Hirokazu Kobayashi
CEO, Green Insight Japan, Inc.
Professor Emeritus and Visiting Professor, University of Shizuoka
2015, the Sustainable Development Goals (SDGs) were adopted at the United Nations summit. It is too late. Humanity has become the dominant species on the planet and seems to have forgotten that our existence is based on coexistence with other organisms. Independent of different organisms for their survival are "autotrophs," which can survive on sunlight, electron donors such as water, carbon dioxide, and a few minerals. Cyanobacteria and plants are examples of such organisms, and this activity is called "photosynthesis." This process's carbohydrates and oxygen gas allow other organisms to survive. These are called "heterotrophs," to which humans belong.
Apart from this kind of interdependence, we can also observe examples of direct "symbiosis" between living organisms, such as the following:
Leguminous plants and rhizobia: Leguminous plants obtain nitrogen by having rhizobia fix nitrogen. In return, rhizobia receives carbon sources from the plants. The nodule formation mechanism in leguminous roots and the nitrogen fixation function of rhizobia have coevolved.
Japanese black pine (Pinus thunbergii) and arbuscular mycorrhizal fungi: The Japanese black pine benefits from the mycorrhizal fungi by more effectively absorbing nutrients (especially phosphate) from the soil. In return, the fungi receive carbon sources from the Japanese black pine's roots. The black pine and mycorrhizal fungi have developed a symbiotic relationship and coevolved in a mutually dependent manner.
Cattle (Bos taurus) and symbiotic gut bacteria: In cattle's digestive systems, bacteria that break down cellulose coexist, allowing cattle to digest grass. The bacteria utilize the environment inside the cattle's digestive tract to reproduce. The digestive system of livestock has evolved specialized structures for symbiosis with these bacteria.
Sloths (Bradypus) and cyanobacteria: Cyanobacteria living on sloths' fur provide green camouflage, enhancing protection against predators. In return, the cyanobacteria gain a stable habitat on the sloth's fur. The ecology of sloth fur and cyanobacteria has specialized through coevolution.
Ants (Formicidae) and aphids (Aphidoidea): Ants feed on the sweet honeydew secreted by aphids, while ants protect aphids from predators. Ants have evolved the ability to safeguard aphids, and aphids have evolved to provide honeydew.
Fig trees (Ficus) and fig wasps (Agaonidae): Fig wasps lay their eggs inside figs, where the larvae develop. In return, the wasps pollinate the figs. Figs and wasps have evolved a unique reproductive system that is mutually dependent.
Honeybees (Apis) and flowers: Honeybees collect nectar from the flowers of fruit trees and other plants, which they pollinate, leading to fruit formation. The flowers of fruit trees and other plants bloom at times and are suitable for honeybees, and honeybees have evolved behavior patterns adapted to these flowers.
Hummingbirds (Trochilidae) and flowers: Hummingbirds consume nectar from flowers and transport pollen to other flowers. They have evolved long beaks, and flowers have evolved shapes and colors suitable for hummingbirds.
Sea anemones (Actiniaria) and clownfish (Amphiprioninae): Clownfish hide among the tentacles of sea anemones to protect themselves from predators. At the same time, they provide food to the anemones and remove parasites from their tentacles. Clownfish have evolved to withstand the venom of the anemones through their mucus, and sea anemones have evolved to accommodate clownfish.
Ants and acacia trees (Vachellia): Acacia trees provide food and shelter to ants, while ants protect the acacia trees from herbivores and other plants. Acacia trees have evolved specific chemicals and housing structures to attract ants, and ants have evolved specialized defensive behaviors for the trees.
The above are examples of "symbiosis," where the interaction benefits both organisms, meaning mutual dependence. In contrast, interactions where one organism benefits at the expense of the other are "parasitism." This involves the relationship between a "host" and a "parasite," and in the context of diseases, parasites are "pathogens." An intriguing case involves Wolbachia (Rickettsia) in arthropods, which is known to affect the host's reproductive system. Wolbachia infects various host organs but cannot exist in mature sperm, so only females infected with Wolbachia can pass it on to their offspring. Wolbachia enhances its transmission and reproduction by manipulating the host's reproductive system in various ways.
A more complex three-way interaction can be observed among cabbage (Brassica oleracea), its herbivore, the cabbage caterpillar (larva of the Pieris rapae butterfly), and its natural enemy, the parasitoid wasp (Cotesia glomerata), as well as among cabbage, the larva of the diamondback moth (Plutella xylostella), and its natural enemy, the parasitoid wasp (Cotesia vestalis). The cabbage emits volatile compounds that attract the parasitoid wasps, which are natural enemies of the herbivores. These wasps lay their eggs in the larvae, effectively eliminating them. In the case of herbivory by the cabbage caterpillar, when a large number of caterpillars are present, and the concentration of volatile compounds is high, more C. glomerata wasps are attracted. Substances produced by plants that act to attract or repel other organisms are called "allelochemicals," and this phenomenon is referred to as "allelopathy." Such coevolution may have been the result of selection for accidental mutations.
There are mutual recognition mechanisms between hosts and pathogens close to us. The novel coronavirus (SARS-CoV-2), a human pathogen, enters and proliferates in cells via the ACE2 (angiotensin-converting enzyme 2) receptor localized on the surface of human cells. ACE2 suppresses the renin-angiotensin-aldosterone system that raises blood pressure by converting angiotensin II to angiotensin-(1-7). ACE2 has been found to protect against lung injury caused by sepsis and other conditions. Furthermore, in the small intestinal epithelium, ACE2 binds to the sodium-dependent neutral amino acid transporter (B0AT1) and functions as a neutral amino acid transporter that absorbs tryptophan, which is believed to trigger the expression of antimicrobial peptides. It can be said that the spike protein of SARS-CoV-2 has evolved to have an affinity for the ACE2 receptor, but it is believed that this arose from random mutations in the spike protein that resulted in binding to the ACE2 receptor.
In plant diseases, only susceptible varieties, such as the apple cultivar “Orei,” react to AM-toxins produced by the Alternaria blotch of apple pathogen (Alternria alternata apple pathotype), leading to infection. This was the theme of my BS thesis at Tottori University under the guidance of Syoyo Nishimura (1929-1989). It has recently been revealed that the factor on the apple side involved in this infection is the CC-NB-LRR protein. On the other hand, the HV-toxin (Victorin) produced by the pathogen of oat Victoria blight, Cochliobolus victoriae (formerly Helminthosporium), binds to thioredoxin h5 (TRXh5), and HV-toxin-binding TRXh5 activates the CC-NB-LRR protein (LOV1), which induces host cell death (susceptibility). Thioredoxin is an enzyme that transmits redox signals in the chloroplasts and cytoplasm of plant cells. Although AM-toxin and HV-toxin are cyclic peptides, they are secondary metabolites not synthesized by the genetic codon-based protein synthesis system, making it unlikely that various peptides are synthesized solely by genetic mutations. From this perspective, the coevolution by which HV-toxin acquired the ability to bind to TRXh5 is filled with mysteries. In the case of Alternaria, the toxin-producing genes are encoded on a "conditionally dispensable (CD) chromosome," which is predicted to undergo horizontal transfer differently from chromosomal DNA. Thus, coevolution at the molecular level in organisms is fascinating as a life dynamism.
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