A new study by researchers at the School of Biological and Chemical Sciences is the first to show that the process of evolving dependence on bacteria for nutrients occurs in a series of predictable steps.
The study, published in the Journal Current Biology examined how several different aphid lineages have evolved dependence on the bacteria Buchnera aphidicola and Serratia symbiotica for nutrients. This is known as co-obligate symbiosis, whereby hosts rely on multiple symbionts for survival.
Symbiosis is the interaction between two different organisms living in close physical association, typically to the advantage of both.
The findings, have wider implications for understanding the role symbiosis may have played in the evolution of complex organisms.
The research was carried out by School of Biological and Chemical Sciences academics, David Monnin, Raphaella Jackson, Lee Henry and researchers from Vrije Universiteit in Amsterdam and University of Puget Sound in Tacoma, USA.
Dr Lee Henry, Senior Lecturer at Queen Mary said “All plants and animals have evolved extreme dependence on bacteria and this process has resulted in some of the most important evolutionary transitions including the origins of the Eukaryotic cell, the formation of the mitochondria and chloroplasts.”
Insects often evolve an extreme dependence on microbes for nutrition. This includes cases in which insects depend on multiple symbionts that function collectively as a metabolic unit.
Whilst there are many cases of these polyamorous relationships in sap feeding insects, most aphids are dependent on only one bacterium, Buchnera. In their study, the researchers discovered several cases where aphids have evolved dependence on a second symbiont, Serratia, making them an ideal system to investigate how tripartite symbioses evolve.
By comparing several ancient and recent associations with the two bacteria in aphids, researchers showed that introduction of new symbionts occurs through a predictable step-wise evolutionary process. This includes how they integrate all three partner’s genomes, how they share metabolic pathways, and how the host insects evolve a new organ to house the newly acquired microbial partner.
The researchers found that surprisingly, dependence on a new symbiotic partner evolves through the ancestral symbiont (Buchnera) losing the capacity to produce specific nutrients - peptidoglycan and vitamin B2 - and this locks the second bacteria (Serratia) into a permanent relationship with the host aphid and the other symbiont.
After dependence on Serratia arises, essential amino acid pathways are lost in Buchnera, which coincides with the increases in the number of Serratia. Finally, the creation of specialized organs to host the new bacterium completes the integration into the host aphid.
More generally, the results of the study suggest the energetic costs of synthesizing nutrients may provide a unified explanation for the sequence of gene losses that occur during the evolution of co-obligate symbioses.
Research paper: Monnin et al., Parallel Evolution in the Integration of a Co-obligate Aphid Symbiosis, Current Biology (2020).