A genetic approach to endocellular symbiosis

Abstract

The value of a genetic approach to symbiosis is three-fold. First, it provides a conceptual framework around which to organize a mass of diverse data. For example, we have seen that both the segregation of uninfected bacteria from a strain harboring bacteriophage, and the elimination of symbiotic algae from epidermal cells of hydra during development are relevant to the question of the regulation and control of symbiont transmission. Similarly, the ability of Rhizobium-infected root nodules to fix nitrogen and the appearance of new antigens on the surfaces of virus-infected bacteria both illustrate new phenotypes arising from the acquisition of a symbiont; endosymbioses therefore either create or release genetic information.

Second, a genetic approach to endocellular symbiosis emphasizes the genetic and biochemical ntegration of host and symbionts. Together they form a single biological system, more or less perfectly coordinated, so that distinctions between “invader” and “organelle” may become impossible, or at least meaningless.

Third, the approach has heuristic value. As a result of considering symbiotic associations in somewhat new ways, new questions arise. For example, what factors regulate the respective division rates of host cells and symbionts so that neither outstrips the other and the association is faithfully perpetuated? Just how complex is the genetic control of mutually compatible host and symbiont specificities, and how do these genes operate? Are macromolecules exchanged between symbiont partners, or is macromolecular synthesis necessary for the creation of a suitable intracellular environment for symbiosis? Are new molecular species formed as a result of the juxtaposition of two dissimilar genomes?

Answers to these and similar questions should be forthcoming with the continued application of genetic and biochemical techniques to the biology of host-symbiont relationships.