Coral-associated Symbiodiniaceae exhibit host specificity but lack phylosymbiosis, with Cladocopium and Durusdinium showing different cophylogenetic patterns.

Altering the composition of the Symbiodiniaceae community to adapt to anomalous sea water warming represents a potential survival mechanism for scleractinian corals. However, the processes of Symbiodiniaceae assembly and long-standing evolution of coral-Symbiodiniaceae interactions remain unclear. Here, we utilized ITS2 (internal transcribed spacer 2) amplicon sequencing and the SymPortal framework to investigate the diversity and specificity of Symbiodiniaceae across 39 scleractinian coral species. Furthermore, we tested phylosymbiosis and cophylogeny between coral hosts and their Symbiodiniaceae. In our study, environmental samples exhibited the highest Symbiodiniaceae diversity. Cladocopium and Durusdinium dominated the Symbiodiniaceae communities, with significant β-diversity differences among coral species. Additionally, host specificity was widespread in Symbiodiniaceae, especially Durusdinium spp., yet lacked a phylosymbiotic pattern. Moreover, Cladocopium spp. showed cophylogenetic congruence with their hosts, while there was no evidence for Durusdinium spp. Furthermore, host switching was the predominant evolutionary event, implying its contribution to Cladocopium diversification. These findings suggest that Symbiodiniaceae assembly does not recapitulate host phylogeny, and host specificity alone does not drive phylosymbiosis or cophylogeny. As environmental reservoirs, free-living Symbiodiniaceae may influence symbiotic communities. Additionally, Durusdinium-coral associations lack cophylogenetic signals, indicating more flexible partnerships than Cladocopium. Overall, our results enhance understanding of Symbiodiniaceae assembly and coral-Symbiodiniaceae evolutionary interactions.