Takema Fukatsu (Leader) and Takahiro Hosokawa (Collaborative Research Scientist) of the Biological Interactions and Symbiosis Research Group, the Institute for Biological Resources and Functions (Director: Masahiro Iwakura) of the National Institute of Advanced Industrial Science and Technology (President: Hiroyuki Yoshikawa) (hereinafter referred to as AIST), demonstrated that the ability of Megacopta punctatissima, a pest insect of crop legumes, to exploit soybean as food plant is determined by its intestinal symbiotic bacterium rather than by its own genome.

Herbivorous insects naturally feed on wild plants. Upon monoculture of agricultural plants, some insects may exhibit or acquire the ability to exploit the crop plants, which would cause serious agricultural problems, and such insects are regarded as “emerging pests”. Conventionally, it has been presupposed that such pest-related insect traits are determined by genes encoded in their own genomes. This research, however, demonstrated that pest status of an insect can be determined by its gut symbiotic bacterium rather than by its own genome.

This finding sheds new light on the evolutionary origin of insect pests, suggests the possibility of targeting intestinal symbiotic bacteria for pest control, and therefore will attract much attention not only from basic sciences such as microbiology, entomology, and ecology, but also from applied aspects such as agriculture and pest control.

The work was published on the website of the British academic journal, Proceedings of the Royal Society B, on June 13, 2007*.

*Takahiro Hosokawa, Yoshitomo Kikuchi, Masakazu Shimada and Takema Fukatsu (2007) Obligate symbiont involved in pest status of host insect. Proc. R. Soc. B 274: 1979-1984.

Megacopta punctatissima and their eggs

Agricultural production can be severely damaged by insects, and measures to prevent insect damage are important for the economy and the society. The practical research fields such as agriculture, plant breeding, and pest control have been engaged in development of counter measures against the problem.

Crop plants have been highly modified through breeding, and are very different from their wild-type species. In most cases, they are cultivated in regions around the globe far from their place of origin. Thus, target crops of many pest insects are not the original host plants that were naturally eaten by the insects. There are many examples where insects that live on indigenous wild plants have adapted to introduced crops and have spread as pests.

Crop breeding aims at developing crop resistance to pest insects. The study of pest control develops new insecticides effective to pest insects in the field. However, although suppression of the insects may be initially successful, they eventually develop resistance to those means and spread once again. The history of insect pest control has been a repetition of this kind of cat-and-mouse game.

Analysis of the processes by which insects acquire the ability to utilize agricultural plants as food is extremely important to the understanding of how pest insects arise and to the development of strategies to control them. Conventionally, it has been presupposed that pest insects arise through genetic variation in the insects themselves.

AIST has been promoting various research projects targeting unculturable symbiotic microbes, which are a treasury of unexplored genetic resources. As a new and useful model system for the study of symbiosis, we have been studying the capsule transmission system of gut symbiotic bacteria of stinkbugs of the family Plataspidae.

Plataspid stinkbugs harbor specific bacteria (which we have named Ishikawaella) in their gut cavity, and the symbiosis is so intimate that the stinkbugs cannot grow and reproduce normally without these microbial associates. Interestingly, stinkbug mothers deliver small brown particles with eggs they lay. These particles are capsules containing the gut symbiotic bacteria, and the hatched nymphs ingest the contents of the capsules via their piercing mouthparts to acquire the symbiotic bacteria (Figure 1).

Figure 1: Newborn nymphs of M. punctatissima probing symbiont capsules for acquisition of symbiotic bacteria. Red, symbiont capsules; blue, eggshells; Bar, 1 mm.

This maternal provision of a symbiont-containing “lunch box” for nymphs is a very unique system found nowhere else in the biological world. Utilizing this system, it became possible to conduct various experiments that we cannot otherwise conduct. For example, we are able to experimentally exchange symbiotic bacteria between populations, species, and genera of the stinkbugs by generating new combinations of eggs and capsules under a binocular microscope using forceps. By collecting isolated capsules, it is possible to prepare pure DNA samples of symbiotic bacteria without contamination of DNA of host insects.

Megacopta punctatissima (Figure 2 left) is found in the area ranging from mainland Japan to Yakushima Island, and it feeds mainly on “kudzu” (Pueraria lobata). However, it frequently invades soybean fields, sucks soybean sap and proliferates, causing agricultural problem. In contrast, Megacopta cribraria (Figure 2 right), found in the southwestern Japanese islands, eats mainly “taiwan-kudzu” (Pueraria montana), and scarcely causes such problem.

Figure 2: Adult insects of M. punctatissima (left) and M. cribraria (right). Normal, insects with original symbiont; Replaced, insects with heterospecific symbiont; bar, 1 mm.

Why does M. punctatissima cause agricultural problem, yet the closely related M. cribraria does not? We reared both species on potted soybean plants under the same conditions in the laboratory. Eggs of the former hatched normally, but the latter suffered low egg hatch rates, despite the fact that each bug grew and laid eggs normally (Figures 3 and 4). These results indicate that M. cribraria cannot reproduce properly on the crop plant.

Then we exchanged the symbiotic bacteria between the species by recombining their eggs and capsules, and raised them on potted soybean plants. Surprisingly, the hatching rate of M. cribraria eggs recovered to normal, whereas that of M. punctatissima was greatly reduced (Figures 3 and 4). So, by exchanging the symbiotic bacteria between the two species, we rendered “harmless” M. cribraria harmful, and “harmful” M. punctatissima harmless to the crop plant.

Figure 3: Egg hatch rates of M. punctatissima and M. cribraria when reared on soybean plants. When their symbiotic bacteria were exchanged between the insect species, their egg hatch rates exhibited complete reversal.

Figure 4: Hatching egg masses of M. punctatissima and M. cribraria. Normal egg hatch was associated with symbiotic bacteria from M. punctatissima. Bar, 1 mm.

These results indicate that the ability of M. punctatissima to feed on the crop plant is determined not by its own genome but by its gut symbiotic bacterium. This research shows for the first time that symbiotic bacterium living in the body can make its host insect pest. Therefore, it appears plausible that some pest insects might have originated from variation of their symbiotic bacteria or from acquisition of new symbiotic bacteria.

We aim to clarify the physiological and molecular mechanisms by which symbiotic bacteria make their host insects pest. We are analyzing the genomes of the symbiotic bacteria from M. punctatissima and M. cribraria. Detailed comparison of the genome sequences should provide useful insights into how symbiont genetic variations are related to the insect ability to utilize the crop plant.

This research presents a novel viewpoint that symbiotic bacteria may influence the evolution of new pest insects, and it is important to examine such a possibility in various pest insects.