Research News

Evolutionary twist: A parasite’s far-reaching effects


Research by John Jaenike, professor of biology, suggests that a prolific parasite is helping shape the destiny of a species of fruit flies (picture above) it does not even infect. It’s the first documented evidence of a convoluted evolutionary pressure that may play a role in splitting a species.

University scientists recently reported that a prolific parasite is helping shape the destiny of a species it does not even infect. The complex relationship between the parasite, its host, and the unconnected species is the first known example of evolutionary pressure from such a remote source, and appears in the October 10 issue of PLoS: Biology.

Five years ago, University scientists linked the bacterial parasite, Wolbachia, to the separation of a single wasp species into two distinct species. Now, researchers have found that this same parasite in fruit flies is not only meddling with the sexual behavior of its host, but may be causing a change in the sexual behavior of a species that is not infected.

“Darwin’s model of evolution is based on genetic variation that causes differences in survival and reproduction,” says John Jaenike, professor of biology. “However, this apparently simple scheme can operate in very complex and indirect ways.” Jaenike found that two very closely related species of fly differed in that one was thoroughly infected with Wolbachia while the other was Wolbachia-free. Each species resided in coniferous forests on opposite sides of North America, and, knowing that Wolbachia can affect reproductive isolation between species, he wondered whether the two species actually met each other in nature. A stretch of forest in Canada physically connected the eastern and western communities, so he set out to discover whether these two species’ territories overlapped.

In June of 2002, Jaenike sent two under-graduate students on a summer-long camping trip across the northern United States and Canada. Armed only with nets, store-bought mushrooms, and Fed Ex boxes, Chad Cornish and Paul Gibas camped at designated spots, set out the mushrooms, collected the flies that gathered on them, and mailed them back to Rochester.

As the boxes came in, Jaenike was gratified to find that as Cornish and Gibas worked their way through Canada, a region of geographic overlap between the two species emerged. But he wasn’t expecting what came next.

The mating behavior of females of the uninfected species in overlap zone had changed dramatically. The flies were put into large glass boxes where hundreds of both species could mingle and mate. The males of both species mated with females from both species, and the females from the infected species mated with both kinds of males. But the females from the uninfected species refused to mate with any but their own kind.

And most importantly, these females had become so picky about their mates that they were beginning to shun some males of their own species.

“In that area in Canada where the two species overlap, the uninfected females’ mating practices have changed dramatically,” says Jaenike. “When we mixed males from the west coast with the females from the same species in central Canada, the females would often refuse to mate. This could be a first step on the road to the uninfected species splitting into two new, individual species.”

In 2000, a colleague of Jaenike’s, Professor of Biology Jack Werren, showed that Wolbachia’s species-splitting effect on its host came before any other evolutionary splitting pressure. Jaenike and his colleagues have now presented evidence that this same parasite might push a completely uninfected species down the same road—the first time such a convoluted evolutionary pressure has been documented.

Race for neuroAIDS treatment

Experimental drugs are showing promise against neuroAIDS, the nerve damage caused by HIV infection that lessens many patients’ ability to think and move. As evidence of the progress, researchers at the Medical Center in October received a $7 million grant to confirm that two new drug classes can protect the brain from HIV-related nerve damage. Driving their approach is the realization that antiviral drugs that work against AIDS do not cure neuroAIDS.

As many as 900,000 Americans are infected with the human immunodeficiency virus (HIV), which attacks immune cells and leaves patients vulnerable to infection. Before the arrival of modern antiviral therapies in 1996, HIV also had a devastating effect on the brain known as HIV-associated dementia (HAD) or neuroAIDs. The current standard combination of treatments has extended the lives of most AIDS patients in the United States but has not cured neuroAIDS, despite early reports to the contrary. Antiviral combinations only slow the onset of HIV-related nerve damage that is becoming more common the longer HIV patients live.

The realization that anti-viral drugs do not cure HAD led researchers to ask whether there is something else about HIV besides its attack on immune cells that causes disease in the brain. The emerging answer is that the indirect effects of infection, like proteins released by the virus and chemicals released by human cells reacting to them, are toxic in themselves.

“The number of HIV patients that suffer brain damage is usually estimated at one in five, but I believe that nearly all of them, if they live long enough, will be affected,” says Harris Gelbard, professor of neurology and principal investigator on the new grant. “As the sensitivity of our measurements improves, so does number of people known to have more subtle cases of HAD. With HIV patients living long enough to re-engage in life and return to work, a condition that often reduces mental function by about 25 percent, and for which there is no approved treatment, deserves attention.”

The grant, from the National Institute of Mental Health, is a competitive renewal of a previous grant, but with a shift in focus. The funding will support a a partnership between the Medical Center and the Center for Neurovirology and Neurodegenerative Disorders at the University of Nebraska Medical Center to investigate ways to reverse nerve damage and to develop a more effective treatment for the causes of neuroAIDS.

Ethnic disparities in Medicare

African Americans and Latinos are more likely to underestimate how long it has been since their last routine screening test for disease, such as a mammogram, Pap smear, or cholesterol test, according to a University study that matched patient recollections with Medicare documentation. Researchers are concerned this might be harming the health of minorities in the long run.

“If you ask anyone, ‘When was the last time you had a mammogram or a cholesterol test?’ almost all people will think it’s been a shorter time than it’s actually been. Over-reporting happens to all of us,” says lead author Kevin Fiscella, associate professor of family medicine and community and preventive medicine. “But our study found that some minorities tend to over-report even more often than other groups of patients. This is a problem that has to be recognized and fixed.”

Fiscella says he is not sure why the discrepancy in reporting exists. One theory supported by social psychology research is that African American or Latino patients may feel some pressure to defy stereotypes. So, when doctors or nurses ask them about an event for which they are uncertain, they err on the side of caution and reply affirmatively, he said.

The study was published September 29 in the open access journal BMC Health Services Research.

The University collaborated with colleagues from the University of California at Davis to analyze 49,645 patient records, matching Medicare claims with patient surveys of the last preventive screening test they had received. The researchers made statistical adjustments for age, gender, income, education level and health status.

Researches say the study helps to shed light on vexing questions such as why black women are dying from breast cancer in higher numbers than white women, despite national data suggesting that black women are being screened with mammograms. The authors recommend further studies to clarify the causes of discrepancies in disparity estimates.