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This study presents a chromosome-level genome of the corallimorpharian Ricordea yuma, demonstrating through genomic analyses that corallimorpharians are the sister group to stony corals and were never ancestrally skeletonized. Using single-cell transcriptomics from both corallimorpharian and stony coral species, researchers discovered that coral calcification evolved through the emergence of a specialized cell type called calicoblasts in stony corals, supported by both new genes and co-opted ancestral components. The findings reveal that coral skeleton formation represents a major evolutionary transition involving coordinated changes in genome structure, gene content, and cellular identity.
Why it matters
Understanding how corals evolved their calcium carbonate skeletons is fundamental to comprehending reef ecosystem development and may inform conservation strategies for protecting modern coral reefs facing climate change. The identification of specific genes and cell types responsible for calcification could potentially aid in predicting coral responses to ocean acidification and warming.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
The production of calcified skeletons by stony corals is the cornerstone of reef ecosystems and the persistence of marine biodiversity. Corallimorpharians, sometimes described as naked corals, resemble stony corals in many respects but lack hardened skeletons. However, whether they are stony corals that secondarily lost their skeletons has been a subject of much debate. Here, we present a chromosome-level genome of the corallimorpharian Ricordea yuma and show that macrosynteny and microsynteny analyses provide strong support for corallimorpharians as the sister group to stony corals. This phylogenetic placement indicates that corallimorpharians were not ancestrally skeletonized and therefore occupy a critical outgroup position for reconstructing the origin of coral calcification. Guided by this phylogenetic context, we sequenced whole-polyp single-cell transcriptomes from R. yuma and the complex coral Galaxea fascicularis and found that the scleractinian skeleton evolved through the emergence of a derived calicoblast cell state in stony corals. A combination of comparative genomics and regeneration experiments in G. fascicularis show that the emergence of the calicoblast cell state was accompanied by the evolution of a biomineralization toolkit assembled from lineage-specific innovations and co-opted ancestral components. Although corallimorpharians retain some ancestral genes later incorporated into coral biomineralization, they lack a detectable calicoblast-like transcriptional state and many core calcification genes. Together, our findings indicate that coral calcification arose through coordinated changes in genome architecture, gene repertoire, and cell identity, highlighting this process as a major evolutionary transition driven by the integration of genomic and cellular innovations.