Holobionts and the Hologenome Theory

The term superorganism was coined in 1928 by the American ant expert William Morton Wheeler to describe a colony of eusocial insects such as ants and bees. The prefix “super” indicates a higher level of organization among multiple individuals of the same species where individual members perform specific duties – analogous to the different tissues of an organism.

Somewhat more recently the term superorganism has been used to describe symbiotic relationships between different species. This rather inappropriate use of the word has prompted use of a new, more appropriate term. Lynn Margulis proposed any physical association between individuals of different species for significant portions of their lifetime constitutes a “symbiosis” and that all participants are bionts, such that the resulting association is a holobiont (Margulis, 1991). Every macrobe we know of is associated with diverse microbes, including viruses. So while the macrobe typically carries the name for the entire organismal association, each macrobe is just one biont that comprises a functional holobiont.

Coral; Photo: 3ric Johanson

Holobionts are everywhere: you, me, mosquitoes, every macrobe qualifies. The first recipient of the label holobiont was the coral/zooxanthellae symbiosis. The zooxanthellae live within the coral cells. Corals live in nutrient poor waters; the zooxanthellae are stable cohabitants providing the coral with essential nutrients during their lifetime unless an environmental perturbation disrupts this association. (For more information see Mbiosphere: Algae Capture, Store, and Release Nitrogen to Feed Reef-building Coral). Elevated sea temperatures are the primary cause of coral “bleaching”, or loss of the zooxanthellae, which is detrimental to the coral since they provide the coral with much needed nutrients. More recent research has showed development of resistance within coral populations following the worst bleaching event in modern history, in 1998. It was postulated that changes in the coral microbial community, due to natural selection pressure of the changing water temperature, was responsible for the large-scale adaptation of the coral (Reshef, Koren, Loya, Zilber-Rosenberg, & Rosenberg, 2006).

Colpophyllia natans (brain coral); Photo: 3ric Johanson

Studies of insect holobionts provide incredible stories of the adaptive strategies conferred from these associations. For example, a bacteriophage (bacterial virus) that infects bacterial symbionts of the pea aphid protects the holobiont against parasitoid wasps. An ant colony is still considered a superorganism, but what better example of a holobiont than the fungi cultivating ants? The holobiont includes attine ants, the farmed monoculture fungal bionts, and various antibiotic producing Bacteria, some of which protect the farm from invading fungal species. Watch these ants work at cultivating their gardens in this fascinating video.

Wikimedia Commons; Photo: Adrian Pingstone

This term holobiont, gave rise to the hologenome theory which states that certain microbiota are selected for based on the host environment, as well as the other types of microbes present (Arumugam et al. 2011). Because the microbes associated with the macrobe are so intimately involved with the organism, changes in the host environment can cause changes in the microbial genomes such that the genome that is being selected is not only the host’s but the hologenome – all the genes of the host and the microbiota. We see evidence of this in nearly all microbiology and human medical journals today with investigations into the human microbiome and its interplay with human health and development.

So what does all this have to do with IV Lab? One of our most well-known macrobes, the mosquito, similarly is a holobiont with a hologenome. Bacterial and viral members of the mosquito holobiont may provide researchers with novel methods of interfering with thePlasmodium life-cycle, thereby disrupting the cycle of human infections. (For more information see Plos Pathogens: Implication of the Mosquito Midgut Microbiota in the Defense against Malaria Parasites; Wikipedia; and Science Magazine; Wolbachia InvadesAnopheles stephensi Populations and Induces Refractoriness to Plasmodium Infection).

Works Cited

Margulis, L. (1991). Symbiogenesis and Symbionticism. In L. Margulis, & R. Fester (Eds.), Symbiosis as a Source of Evolutionary Innovation: Speciation and Morphogenesis (pp. 1-14). Boston: Massachusetts Institute of Technology.