Sex (As We Know It) Works Thanks to Ever-Evolving Host-Parasite Relationships, Biologists Find

Sex (As We Know It) Works Thanks to Ever-Evolving Host-Parasite Relationships, Biologists Find

  1:55:00 PM    Science Lover

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It seems we may have parasites to thank for the existence of sex as we know it. Indiana University biologists have found that, although sexual reproduction between two individuals is costly from an evolutionary perspective, it is favored over self-fertilization in the presence of coevolving parasites. Sex allows parents to produce offspring that are more resistant to the parasites, while self-fertilization dooms populations to extinction at the hands of their biological enemies.

The relationship between the roundworm Caenorhabditis 
elegans and the pathogenic bacteria Serratia marcescens, 
pictured here together in a Petri dish, is helping scientists 
understand why sexual reproduction occurs as it does. 
(Credit: Image courtesy of Indiana University)

The July 8 report in Science, "Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex," affirms the Red Queen hypothesis, an evolutionary theory who's name comes from Lewis Carroll's Alice in Wonderland text: "It takes all the running you can do, to keep in the same place." The idea is that sexual reproduction via cross-fertilization keeps host populations one evolutionary step ahead of the parasites, which are coevolving to infect them. It is within this coevolutionary context that both hosts and parasites are running (evolving) as fast as they can just to stay in the same place.

"The widespread existence of sex has been a major problem for evolutionary biology since the time of Charles Darwin," said lead author Levi T. Morran. Sex does not make evolutionary sense, because it often involves the production of males. This is very inefficient, because males don't directly produce any offspring. Self-fertilization is a far more efficient means of reproduction, and as such, evolutionary theory predicts that self-fertilization should be widespread in nature and sex should be rare. However, as we all know, this is not the case.

The Red Queen Hypothesis provides one possible explanation for the existence of sex.

"The Red Queen Hypothesis predicts that sex should allow hosts to evade infection from their parasites, whereas self-fertilization may increase the risk of infection," said co-author Curtis M. Lively.

By combining the DNA of two parents, sex allows parents to produce offspring that are genetically diverse and different from their parents. Parasites that have adapted to infect one generation may have difficulty infecting the next generation. However, offspring produced through self-fertilization inherit the DNA of their single parent, thus any parasites adapted to infect the parent should also be capable of infecting the offspring.

Morran, a post-doctoral researcher, and Lively, a distinguished professor of biology, both in the IU Bloomington College of Arts and Science's Department of Biology, authored the report with biology undergraduates Olivia G. Schmidt, Ian A. Gelarden and Raymond C. Parrish II.



The team used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis. They genetically manipulated the mating system of C. elegans, causing populations to mate either sexually, by self-fertilization, or a mixture of both within the same population. Then they exposed those populations to the S. marcescens parasite. The parasites were either allowed to coevolve with C. elegans or were prevented from evolving. The researchers then determined which mating system gave populations an evolutionary advantage.

"We found that the self-fertilizing populations of C. elegans were rapidly driven extinct by the coevolving parasites, a result consistent with the Red Queen Hypothesis," Morran said. On the other hand, sex allowed populations to keep pace with their parasites. "Sex helped populations adapt to their coevolving parasites, allowing parents to produce offspring that were resistant to infection and ultimately avoid extinction," he noted.

In host populations where either sex or self-fertilization were possible, the evolutionary state of the parasite determined the most effective reproductive strategy. When the parasite did not coevolve, self-fertilization evolved as the dominant form of host reproduction. However, when the parasite was allowed to coevolve with the hosts, then sex became the favored reproductive strategy.

"Coevolution with the pathogen not only favored sex over self-fertilization, but also allowed sex to be maintained throughout the experiment," Morran said.

These results are consistent with the Red Queen Hypothesis and may go a long way toward explaining the widespread existence of sex.

"Coevolving parasites seem to be very common in nature," said Lively. "The experiment shows that coevolution with parasites, but not the presence of parasites per se, selects for higher levels of outcrossing. Thus the coevolutionary struggle between hosts and their parasites could explain the existence of males."

