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

Area of Science:

• Marine Biology
• Molecular Ecology
• Evolutionary Biology

Background:

• Molecular genetics advances species definition, particularly for cryptic microorganisms.
• Understanding coral-dinoflagellate mutualisms is vital for reef ecology and evolution.
• The Symbiodiniaceae family, including Cladocopium, are key symbionts of reef-building corals.

Purpose of the Study:

• To define new species of Cladocopium symbionts in Pocillopora corals using integrated evidence.
• To investigate the ecological and evolutionary dynamics of these coral-algal mutualisms.
• To assess the impact of past climate change on symbiont speciation and host-symbiont co-evolution.

Main Methods:

• Combined genetic, ecological, and morphological analyses for species recognition.
• DNA sequencing and phylogenetic analysis to delineate Cladocopium species.
• Molecular clock dating to estimate speciation times.

Main Results:

• Defined two new sibling Cladocopium species: C. latusorum (with P. grandis/meandrina) and C. pacificum (with P. verrucosa).
• Identified oogenic transmission and high host fidelity in Pocillopora-Cladocopium mutualisms.
• Demonstrated genetic connectivity across the Pacific for both symbiont species and their hosts, with speciation during the Pliocene-Pleistocene.

Conclusions:

• New species definitions enhance understanding of coral-algal symbiosis.
• Co-speciation with Pocillopora hosts and long-term genetic connectivity suggest limited adaptive capacity to climate warming.
• Distinct ecological attributes and evolutionary histories of symbionts and hosts may constrain responses to environmental change.