Systematic Revision of Symbiodiniaceae Highlights the Antiquity and Diversity of Coral Endosymbionts

TLDR

Molecular data have transformed the study of life, revealing that dinoflagellate endosymbionts powering stony corals are diverse, not a single species, but a rich array of clades, yet formal systematics have lagged, necessitating a major revision. The authors propose that evolutionarily divergent Symbiodinium clades are equivalent to genera in Symbiodiniaceae and provide formal descriptions for seven of them. They base this proposal on molecular, morphological, physiological, and ecological data. The authors recalibrate the molecular clock, placing the earliest diversification of Symbiodiniaceae at ~160 Mya in the middle Mesozoic, aligning with the Jurassic radiation of shallow‑water stony corals and highlighting the long evolutionary history of these symbionts.

Abstract

The advent of molecular data has transformed the science of organizing and studying life on Earth. Genetics-based evidence provides fundamental insights into the diversity, ecology, and origins of many biological systems, including the mutualisms between metazoan hosts and their micro-algal partners. A well-known example is the dinoflagellate endosymbionts ("zooxanthellae") that power the growth of stony corals and coral reef ecosystems. Once assumed to encompass a single panmictic species, genetic evidence has revealed a divergent and rich diversity within the zooxanthella genus Symbiodinium. Despite decades of reporting on the significance of this diversity, the formal systematics of these eukaryotic microbes have not kept pace, and a major revision is long overdue. With the consideration of molecular, morphological, physiological, and ecological data, we propose that evolutionarily divergent Symbiodinium "clades" are equivalent to genera in the family Symbiodiniaceae, and we provide formal descriptions for seven of them. Additionally, we recalibrate the molecular clock for the group and amend the date for the earliest diversification of this family to the middle of the Mesozoic Era (∼160 mya). This timing corresponds with the adaptive radiation of analogs to modern shallow-water stony corals during the Jurassic Period and connects the rise of these symbiotic dinoflagellates with the emergence and evolutionary success of reef-building corals. This improved framework acknowledges the Symbiodiniaceae's long evolutionary history while filling a pronounced taxonomic gap. Its adoption will facilitate scientific dialog and future research on the physiology, ecology, and evolution of these important micro-algae.