Rice graduate student choreographs cross-species cellular ballet

Amoebae play well together
Rice graduate student choreographs cross-species cellular ballet

BY MIKE WILLIAMS
Rice News staff

As Chandra Jack absentmindedly gripped her son Jacorri’s cardboard building blocks, she had much tinier building blocks on her mind.

The second-year doctoral student at Rice University, with her 15-month-old son in tow just before Thanksgiving, was immersed in a discussion about amoeba.

The primary author of a recently published paper in BMC Evolutionary Biology, she wondered why two species of amoeba, Dictyostelium discoideum and Dictyostelium purpureum, are willing to work together even though there’s no obvious advantage in doing so.

	CHANDRA JACK

Jack and her colleagues are looking for clues as to how and why single-cell beings cooperate with other species and what they stand to gain or lose in the process. ”They start out living on their own but they give up some of their independence to form a multicellular structure, which is how life progressed from single-celled to multicellular beings — basically, us,” she said.

”Chandra is able to see the big picture and to understand the importance of her questions,” said Joan Strassmann, Rice’s Harry C. and Olga K. Wiess Professor of Ecology and Evolutionary Biology and chair of the Department of Ecology and Evolutionary Biology, and Jack’s co-adviser on the project.

”She has succeeded with some very difficult experiments that involve two species of social amoebae,” said Strassmann. ”The experiments are difficult because of timing. They have to be conducted on cells at a certain stage, which can take about 30 hours to attain. If either species is at the wrong stage, then the experiment cannot be done. Chandra succeeded at this difficult cellular ballet enough times to produce an excellent study.”

A free-range amoeba is less likely to play well with others when there’s plenty of food, primarily bacteria. But when the chow runs out, Jack found that D. discoideum and D. purpureum might join forces to find greener pastures.

Through her lab work, she showed that while the two species of ”Dicty” generally keep to themselves, they are able to combine to form a slug that is chimeric — that is, it has the characteristics of both species, a situation sometimes found even in mammals. The slug gains the ability to migrate and reproduce via spores in what’s called a fruiting body, which looks like a balloon atop a very long stalk.

About one-fifth of the amoeba in a chimeric slug die to form the stalk, an act of self-sacrifice also characteristic of more homogeneous slugs.

”Both of these species aggregate to the same chemical signal,” said Jack, ”and they are also found in the same soil samples, but in previous studies, nobody had ever found cooperation between them.”

Her study found half of the fruiting bodies that were primarily D. discoideum and 22 percent that were primarily D. purpureum were chimeric, though the production of spores in either case suffered as a result. ”The surprising finding … cannot be explained by increased spore production,” she wrote. ”It may simply be a mistake or it may be making the best of a bad job.” She found, however, that cooperating species are able to make larger fruiting bodies, ”which are advantageous for migration and dispersal.”

In the long run, Jack hopes to discover what specific benefits each species gains. ”We believe we have a mutualism — an interaction between two different species in which both get a net benefit.

”Usually, in a mutualism, the species get different benefits,” she said. Bees and flowers, for instance, trade pollination for food. ”But here we’re saying they’re providing the same benefit. When they’re able to operate together, they form a bigger slug, which allows them to potentially move further and form a bigger fruiting body.”

In her observations, Jack found slugs travel an average of 1-2 millimeters an hour and can go as far as 10-20 centimeters. ”My next step will be to see if the chimeric slugs migrate farther than slugs that have only one species but are half the size.”

Jack is a San Antonio native who also earned her undergraduate degree at Rice. Her co-authors on the paper are Rice senior Julia Ridgeway and graduate student Tracy Edwards; research associate Natasha Mehdiabadi; Emily Jones ’04, now a graduate student at the University of Arizona; and co-advisers Strassmann and David Queller, Rice’s Harry C. and Olga K. Wiess Professor of Ecology and Evolutionary Biology.