Clustering of eight electric eel tissues by gene expression profile. Gene expression values for the eight tissues were normalized, variance filtered, log2-transformed and median-centered as described previously . Values shown are Euclidean distances based on ca. 6,000 genes passing the covariance filter, also indicated by blue shade (darker indicates shorter distance). Clustering was performed using complete linkage hierarchical clustering. Colored bars indicate a general grouping by tissue and cell type that is suggested by the data, with electric organ tissues (yellow) clustering most closely with skeletal and heart muscle (red). SPN = spinal cord; BRN = brain; KID = kidney; HRT = heart; SKM = skeletal muscle; HEO = Hunter’s EO; SEO = Sachs’ EO; MEO = main EO.