The diversity of life on earth is not only astounding but inundated with myriad expressions of host-symbiont relationships. Among these, microbial symbionts are so pervasive that they could be considered the rule, rather than the exception, among multicellular eukaryotes. These miniscule microbial partners have in many cases become essential for the host's capacity to interact with the environment, for its adaptation and evolution.
Symbiosis refers to a close and long-term association between two organisms of different species. These associations can be mutualistic, commensal, or parasitic. The term was first coined in the late 19th century by Anton de Bary to signify "the living together of unlike organisms," following work on lichens that result from the intimate association between a fungus and an alga. Soon enough, many other cases of symbiosis were described. Perhaps the most notable interaction of unlike species is that which gave rise to the modern eukaryotic cell. The now accepted theory of symbiogenesis (as nicely described here), proposed that eukaryotic organelles like mitochondria and chloroplasts arose by the engulfment of formerly free-living bacteria. This symbiosis proved to be crucial for the transition to a eukaryotic lifestyle and a major driving force behind evolution of life on this planet.
Many well-known symbiotic associations involving microbes have been covered in this blog. Plants associate with nitrogen-fixing bacteria or with mycorrhizal fungi to improve nutrient uptake; microbial symbionts can supply essential amino acids and provide protection to their insect hosts. Closer to our hearts is the human gut microbiome, that complex community of microbes that is gradually proving an essential contributor to our well-being, from poorly understood effects on mood and behavior to immune development. These past decades have made us keenly aware that microbial symbionts expand the functional capacity of a eukaryotic host and allow adaptation by providing a broad range of traits that are not encoded in the host genome. It is therefore not surprising that many of these host-symbiont relationships become so deeply intertwined that they become interdependent, or what is known as an obligate symbiosis in which at least one of the partners cannot live without the other.
Insects, one of the largest and most diverse groups of organisms on this planet, also carry microbial symbionts, many of which are obligate partners. The diversity of insects and their symbiotic relationships represent a vast resource for the study of symbiosis and its impact on host evolution and adaptation. Which is exactly what Cornwallis and colleagues aimed to do in a recent study: look for possible guiding principles underlying evolution of obligate symbiosis in insects.
As beautifully depicted in the figure, the authors found that obligate symbiosis evolved multiple times across the 89 insect families they studied. They then looked for possible explanations by examining the composition of insect diets, which differ widely among species. Some insects feed on a variety of food sources while others have specialized diets like blood or wood. I found two results striking. First, symbionts allowed insects to specialize on a range of nutrient-deficient diets. Second and particularly interesting, the symbionts overcame their host's lack of B vitamins.
Insects which cannot produce B-vitamins, rely on their bacterial symbionts to solve this deficiency. As it turns out, this is not the only effect these symbionts have on their hosts: they also influence the diversity of species. In the case of insects that use diverse food sources, the presence of obligate symbionts resulted in a proliferation of species. In contrast, the number of species was much smaller among insects that specialized on a single food source like blood.
Even though there are still many aspects of symbiosis that are poorly understood, there is no doubt that microbial symbionts influence a host's capabilities and adaptations. What remains unclear to me is how such a relationship benefits the obligate symbionts. There could of course be as many answers as there are examples of symbiosis.