Meeting Abstract
An axial column with segmented vertebrae is a key feature of the vertebrate body plan, but the diversity of developmental mechanisms that give rise to the axial column remains poorly understood. Vertebral elements derive from a series of mesodermal segments called somites. In tetrapods, somites are polarized, with adjacent somite halves recombining to form a single vertebra through a process known as “resegmentation”. However, in teleost fishes, strict resegmentation is less apparent, with cells from multiple somites giving rise to a single vertebral half. To determine whether the process of resegmentation is ancestral for jawed vertebrates, we tested the relationship between somites and vertebrae in an outgroup to the bony fishes – a cartilaginous fish, the little skate (Leucoraja erinacea). We first demonstrate that skates and tetrapods share molecular markers of somite polarity. Next, using cell lineage tracing approaches, we show that the anterior and posterior halves of single skate trunk vertebrae derive from adjacent somites – a condition reminiscent of tetrapod resegmentation. Interestingly, the tails of cartilaginous fishes exhibit a condition known as diplospondyly, in which two vertebral units correspond with a single body segment. Lineage tracing of adjacent skate tail somites points to resegmentation in this region as well, though with each somite giving rise to half of one vertebra, an entire adjacent vertebra, and half of a third vertebra. These data demonstrate that the developing skate axial skeleton undergoes tetrapod-like resegmentation, that this process occurs across the entire axial skeleton (regardless of the mono- or diplospondylous conditions of the trunk and tail, respectively), and that resegmentation is likely an ancestral feature of the vertebrate axial skeleton.
