Not without my microbe

An entomological puzzle that began more than 80 years old has now been solved, the information gained from the smallest genome ever sequenced.

The reason that a particular species of beetle can chow down on thistle leaves is because a type of bacteria living its gut provides it with the tools to do so.

The puzzle was first framed by the German zoologist Hans-Jurgen Stammer (1899-1968), who studied symbiotic relationships between insects and bacteria through the 1920s and 1930s.

He found, among other things, that a family known as leaf beetles (Chrysomelidae) did not generally form such interdependent partnerships. However, there were exceptions. One species, called the thistle tortoise beetle (Cassida rubiginosa), did maintain a symbiosis with a species of bacteria. Indeed, in a study in 1936, Stammer discovered that the beetle contained a special internal sac in which the bacteria lived.

The importance of the relationship was reinforced by the discovery that female thistle tortoise beetles deposit a tiny colony of the microbes onto each of her eggs before laying them.

Now, a study led by Hassan Salem of the Alexander von Humboldt Foundation at Emory University in Atlanta, Georgia, US, together with colleagues from the Max Planck Institute for Chemical Ecology, has revealed the exact nature of the relationship.

The team sequenced the genome of the microbe, which they named Candidatus Stammera capleta, and found that it contained genes that produced a type of enzyme called pectinase – which functions to break down the tough cell walls of the thistle plant’s leaves.

“The beetle host possesses the genes responsible for producing cellulases to digest cellulose, whereas the symbiont provides the pectinases,” explains Salem. “Together they have the necessary enzymes to break down the plant cell wall.”

To test their findings, the scientists removed the bacteria from beetle specimens and, from the point of view of the beetles, the results were catastrophic.

“When we compared enzyme activity in tortoise beetles with and without symbiotic bacteria, we found that beetles without symbionts were not able to digest pectin in order to gain access to the nutrients in the cell and as a consequence their chances of survival decreased,” says co-author Roy Kirsch from the Max Planck Institute for Chemical Ecology.

As well as thus resolving the question initially framed almost a century ago, the researchers uncovered a surprise.

The genome of Candidatus Stammera capleta contains just 270,000 base pairs – the lowest number ever discovered for an organism that doesn’t require a host cell in which to survive. In comparison, the genome of another gut-dwelling bacterium, Escherichia coli, contains more than 4.6 million base pairs.

Many other species of leaf-eating beetle have genes that produce pectinase lodged in their own genomes. They are thought to have been originally acquired from other organisms, such as bacteria and fungi, and fully incorporated.

“It is fascinating that insects have solved the problem of how to break up plant cell walls so differently,” says another co-author, Martin Kaltenpoth from the University of Mainz.

“Why some insects acquired genes from microbes horizontally, while others maintain symbionts to do the same job is an interesting question that remains to be answered in future studies.”

The research is published in the journal Cell.