Comparative genomic analysis of a photosymbiont evolution

The unicellular green alga Oophila amblystomatis engages in a resource-exchange mutualism with egg masses of several amphibians. These algae gain access to the inside of gelatinous egg masses and grow inside individual egg capsules, resulting in a (nearly) monophyletic algal population in direct contact with the developing embryo. In egg capsules of the spotted salamander Ambystoma maculatum some cells of O. amblystomatis also invade embryonic tissues and cells where they persist in an altered transcriptional state for an unknown duration. The purpose of this unusual cellular arrangement is not known and is unique among vertebrates. We conducted whole genome sequencing of O. amblystomatis to support further inquiry into the biological basis of this mutualism. Our long-term goal is to identify and characterize changes to the structure and content of the genome of Oophila amblystomatis that are associated with the evolution of a mutualistic association with amphibian egg masses from free-living ancestors. To facilitate comparisons and to establish the polarity of evolutionary transformations the genomes of Chlamydomonas nastuta and Chlamydomonas pseudogloegama (both in the Oophila clade) and Chlamydomonas moewusii (a closely related outgroup) were also sequenced. It is anticipated that an analysis of these four genome sequences will provide new insights into the evolution of this mutualism specifically, and that of symbioses generally.