Transcriptomic signature of differentiating catshark cartilage unravels the co-evolution of the Spp2 gene family with skeletal mineralisation in cartilaginous fish

BACKGROUND: Skeletal mineralisation, achieved through the controlled precipitation of calcium phosphate, represents a major evolutionary innovation in early vertebrates. While this process has been extensively studied in bony species, it remains poorly understood in cartilaginous fishes, which lack endochondral bone. In this study, the small-spotted catshark (Scyliorhinus canicula) was used as a model to investigate the genetic basis of cartilage differentiation and subsequent mineralisation. RESULTS: Two stages in late embryogenesis were examined: one preceding and one following the onset of vertebral mineralisation. Using a bulk RNA sequencing approach, we identified a set of genes characterising the early differentiation of chondrocytes and cartilage synthesis, shared among jawed vertebrates. Strikingly, only a very limited number of genes previously identified in mammalian skeletogenesis showed upregulated expression in the catshark mineralising chondrocytes. Among these, particular attention was given to the spp2 (secreted phosphoprotein 2) gene family. Unlike in bony vertebrates, where spp2 exists as a single and rather little-studied gene, cartilaginous fishes possess multiple gene duplicates with contrasted sites of expression. Through phylogenetic analyses and gene expression studies in the catshark, we showed how, despite the presence of shared molecular components likely inherited from their last jawed vertebrate ancestor, cartilaginous and bony fishes have independently evolved distinct cartilage mineralisation strategies. CONCLUSIONS: This work highlights the diversity of skeletal development mechanisms and underscores the importance of cartilaginous fishes in broadening our understanding of how vertebrate mineralisation evolved. By contrasting conserved and lineage-specific features, the study provides new insights into the independent trajectories that shaped the skeletal structures of modern vertebrates.