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Mutualistic microalgae co-diversify with reef corals that acquire symbionts during egg development


The application of molecular genetics has reinvigorated and improved how species are defined and investigated scientifically, especially for morphologically cryptic micro-organisms. Here we show how species recognition improves understanding of the ecology and evolution of mutualisms between reef-building corals and their mutualistic dinoflagellates (i.e. Symbiodiniaceae). A combination of genetic, ecological, and morphological evidence defines two sibling species of Cladocopium (formerly Symbiodinium Clade C), specific only to host corals in the common genus Pocillopora, which transmit their obligate symbionts during oogenesis. Cladocopium latusorum sp. nov. is symbiotic with P. grandis/meandrina while the smaller-celled C. pacificum sp. nov. associates with P. verrucosa. Both symbiont species form mutualisms with Pocillopora that brood their young. Populations of each species, like their hosts, are genetically well connected across the tropical and subtropical Pacific Ocean, indicating a capacity for long-range dispersal. A molecular clock approximates their speciation during the late Pliocene or early Pleistocene as Earth underwent cycles of precipitous cooling and warming; and corresponds to when their hosts were also diversifying. The long temporal and spatial maintenance of high host fidelity, as well as genetic connectivity across thousands of kilometers, indicates that distinct ecological attributes and close evolutionary histories will restrain the adaptive responses of corals and their specialized symbionts to rapid climate warming.

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Fig. 1: Collection locations, light micrographs, and cell sizes of symbiotic dinoflagellates from corals in the genus Pocillopora.
Fig. 2: Phylogenetic and population genetic data resolving two species of Cladocopium.
Fig. 3: High-resolution phylogenetic analysis of Cladocopium latusorum and C. pacificum.
Fig. 4: Age estimates for the co-diversification of Cladocopium with their pocilloporid hosts.


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The authors wish to thank Jorge Pinzon, Andie Chan, Chaolun Allen Chen, Linda Tonk, Iliana Baums, Rachel Silverstein, Sarah Davies, David Suggett’s lab, and Adrienne Correa’s lab, who contributed Pocillopora samples, as well as Ove Hoegh-Guldberg and Allison M. Lewis for photos of Pocillopora. We are grateful to Allison M. Lewis and Scott R. Santos and for assistance with data analysis. We are grateful for the valuable feedback from Adrienne Correa and two anonymous reviewers, which improved the paper. We would like to also thank Palau International Coral Reef Center (PICRC) for providing support during sample collections, which were made possible with permission from the Ministry of Natural Resources, Environment and Tourism, Palau, and from the Koror State Government, Department of Conservation and Law Enforcement. This project was supported by the USA National Science Foundation (IOS-1258058 and OCE-1636022 to TCL), NTRGP Australian Biodiversity Resources Study (ID 4-EHOJ1F5) to ES and TCL, the Society of Systematic Biologists (to KET), and the Pennsylvania State University.

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Correspondence to Kira E. Turnham or Todd C. LaJeunesse.

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Turnham, K.E., Wham, D.C., Sampayo, E. et al. Mutualistic microalgae co-diversify with reef corals that acquire symbionts during egg development. ISME J (2021).

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