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Gill arch axial patterning, serial homology and the origin of jawed vertebrates

The jawed vertebrate body plan is defined largely on the basis of two anatomical features: jaws and paired appendages. In the late 19th century, Carl Gegenbaur proposed that both jaws and paired fins were derived members of a primitive series of gill arches. These controversial hypotheses of serial homology were based largely on the pharyngeal endoskeletal anatomy of chondrichthyan fishes. We are using comparative transcriptomic and experimental embryological approaches to investigate and compare jaw, gill arch and paired fin axial patterning mechanisms in skate embryos. This research program will yield insight into whether similarities in endoskeletal organization that led Gegenbaur to propose gill arch origins of jaws and fins reflect constraints imposed by common developmental mechanisms (i.e. serial homology), or rather convergent evolution, by bringing modern molecular developmental approaches to bear on a century-old controversy in vertebrate evolutionary biology.


Shark head skeleton illustrating the putative serial homology of the jaw, gill arch and paired fin skeleton. Modified from Owen (1866).

Fate mapping the chondrichthyan neural crest

The neural crest is an embryonic cell population that gives rise to much of the vertebrate craniofacial skeleton, and its evolutionary origin is generally regarded as a key step in the diversification of vertebrates. Neural crest fate maps have been generated for a number of osteichthyan model systems (e.g. mouse, chick, frog and zebrafish). However, nothing is known about the fates of neural crest cells in chondrichthyans. We have developed methods for long-term lineage tracing of cell populations in early skate embryos, and we are using these methods to generate fate maps of chondrichthyan cranial and trunk neural crest cells. This work will allow us to infer primitive fates of neural crest cells in the last common ancestor of jawed vertebrates (e.g. neural crest vs. mesodermal contributions to the craniofacial skeleton and pectoral girdle, and the skeletogenic potential of trunk neural crest cells), thereby resolving a number of outstanding controversies relating to the early evolution of the jawed vertebrate skeleton.


Neurula-stage skate embryo (anterior to the left) with DiI-labeled trunk neural tube (magenta).

Development, growth and repair of chondrichthyan cartilage

Adult chondrichthyans possess an endoskeleton that is composed entirely of hyaline cartilage, and this skeleton grows indefinitely throughout life. This is unlike adult mammalian hyaline cartilage (e.g. articular cartilage), which ceases growth at adolescence, and also has very limited capacity for repair. We are investigating the cellular and molecular mechanisms underlying cartilage development and growth in chondrichthyans, to determine whether cartilage growth occurs by extracellular matrix expansion, proliferation of differentiated chondrocytes or recruitment of chondroprogenitors from the perichondrium - and if the latter, whether chondroprogenitors may also be recruited to repair cartilage at sites of injury. This work will establish chondrichthyans as a model for hyaline cartilage development, indeterminate cartilage growth, and - perhaps uniquely among vertebrates - spontaneous cartilage repair.

Wholemount preparation of the cartilaginous pectoral fin endoskeleton of a bamboo shark embryo.