COLE, A.G.; HALL, B.K.: Non-vertebrate cartilage: A phylogenetic analysis

Cartilage is often thought of as solely a vertebrate tissue, although cartilage-like tissue is found in a number of species, from a diversity of groups [eg. molluscs, sabellid polychaetes, and arthropods]. The relationship between these tissues and vertebrate cartilage is not clear, although I will discuss distinct similarities in macromolecular constituents, cellular and extracellular components, and histological appearance of the cartilaginous tissues in cephalopod molluscs and sabellid polychaetes using techniques similar to those used to study vertebrate cartilage. Despite the widespread occurrence of cartilage-like tissues in many groups, little is known of their phylogenetic relationships. Addressing issues of homology, such as that of vertebrate and non-vertebrate cartilaginous tissues, requires the use of robust phylogenetic hypotheses, however, the status of metazoan phylogeny has been in a state of flux since the rise of molecular data. To evaluate the current status of metazoan phylogeny, I will present a supertree created using matrix representation with parsimony, from phylogenetic hypotheses based upon molecular data. The distribution of different features of cartilage-like tissues will then be examined in the context of the phylogeny proposed by the supertree analysis. One of the most interesting observations to be made from this analysis is the presence of cartilage-like tissues in all 3 major bilaterian groups: Deuterostomia [Chordata and Hemichordata], Lophotrochozoa [Bryozoa, Mollusca, and Annelida], and Ecdysozoa [Arthropoda]. The presence of cartilage-like tissue in the mesoglea of some members of Cnideria, sister group to the bilaterians, suggests that the ability to make a tissue that is structurally very similar to cartilage appeared early in metazoan evolution.