Research by Pitzer College’s W.M. Keck Science Professor on Genome Conflict Publishes in Leading Science Journal

A study co-authored by W.M. Keck Science Department’s Associate Professor of Biology Patrick Ferree was published in Science Advances, a member of the prestigious Science family of academic journals. It presents an extraordinary genome conflict that forces a sex change in jewel wasp embryos. The W.M. Keck Science Department is the integrated center for Pitzer, Scripps, and Claremont McKenna Colleges.

It’s exceptional for liberal arts faculty to publish in a Science journal, but the six-member team broke new ground in the area of “unnecessary” B chromosomes, which have been studied for over a century.

The team studied Nasonia vitripennis, a tiny iridescent wasp, and found that the expression of a gene carried by a B chromosome, delivered by males, could change the sex of embryos.

“The chromosome ends up destroying half of the wasp’s genome, that which is inherited from ‘dad,’” Ferree says. The result is a conversion of the offspring’s sex from female to male.

A different species of female jewel wasps is notorious for outlandish reproductive behavior. She injects a toxin in the brain of a cockroach several times her size. That creates a zombie-like state that allows the wasp to lead the still-live cockroach to a protected area and use its body as a carapace where it lays eggs. The larvae eventually use the cockroach for food and emerge from the shell after they are born.

Although the behavior Ferree’s team studied is invisible to the unassisted eye, the end result is just as striking and destructive, he says. The “selfish” chromosome that changes the sex of offspring is transmitted only by males. “Thus, it’s a vicious cycle, and a very effective one,” Ferree says. “It happens with 100% effectiveness. It is pretty much perfect at what it does.”

“Selfish” chromosomes have been detected in many plants and animals. The question was how they override normal reproductive processes and are transmitted at higher than expected levels. “One very big idea is that such selfish chromosomes express genes that function in these selfish behaviors. However, no such example has been shown until now,” Ferree says.

To determine that, they used RNA interference to degrade RNA made by the gene. When the gene is knocked out, “We see a strong loss of the sex-conversion effect,” Ferree said.

The study co-authors are: Elena Dalla Benetta (a postdoc with Ferree), Igor Antoshechkin (CalTech), Ting Yang (UC Riverside) Hoa Quang My Nguyen (UC San Diego), Ferree, and Associate Professor of Biology Omar S. Akbari (UC San Diego).

The project was funded in part by Ferree’s National Science Foundation (NSF) five-year CAREER grant (MCB-1451839). Such grants are considered a springboard for promising non-tenured faculty. He is now writing a proposal to study how the B chromosome’s expressed gene works to destroy the wasp’s hereditary material. As with many of his articles, Ferree had student co-authors, and roughly 15 students earned experience and summer stipends over the course of the study.

“Now that we know that this gene is involved … we want to understand what it’s doing at the cellular and molecular levels,” he said. 

“This project is an outstanding example of what NSF strives to achieve through its CAREER program, which supports development of early-career researchers as next-generation scientist-educators,” said NSF Program Director Karen Cone. “Dr. Ferree’s findings help answer a long-standing question of how self-serving chromosomes promote their own propagation, and the results provide clues about how such selfish systems might be harnessed for beneficial uses.”