Thursday, July 24, 2008

1 missing gene leads to fruitless mating rituals

Male fruit flies missing a gene for one particular odor receptor become clueless in matters of love, scientists at Duke University Medical Center have discovered.

Because they lack the ability to read important chemical cues, these flies will indiscriminately attempt to have sex with other males, and with females who have already mated. The signals they're missing are pheromones wafting from mated females and male flies. The work appears online in Nature Neuroscience.

The researchers found that the signals from this pheromone receptor are so important to the flies that they are wired directly into the higher-order processing center of the fly's brain, which governs behavior. This direct connection surprised the scientists, who have studied other fruit fly courtship genes.

"It goes against the dogma that was established for the olfactory and taste systems," said Hubert Amrein, Ph.D., of the Duke Department of Molecular Genetics and Microbiology. "Our finding implies that signals from the outside don't have to go through processing stations in the chemosensory system before being connected to the higher-order brain structures."

Males without a gene called Gr32a, the gustatory receptor gene, showed normal levels of courtship with virgin females. But in competition with normal (or wild-type) male fruit flies, they were outperformed by 4 to 1. In fact, the Gr32a-lacking flies courted the male competitors in addition to the females.

To further investigate the role of the gene, researchers used decapitated, passive flies of both genders, because these do not provide any behavioral feedback that could confound the precise measurement of the sex appeal they held for the male flies being studied. Both types of males courted the decapitated virgin females equally. However, courtship attempts toward decapitated males increased only in the males lacking the Gr32a gene, and these flies attempted copulation, behavior not seen in the wild-type males.

The scientists also found that the males lacking the Gr32a gene courted females who had already mated. Wild-type males, however, were significantly less attracted by mated females, because mated females have received male pheromones during the first mating.

The hapless Gr32a-negative males tried to mate with virgin females even when they had been covered with male pheromones, behavior that the wild-type flies avoided.

"This gene was very powerful for distinguishing between genders and for determining mating status," said co-author Tetsuya Miyamoto, Ph.D., also of the Department of Molecular Genetics and Microbiology. "Male pheromone is so effective that Gr32a mutants court males with almost the same intensity as they do females."

The GR32a gene is not found in humans। "In general, the development of pheromones in human sexual behavior is not as clear-cut as one would hope," Amrein said. "We know that males and females have preferences for certain olfactory cues. The mouse has an olfactory organ, and humans have a remnant of this in the nose, but it doesn't function in people. So I think it is very difficult to make any direct connections between these gene findings in fruit flies and what happens in people."

Source: Duke University Medical Center

Study shows parasites outweigh predators

In a study of free-living and parasitic species in three estuaries on the Pacific coast of California and Baja California, a team of researchers from the University of California, Santa Barbara, the United States Geological Survey, and Princeton University has determined that parasite biomass in those habitats exceeds that of top predators, in some cases by a factor of 20. Their findings, which could have significant biomedical and ecological implications, appear in the July 24 issue of the science journal Nature.

According to Armand Kuris, a professor of zoology in UCSB's Department of Ecology, Evolution, and Marine Biology and a lead author of the paper, the study's findings have a potential impact on the perceived role of parasites in an ecosystem. From an ecological perspective, parasites serve both as regulators to prevent species from becoming numerically dominant and as indicators of the health of a particular ecosystem. The study shows for the first time that parasites might drive the flow of energy in ecosystems.

"The total amount of energy flow in ecosystems due to infectious processes must be enormous - even greater than we'd expect given the large parasite biomass," Kuris said. "I expect the amount of energy going into host tissue repair and replenishment is also huge. An implication of our study is that we should pay more attention to the energetics of disease."

Biomass is the amount of living matter that exists in a given habitat. It is expressed either as the weight of organisms per unit area or as the volume of organisms per unit volume of habitat. Until now, scientists have believed that because parasites are microscopic in size they comprise a small fraction of biomass in a habitat while free-living organisms such as fish, birds, and other predators comprise the vast majority.

The researchers quantified the biomass of free-living and parasitic species in the three estuaries and demonstrated that parasites have substantial biomass in these ecosystems. "Parasites have as much, or even more, biomass than other important groups of animals - like birds, fishes, and crabs," said Ryan Hechinger, a researcher at UCSB's Marine Science Institute and co-lead author of the paper.

The article grew out of a five-year study supported by a $2.2 million grant from the National Science Foundation and the National Institutes of Health through the agencies' joint Ecology of Infectious Diseases Program. In addition to Kuris, principal investigators include Kevin Lafferty, a marine ecologist with the United States Geological Survey; and Andrew Dobson, a professor of ecology and evolutionary biology at Princeton University. Other important collaborators included Leopoldina Aguirre-Macedo, of the Centro de Investigación y Estudios Avanzados Unidad Mérida, and Mark Torchin, a staff scientist with the Smithsonian Tropical Research Institute.

The researchers quantified parasites and free-living organisms in the Carpinteria Salt Marsh and in the Bahia San Quintín and Estero de Punta Banda estuaries in Baja California. Their study included 199 species of free-living animals, 15 species of free-living vascular plants, and 138 species of parasites.

"The reason we wanted to complete this study is because a lot of work we've done has suggested that parasites are important in ecosystems. But no one's actually looked at them as a group throughout an ecosystem," said Lafferty. "Also, no one's considered parasites from the perspective of how much they weigh because it's always been assumed they weigh almost nothing. Now we know that's not true.

"For example, in an estuary there are more kilograms of trematode worms - parasites - than kilograms of birds," he noted. "If you could see the trematodes with binoculars, you might not bother bird watching."

Said Hechinger: "No one debates whether it's important for ensuring human welfare to understand how ecosystems work. How can we possibly understand something without accounting for its major parts? Because our findings indicate that parasites control a massive amount of biomass, it would seem future research can't ignore them."

According to Kuris, understanding the enormity of parasite biomass and the burden it places on available hosts could lead to new strategies in the management of infectious diseases. Treatment protocols might put greater emphasis on enhancing the host's ability to defend itself against parasitic disease and slow the rate of energy uptake by the parasites and pathogens.

Source : University of California - Santa Barbara