Although hagfish have no jaws and no vertebrae, they do have other characters that identify them as fish. These traits make them an interesting candidate for being in some way transitional between chordates and vertebrates. Research has not been helped by the extreme difficulty of breeding these animals in captivity. A recent study has, however, succeeded in analyzing hagfish embryos and late-stage specimens have revealed the presence of a neural crest, a distinctive characteristic of vertebrates. The neural crest is a remarkable tissue that differentiates into a great variety of specialist cells, and is consequently very important for developmental biologists.
From an evolutionary perspective, this implies that the common ancestor of vertebrates would have possessed a neural crest. The authors suggest that "the neural crest probably existed as a population of delaminating and migrating cells in the common ancestor of the entire vertebrate clade, and thus its origin should be sought in non-vertebrate chordates."
Where do these animals fit into our understanding of living things? Are they (as many like to think) primitive vertebrates? This question has not been answered by palaeontology, because the earliest hagfishes are very similar to living forms. Two possibilities have been suggested: they could be primitive vertebrates (lacking vertebrae and jaws), or they could be degenerate organisms after losing several vertebrate characters (an option which also gets support from DNA and RNA sequencing). These possibilities are discussed by Janvier in a useful "News & Views" essay. In his judgment, "with this discovery, hagfishes become more 'conventional' vertebrates". The issues now are clearer than they were. But the big question remains unresolved. "Further analyses of the developmental genetics of hagfish embryos might enable us to discover whether hagfish anatomy is primitive or degenerate, and may help in reconstructing the theoretical common ancestor to all vertebrates."
At present, it is necessary to acknowledge that this theoretical "common ancestor to all vertebrates" is highly elusive. The search itself is theory-driven: based on the conviction that there must be a common ancestor. The authors say that their work "opens up new approaches to clarifying the evolutionary history of vertebrates", and this is true. However, should we adopt the restrictive approach that insists on a common ancestor in a Darwinian sense? Is there not scope here for an information-perspective on the basic types of life?
Hagfish embryology with reference to the evolution of the neural crest
Kinya G. Ota, Shigehiro Kuraku & Shigeru Kuratani
Nature 446, 672-675 (5 April 2007) | doi:10.1038/nature05633
Hagfish, which lack both jaws and vertebrae, have long been the subject of intense interest owing to their position at a crucial point in the evolutionary transition to a truly vertebrate body plan1, 2, 3, 4. However, unlike the comparatively well characterized vertebrate agnathan lamprey, little is known about hagfish development. The inability to analyse hagfish at early embryonic stages has frustrated attempts to resolve questions with important phylogenetic implications, including fundamental ones relating to the emergence of the neural crest1, 5, 6. Here we report the obtainment of multiple pharyngula-stage embryos of the hagfish species Eptatretus burgeri and our preliminary analyses of their early development. We present histological evidence of putative neural crest cells, which appear as delaminated cells that migrate along pathways corresponding to neural crest cells in fish and amphibians2, 7, 8, 9, 10, 11. Molecular cloning studies further revealed the expression of several regulatory genes, including cognates of Pax6, Pax3/7, SoxEa and Sox9, suggesting that the hagfish neural crest is specified by molecular mechanisms that are general to vertebrates. We propose that the neural crest emerged as a population of de-epithelialized migratory cells in a common vertebrate ancestor, and suggest that the possibility of classical and molecular embryology in hagfish opens up new approaches to clarifying the evolutionary history of vertebrates.
Janvier, P. Born-again hagfishes, Nature 446, 622-623 (5 April 2007) | doi:10.1038/nature05712
Abstract: The strange, slimy creatures called hagfishes are of abiding interest to students of vertebrate evolution: just where do they fit in? Investigations of hagfish development take the story forward.
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