The Institute of Biological Resources and Functions (BRF) of the National Institute of Advanced Industrial Science and Technology (AIST) discovered that plant specialization of a herbivorous insect, pea aphid, is determined by a symbiotic bacterium inside the insect.

In Japan, pea aphids (Acyrthosiphon pisum; fig.1 left) feed mainly on vetch (Vicia sativa; fig.1 middle) and white clover (Trifolium repens; fig.1, right), and are able to reproduce on both. Pea aphids in natural Japanese populations exhibited a characteristic geographical distribution of endosymbiont infection with a facultative γ-proteobacterium, called pea aphid U-type symbiont (PAUS; fig.2, red). The endosymbiont was more common in northern regions where vetch plants are rare. At three locations in central Japan where vetch and white clover are sympatrically available, the occurrence of PAUS was consistently higher on white clover than on vetch.

By using a newly developed selective elimination technique, we generated aphid strains that are genetically identical and differ only in PAUS infection. The PAUS-eliminated strain lost almost 50 percent fecundity compared with the infected strain only on white clover. Notably, re-introduction of PAUS by micro-injection resulted in an almost complete recovery of the fecundity in the recipient aphids on white clover. These results indicated that PAUS infection improves the fitness of the host aphid specifically on white clover.

Thus far, macroscopic biological traits of an insect like plant specialization have been, needless to say, regarded as traits of the insect itself. However, this finding provides a new viewpoint that a community of endosymbiotic microorganisms may sometimes substantially affect the biological phenomena that we observe without noticing the unseen force. Hence, the discovery has a potential impact on basic and applied aspects of many fields in biology such as microbiology, entomology, ecology, agriculture and pest management.

This study was published in Science on March 26, 2004*.

* Tsuchida T., Koga R. and Fukatsu T. (2004) Host plant specialization governed by facultative symbiont. Science 303: 1989.

Relevant papers are available below.

Tsuchida T., Koga R., Meng X. Y., Matsumoto T., Fukatsu T. (2004) Characterization of a facultative endosymbiotic bacterium from the pea aphid Acyrthosiphonpisum. Microbial Ecol. 47: in press.

Koga R., Tsuchida T., Fukatsu T. (2003) Changing partners in an obligate symbiosis: a facultative endosymbiont can compensate for loss of the essential endosymbiont Buchnera in an aphid. Proc. R. Soc. Lond. B 270: 2543-2550.

Tsuchida T., Koga R., Shibao H., Matsumoto T., Fukatsu T. (2002) Diversity and geographic distribution of secondary symbiotic bacteria in natural populations of the pea aphid Acyrthosiphonpisum. Mol. Ecol. 11: 2123-2135.

Figure 1. Pea aphid (left) and host plants, vetch (middle) and white clover (right).

Figure 2. Endosymbiotic system of pea aphid. Red, the facultative symbiont PAUS in small and flat sheath cells; green, the essential symbiont Buchnera in large mycetocytes; blue, nuclei of aphid cells. Bar shows 50μm.