Fig. 4: The evolution of spines.

a Left, Species tree displaying the independent evolution of spines in seahorses. The branch length indicates number of substitutions per site. Four spiny seahorse species are highlighted in blue. Thicker branches correspond to higher rates of nonsynonymous-to-synonymous substitutions (dN/dS) for bmp3 gene. Canonical and generalized McDonald and Kreitman test (MKT) for bmp3 gene was performed for three pairwise sister species with divergent spiny and non-spiny features highlighted by background colors, whose significance levels were indicated by p value with blue and red font, respectively. Right, comparison of amino acid substitutions in bmp3 protein, polymorphic and fixed substitutions in spiny seahorses are indicated with red and blue circles, respectively. b Distribution of dN/dS values in bmp3 in spiny seahorses compared to non-spiny species. c Independent evolution in the phylogenetic tree reconstructed for the protein encoded by bmp3. Seahorses illustrations by Geng Qin. d Whole-mount in situ hybridization of bmp3 in Hippocampus erectus. In situ photos of seahorses by Ralf F. Schneider. Source data are provided as a Source Data file.