9The vertebral skeleton is a defining feature of vertebrate animals. However, the mode of 10 vertebral segmentation varies considerably between major lineages. In tetrapods, adjacent 11 somite halves recombine to form a single vertebra through the process of "resegmentation".
12However, in teleost fishes, there is considerable mixing between cells of the anterior and 13 posterior somite halves, without clear resegmentation. To determine whether resegmentation is 14 a tetrapod novelty, or an ancestral feature of jawed vertebrates, we tested the relationship 15 between somites and vertebrae in a cartilaginous fish, the skate (Leucoraja erinacea). Using cell 16 lineage tracing, we show that skate trunk vertebrae arise through tetrapod-like resegmentation, 17 with anterior and posterior halves of each vertebra deriving from adjacent somites. We further
18show that tail vertebrae also arise through resegmentation, despite a duplication of the number 19 of vertebrae per body segment. These findings resolve axial resegmentation as an ancestral 20 feature of the jawed vertebrate body plan. 21 22 24 25 30 segmentation of paraxial mesoderm into epithelial blocks called somites (Figure 1a). Cells from 31 the ventromedial portion of each somite then undergo an epithelial to mesenchymal transition 32 and migrate around the notochord and neural tube, where they condense into vertebrae. 33 34 2 Cell lineage tracing studies in tetrapods have shown that there is not a 1:1 correspondence 35 between somites and vertebrae. Rather, cells from adjacent somite halves recombine to give 36 rise to a single vertebra, through a process known as "resegmentation" (Remak, 1855; Verbout, 37 1976). Somite lineage tracing in chick using chick-quail chimeras or lipophilic dyes (Aoyama and Ward et al., 2017) has shown that cells from the rostral half of one somite 40 combine with cells from the caudal half of the adjacent somite to give rise to a single vertebra, 41 with sharp compartment boundaries in the middle of vertebrae reflecting original somite 42 boundaries. Additionally, lineage tracing experiments in axolotl using injections of fluorescent 43 dextrans or grafts of GFP+ somites into wildtype hosts point to conservation of resegmentation 44 during vertebral development in lissamphibians (Piekarski and Olsson, 2014). However, in 45 teleost fishes, resegmentation is less apparent, with cells from adjacent somite halves 46 undergoing substantial mixing, resulting in vertebrae without clear lineage-restricted 47 4 that in d), one set of spinal nerves spans two vertebrae. Scale bars: a) 400 ”m; b) 100 ”ml c) 59 500 ”m; d) and e) 100 ”m. 60 61 The vertebral skeleton in cartilaginous fishes consists of a series of neural and intercalary 62 arches and neural spines, tri-layered centra and haemal arches (the latter restricted to the 63 caudal region -Figure 1b) (Criswell et al., 2017a). Notably, the axial skeleton of the embryonic 64 skate forms initially as a continuous, sclerotome-derived cartilaginous tube, which subsequently 65 subdivides into...