Abstract

The backbones of land vertebrates (tetrapods) are subdivided into up to five distinct regions (cervical, thoracic, lumbar, sacral and caudal), with vertebrae that are different from one another in shape and are patterned by the expression of Hox genes in early embryos. Conversely, the backbones of fishes are thought to have fewer regions, consisting of just a trunk and tail, and the relationship of Hox genes to vertebrae in fishes is unclear. To understand how backbone shape has evolved across fishes and how Hox genes might have patterned vertebrae outside of tetrapods, I used quantitative methods to compare vertebrae along the body and studied Hox gene expression in a cartilaginous fish, the little skate. I found that fish have more vertebral regions than previously appreciated, and that Hox genes were likely responsible for developing these regions in the earliest jawed vertebrates.