards of the flasks, having to coordinate care over weekends, holidays, and even a move from California to Lenski’s current post at Michigan State University in Lansing. Now in his late 50s, Lenski is thinking about ways to continue the experiment. After all, there’s no telling what the bacteria might come up with in the future. As Thompson says, “These experiments provide us with clear evidence that adaptive evolution truly is relentless.”

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