Soldier or worker – what do the genes say?

Research published in Genome Biology this week has uncovered 25 genes that are expressed at different levels in worker and soldier termites. As one of the first molecular studies on termites, it paves the way for investigations into how termite larvae can develop into workers, soldiers or reproductive adults depending on the colony’s needs.

The eastern subterranean termite, Reticulitermes flavipes, exists in colonies, whose members share work and resources among themselves. Different castes of termite have evolved to do different jobs: worker termites dig tunnels and eat wood; soldier termites defend the colony; and reproductive termites fertilise or lay eggs. Each caste has specific features useful for their role. For example, soldier termites are larger than workers and have an enlarged head and hard black mouthparts that they use to fight ants or termites from neighbouring colonies. Interestingly, the fate of an individual is not determined until the larval stage. Little is known about how these fates are specified – especially at the molecular level.

To investigate this, the research team from Purdue University searched for genes that were expressed at different levels in soldier and worker termites, and found 25 candidates.

Workers had higher levels of expression of genes involved in breaking down cellulose, perhaps not surprising given that these animals have to digest wood. These genes belonged both to the termites themselves, and to the bacteria and protozoa that live in the termites’ guts. Soldiers had higher levels of expression of genes central to the formation of muscle or the cytoskeleton. These genes might be involved in making the soldiers’ large mouthparts work.

Michael Scharf and collegues also discovered two genes that may be involved in directing termite development. These genes are related to two genes, called bric-a-brac and bicaudal, which regulate patterning in the fruit fly, Drosophila. Bric-a-brac works by inhibiting the expression of other genes. Increasing the expression of such a gene in workers may turn off ‘soldier-specific’ genes. Bicaudal is involved in the formation of the fly head and thorax. Its increased expression in the soldier caste may help the large soldier head to grow.

The authors said, “While these findings are highly enlightening and suggest numerous future research paths, we feel they are only the ’tip of the iceberg’. Since our experiments examined a non-differentiated caste (worker) and a developmental end point (soldier), many key developmental genes were certainly not identified. If there is a master gene that regulates caste out there in termites, it likely awaits discovery. Our ongoing and upcoming efforts examining intermediary developmental stages will likely yield exciting new information.”