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2015 Annual Science Report

University of Montana, Missoula Reporting  |  JAN 2015 – DEC 2015

Project 2: Function by Reduction: Do Extant Symbiont Enzymes Recapitulate Ancient Metabolic Generalists

Project Summary

The origins of mitochondria and chloroplasts are two of the great unsolved mysteries in biology. It is now clear that these organelles used to be bacteria, but the evolutionary paths taken as they transitioned from bacteria to organelle are not well understood because they happened more than 1.5 billion years ago. Some insect endosymbionts have symbioses with bacteria which resemble organelles in many ways. We use these more recent symbioses as models to better understand the origins of organelles, one of the most critical events in the evolution of complex life.

4 Institutions
3 Teams
4 Publications
2 Field Sites
Field Sites

Project Progress

Since submitting the NAI proposal, the focus of my lab has taken a rather dramatic turn because of one major discovery. My original idea was to use insect endosymbiont systems as a model to understand the evolution of specificity in protein enzymes. This is perhaps still interesting, but in my view pales in comparison to other possible directions for my NAI subproject.

Of particular significance was our publication, Genome expansion via lineage splitting and genome reduction in the cicada endosymbiont Hodgkinia, in PNAS (doi: 10.1073/pnas.1421386112). Writing this paper crystallized for me the parallels that insect endosymbionts have with the cellualr organelles of bacterial origin, the mitochondria and plastids. What the paper describes is a very unusal process whereby the original single bacterial endosymbiont lineage has split into several related lineages inside the cicada. Each individual new genome has become smaller through the expected process of endosymbiotic genome reduction. However, critically, from the perpective of the host insect, the collective endosymbiotic genome has actually grown in size because the number of lineage splitting events has outpaced the genome reductive process found in individual lineages. Thus, genome expansion through lineage splitting and genome reduction.

This is interesting on its own, but has a fascinating parallel to mitochondrial genome evolution. In some eukaryotic lineages, in particular plants and fungi, mitochondrial genomes have undergone similar fragmentation and genome expansion events. While the mechanism of expansion seems different in each case (fungi, plants, and cicada endosymbionts), we hypothesize that they are united in being a result—a syndrome, perhaps—of long-term obligate host association. It has been argued in mitochondrial cases that this is an example of non-adaptive genome expansion, and we favor that idea in our case, too. Collectively these systems are remarkable test cases to study complexity in biology that arises not because it makes the organism more fit, but simply because the organism cannot stop it from happening.

My intention for the next year of my NAI project is to turn towards finding the cell biological mechanisms that are hinted at by our genomic findings. I believe that this is the most interesting question in my field, that it has broad implications that fit squarely into the Astrobiology Roadmap Objectives, and that the NAI structure is the ideal place to do it. My plan is to advertise for a eukaryotic cell biologist in the next couple of weeks, e.g.:

EVOLUTIONARY CELL BIOLOGIST. The McCutcheon lab at the University of Montana seeks a postdoctoral fellow interested in evolutionary cell biology. Our lab values diversity in thought, experience, and personel, and as such individuals from many different backgrounds are appropriate for this position. However, experience in eukaryotic cell biology and a willingness to work in non-model insect systems are required. An ideal candidate would have experience with immunologically based electron or fluorescence microscopy. The position will remain open until filled, but could start immediately.

Other accomplishments of the year included co-editing a special issue of PNAS on the evolution of organelles and endosymbionts (resulting from the National Academy of Sciences Sackler Colloquium, Symbioses Becoming Permanent: The Origins and Evolutionary Trajectories of Organelles); recruiting an NPP fellow, Stephanie Weldon, to the lab; and the award of an NSF CAREER grant.

NPP fellow Weldon is working on a project with strong ties to the NAI. Among other things, she is working to “reboot” an insect-bacterial endosymbiosis with an environmental bacteria in order to perform experimental evolution experiments on an endosymbiosis.

    John McCutcheon John McCutcheon
    Project Investigator
    Objective 4.2
    Production of complex life.

    Objective 5.2
    Co-evolution of microbial communities

    Objective 6.2
    Adaptation and evolution of life beyond Earth