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LCD Projector Used to Control Brain and Muscles of Tiny Organisms Such as Worms

LCD Projector Used to Control Brain and Muscles of Tiny Organisms Such as Worms

  10:22:00 AM    Science Lover

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Researchers are using inexpensive components from ordinary liquid crystal display (LCD) projectors to control the brain and muscles of tiny organisms, including freely moving worms. Red, green and blue lights from a projector activate light-sensitive microbial proteins that are genetically engineered into the worms, allowing the researchers to switch neurons on and off like light bulbs and turn muscles on and off like engines.

Hang Lu, an associate professor in the School of Chemical 
& Biomolecular Engineering at Georgia Tech, and her 
graduate students Jeffrey Stirman (left) and Matthew Crane 
are using inexpensive LCD projectors to control the brain 
and muscles of tiny organisms, including freely moving 
worms. (Credit: Georgia Tech/Gary Meek)

Use of the LCD technology to control small animals advances the field of optogenetics -- a mix of optical and genetic techniques that has given researchers unparalleled control over brain circuits in laboratory animals. Until now, the technique could be used only with larger animals by placement of an optical fiber into an animal's brain, or required illumination of an animal's entire body.

A paper published Jan. 9 in the advance online edition of the journal Nature Methods describes how the inexpensive illumination technology allows researchers to stimulate and silence specific neurons and muscles of freely moving worms, while precisely controlling the location, duration, frequency and intensity of the light.

"This illumination instrument significantly enhances our ability to control, alter, observe and investigate how neurons, muscles and circuits ultimately produce behavior in animals," said Hang Lu, an associate professor in the School of Chemical & Biomolecular Engineering at the Georgia Institute of Technology.

Lu and graduate students Jeffrey Stirman and Matthew Crane developed the tool with support from the National Institutes of Health and the Alfred P. Sloan Foundation.

The illumination system includes a modified off-the-shelf LCD projector, which is used to cast a multi-color pattern of light onto an animal. The independent red, green and blue channels allow researchers to activate excitable cells sensitive to specific colors, while simultaneously silencing others.

"Because the central component of the illumination system is a commercially available projector, the system's cost and complexity are dramatically reduced, which we hope will enable wider adoption of this tool by the research community," explained Lu.

By connecting the illumination system to a microscope and combining it with video tracking, the researchers are able to track and record the behavior of freely moving animals, while maintaining the lighting in the intended anatomical position. When the animal moves, changes to the light's location, intensity and color can be updated in less than 40 milliseconds.

Once Lu and her team built the prototype system, they used it to explore the "touch" circuit of the worm Caenorhabditis elegans by exciting and inhibiting its mechano-sensory and locomotion neurons. Alexander Gottschalk, a professor in the Johann Wolfgang Goethe-University Frankfurt Institute of Biochemistry in Frankfurt, Germany, and his team provided the light-sensitive optogenetic reagents for the Georgia Tech experiments.

For their first experiment, the researchers illuminated the head of a worm at regular intervals while the animal moved forward. This produced a coiling effect in the head and caused the worm to crawl in a triangular pattern. In another experiment, the team scanned light along the bodies of worms from head to tail, which resulted in backward movement when neurons near the head were stimulated and forward movement when neurons near the tail were stimulated.

Additional experiments showed that the intensity of the light affected a worm's behavior and that several optogenetic reagents excited at different wavelengths could be combined in one experiment to understand circuit functions. The researchers were able to examine a large number of animals under a variety of conditions, demonstrating that the technique's results were both robust and repeatable.

"This instrument allowed us to control defined events in defined locations at defined times in an intact biological system, allowing us to dissect animal functional circuits with greater precision and nuance," added Lu.

While these proof-of-concept studies investigated the response of C. elegans to mechanical stimulation, the illumination system can also be used to evaluate responses to chemical, thermal and visual stimuli. Researchers can also use it to study a variety of neurons and muscles in other small animals, such as the zebrafish and fruit fly larvae.

"Experiments with this illumination system yield quantitative behavior data that cannot be obtained by manual touch assays, laser cell ablation, or genetic manipulation of neurotransmitters," said Lu.

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