Can an original be found? Mitochondrial species identity does not predict nuclear genome similarity in the photosymbiotic jellyfish Cassiopea andromeda and C. xamachana

The Upside-Down Jellyfish, Cassiopea, is a popular subject in cnidarian photosymbiosis research. Two closely related species, C. xamachana and the introduced C. andromeda, are commonly utilized in laboratory research across America and Europe. Researchers have now sequenced the genomes of 21 Cassiopea medusae from the Florida Keys, Bocas del Toro in Panama, and a laboratory line in Europe to determine if assigning mitochondrial species predicts nuclear genome similarity.

Surprisingly, medusae from Florida carrying either C. xamachana or C. andromeda mitochondria types cannot be distinguished from one another based on their nuclear genomes, and geographic location is the primary factor influencing nuclear genetic structure. A population tree that groups the Floridian mitotypes as a single interbreeding unit is strongly supported, while a tree that respects mitochondrial species boundaries is rejected.

The European C. andromeda laboratory line, however, clusters with Panamanian C. xamachana rather than with Floridian C. andromeda-mitotype animals.

The study also reveals evidence of variability in the symbiont communities of the hosts in Panama and presents two nearly-complete genomes of the Tenacibaculum and Endozoicomonas organisms. Collectively, these findings suggest that the hybridization zone between C. andromeda and C. xamachana may span across ocean basins, making it difficult to find a pure representative of either species. The researchers recommend that Cassiopea researchers establish new laboratory lines with well-documented genomes to avoid this issue.