‘Caste’ genes evolve quickly in social bugs

GEORGIA TECH (US) — Genes responsible for different sexes, life stages, and castes in fire ants, honeybees, and other social insects evolve faster than others not involved in those functions, according to new research.

All species of life develop in different ways by varying the genes they express, ultimately becoming different shapes, sizes, colors, and sexes—a plasticity that permits organisms to operate successfully in their environments.

For the new study, published in Proceedings of the National Academy of Sciences, researchers found that genes involved in these processes exhibited elevated rates of evolution even before they were recruited to produce diverse forms of an organism.

Shown are fire ants of different castes and life stages: (top-bottom) worker, male and queen castes; (left-right) adult, pupal, and larval stages. (Credit: Amanda L. Palaski and Ariel R. Thompson)


“This was a totally unexpected finding because most theory suggested that genes involved in producing diverse forms of an organism would evolve rapidly specifically because they generated developmental differences,” says Michael Goodisman, associate professor of biology at Georgia Institute of Technology (Georgia Tech). “Instead, this study suggests that fast-evolving genes are actually predisposed to generating new developmental forms.”

Social insects exhibit a sophisticated social structure in which queens reproduce and workers engage in tasks related to brood-rearing and colony defense. By investigating the evolution of genes associated with castes, sexes, and developmental stages of the invasive fire ant Solenopsis invicta, the researchers explored how social insects produce such a diversity of form and function from genetically similar individuals.

“Social insects provided the perfect test subjects because they can develop into such dramatically different forms,” says Goodisman.

Microarray analyses revealed that many fire ant genes were regulated differently depending on whether the fire ant was male or female, queen or worker, and pupal or adult. These differentially expressed genes exhibited elevated rates of evolution, as predicted. In addition, genes that were differentially expressed in multiple contexts—castes, sexes, or developmental stages—tended to evolve more rapidly than genes that were differentially expressed in only a single context.

To examine when the genes with elevated rates of evolution began to evolve rapidly, the researchers compared the rate of evolution of genes associated with the production of castes in the fire ant with the same genes in a wasp that does not have a caste system. The genes were rapidly evolving in the genomes of both species, even though only one produced a caste system. These results were also replicated for the honeybee Apis mellifera.

“This is one the most comprehensive studies of the evolution of genes involved in producing developmental differences,” Goodisman notes.

This study helps explain the fundamental evolutionary processes that allow organisms to develop different adaptive forms. Future research will include determining what these fast-evolving genes do and how they’re involved in the production of different sexes, life stages, and castes, says Goodisman.

Researchers from the University of Lausanne in Switzerland contributed to the study, which was supported by the National Science Foundation.

More news from Georgia Tech: www.gatech.edu/newsroom/