The thistle tortoise beetle has a very simple microbiome - dominated by a single species of bacteria that provides it with important enzymes to digest its plant food. Genetic variations in the microbe can bring the beetle even more advantages. This is because some strains have an additional digestive enzyme that allows the beetle to extend its diet to a greater variety of plant species. With this observation, Hassan Salem of the Max Planck Institute for Developmental Biology in Tübingen, in cooperation with an international team of researchers, now provides evidence of how a symbiote drives the evolution of its host. The scientist published his results in Current Biology.
The human body contains about ten times as many microorganisms as human cells. And the microbiome, as the microbial flat-share is called, has a decisive influence on health and well-being. They protect against pathogens, support digestion, train the immune system and supply the organism with valuable substances. "However, if you want to investigate the interplay between host and microbiome, you usually face the problem that you are dealing with hundreds or even thousands of different microorganisms," said Hassan Salem from the Max Planck Institute for Developmental Biology in Tübingen.
Salem is interested in the basic principles of co-evolution in symbioses. The object of his research is the thistle tortoise beetle (Cassida rubiginosa), a bright green beetle, about 6 to 7.5 millimetres in size, which is found in many parts of the world. What is special about this insect is that its microbiome is dominated by a single species of bacteria, Candidatus Stammera capleta, or Stammera for short. And this bacterium also decides which plant food the beetle can digest. Because Stammera is so important for the beetle, the females inoculate their eggs with the bacterium after laying them to ensure that the next generation is also equipped with the symbiote.
Anyway, herbivorous beetles - that is about half of all beetle species on earth - have acquired their ability to digest such food in the course of the evolution of microorganisms. They have taken genes from bacteria and fungi with which they produce enzymes, for example to break open the cell walls of plant cells.
"In addition, beetles can also gain additional metabolic abilities through suitable symbionts," explains Salem. According to this principle, scientists assume that beetles succeed in tapping new food sources. Salem now wanted to find out what influence Candidatus Stammera capleta has on which plants the beetle can adopt as food source in the different distribution areas. Do different metabolic properties of the symbiote influence the diet of the beetle?
To answer this question, Salem has analysed and compared the genomes of 13 Candidatus Stammera capleta strains. "Interestingly, the genomes were highly conserved over large parts, which also has an effect on functionality," explains the Max Planck scientist. For example, all bacterial strains had an enzyme called polygalacturonase, whose task is to cleave homogalacturonan, the most common pectin in plants. "In some strains we have also found genes for another pectin-cleaving enzyme, rhamnogalacturonan I," Salem says. This may allow the host to digest additional pectins and thus extend its menu to other plant species. In fact, further investigations showed that only beetles harbouring Stammera variants with the additional gene are able to eat leaves of other plants in addition to the ancestral thistles.
The additional properties of the symbiote thus influence the beetle, which opens up new food sources. Thus, the genetic characteristics of the microbe become the evolutionary engine for the beetle - a fundamental principle that probably also plays a role in the evolution of other herbivorous beetles. And ultimately, such co-evolution may also have taken place in mammals up to and including humans - which thus adapt to changing environmental conditions and a different food supply or tap additional sources of nutrients.
However, Hassan Salem warns against thinking too simply. Even in the simple system of the thistle shield beetle and the Stammera, the relationship between host and microbe is not the only motor of evolution: "There is always a co-evolution between beetle and food plant that also influences the overall process.
An international team of scientists from the USA, Panama, New Zealand and Japan as well as from the Max Planck Institute for Chemical Ecology in Jena was involved in the work.
Salem H, Kirsch R, Pauchet Y, Berasategui A, Fukumori K, Moriyama M, Cripps M, Windsor D, Takema Fukatsu T, and Nicole M. Gerardo. Symbiont 1 digestive range reflects host plant breadth in herbivorous beetles. Current Biology Available online 4 June 2020 doi.org/10.1016/j.cub.2020.05.043
Dr. Hassan Salem
Max Planck Research Group Leader
Phone: +49 7071 601-1367