Analysis of skate fin development by RNA-seq. (A) Differential expression of transcripts assembled from RNA-seq of anterior and posterior fin tissue specimens of stage 30 skates. Differential expression score is derived from multiple A–P comparisons of transcript abundance (n = 3; see SI Appendix). Above the red dotted line corresponds to P values less than 0.05. Blue, 5′Hox genes; green, Fgf genes; purple, Gli3; red, 3′Hox genes. (B) Standardized read counts (z-scores) for selected transcripts in the pectoral fin at three developmental stages and locations of the pectoral fin. Transcripts were median-summarized according to their annotations. A, anterior fin; C, center fin; P, posterior fin. Sample developmental stages are listed at the top, and clustering is based on relative expression levels. (C–H) Whole-mount in situ hybridization for canonical tool kit genes for limb development at stage 30. (C) Fgf8. The expression is at the posterior ectoderm. (D) Fgf10. The expression can be seen at the posterior mesenchyme. (E) Gremlin. (F) Hoxd10. (G) Hoxd12. (H) Hoxd13. Note that all expression patterns are limited to the posterior half, not in the anterior fin.