The Role of Symbiosis in Physiology and Evolution

This link provides an overview of the types of symbioses and some of the players involved in their research. From the conference abstract...

A variety of new and exciting models of symbiotic associations have been developed to investigate symbiotic relationships among organisms, their role in physiology, and especially the evolution of the organisms involved. The association host/symbiotes creates a new biological unit, the symbiocosm, itself subject to natural selection.

Different types of association exist as demonstrated by six different models. The symbiotes can be outside (ectosymbiosis) or inside the host, in the intestine or in some invaginations of the integument (endosymbioses), or inside the cells (endocytobioses). In integrated symbioses the symbiote (a bacterium) is perfectly controlled by the host (location and number), and looklikes a new cell organelle, only transmitted to the progeny by the mother. In other cases the symbiote is not perfectly controlled and invades most cells (Wolbachia), but not the entire the host population.

In the Squid-Vibrio model (extracellular symbiosis), the bacterium is transmitted horizontally, or cyclically, and can be grown in vitro. The colonization of the luminous organ modifies processes of recognition and specificity. Another endosymbiosis is that seen among termites. The gut lumen harbours protozoa and/or bacteria. They help the digestion. Transmission is horizontal. A coevolution host/symbiotes is highly probable.

Among endocytobioses another model is a protozoan, the amoeba, living symbiotically with a bacterium. The formation of the symbiosis has been observed in the laboratory. The bacterium, which was originally a parasite, has coevolved with the host cell thereby becoming obligate and thus integrated. The role of symbiote in co-evolution is also spectacular with the Wolbachia model, a bacterium associated with many insects and other invertebrates.

The aphid model presents an integrated and obligate symbiosis. The changes of the symbiotic bacterium during symbiosis are spectacular, most notably the reduction of the genome, which has been recently sequenced.

A last model is a weevil (Coleoptera), where symbiosis is perfectly integrated, but surprinsingly not always obligate. Comparison of symbiotic and aposymbiotic strains that lack symbiotes allows an understanding of the exact role of symbiosis. The symbiocosm is there controlled by interactions of four different genomes: nuclear, mitochondrial and symbiotic (principal endocytobiotes and Wolbachia).

Symbioses appear as important factors in evolution. Consequently, symbiology must be recognized as an important field of Zoology, Botany and Genetics.