It has become increasingly apparent that larger organisms like ourselves cannot live alone. A certain community of bacterial associates must live in, and on a host organism. This relationship is sometimes essential for the host’s well being as well as that of the the microorganisms. Different animals have a specific array of microorganisms that function in roles that offer adaptive advantage in the context of the environment. These roles include digestion, protection from destructive pathogens and so on. They have also been shown to play roles in reproduction and sociality among other things.
This relationship between host and microbial organisms should perhaps come as no surprise because complex organisms such as ourselves arose from cooperative ties between microorganisms and viral components. We are, from a certain perspective, a microbial community ourselves as we are composed of a community of like cells, differentiated slightly into various organ roles that operate as a community. This same principle applies to the more fluid, extended microbial community in the environment.
How this community of relationships forms and develops between a host and the microbiome has been the focus of recent studies on the cutting edge of evolutionary biology. It appears that the relationship between host an microbiome can and does shape the evolutionary path of this collective “community”. Each organism plays a role as a voice in a choir, and the persistent demands of the environment is the choirmaster. The fact that there are severe fitness disadvantages in hosts that don’t have an appropriate blend of microorganism companions is an indication of how crucial this cooperative communal role is to develop. Together, the host and all the corresponding microorganisms that live in together are called a metaorganism.
The host organism actively cultivates a climate to identify microbial friends from enemies. One of the things that emerged in terms of understanding how these communal relationships are forged is that the host’s is able to recognize phylogenic similarities between itself and the various microbial genomes. It does so using its immune system as the sensor to differentiate friend from foe. In other words; the host’s genotype is in part responsible for the composition of microbiota which the metaorganism consists of. The more distantly related species, the less preferred it is. Of particular note is the fact that self similarity in the collective genomes between host and microbe are tied to their inclination to service each other’s needs. Phylogenetic similarity is what appears to incline them to confer advantage toward each other. In other words, the more similar, the more likely their behaviors will center on cooperation.
The host’s immune system is the vehicle that cultivates specific relationships from the environmental microbiome. It is this recapitulation of host phylogeny by microbiota that is called phylosymbiotic relationship. Over time, this relationship field in the metaorganism inclines toward a host-bacterial homeostasis that collectively offers adaptive advantages and in some cases, obligate (necessary) relationships, without which the communal social system would break down.
For more on this, see Seth Bordenstein’s talk on the topic.