Looking for the deuterostome ancestor: Hemichordate phylogeny, cartilage development, and regeneration

Hemichordates are deuterostomes, a group of animals that also includes xenoturbellids, echinoderms, tunicates, cephalochordates, and vertebrates. As the name suggests, hemichordates have some, but not all chordate characteristics, the most recognized being pharyngeal gill slits. This work investigates the origin of deuterostomes by studying evolutionary developmental transitions that occurred in the morphology between hemichordates, cephalochordates, tunicates and vertebrates. First, I characterized the structure and development of the pharyngeal slits and associated cartilages in a solitary hemichordate worm. Developmental origin and gene expression in the pharyngeal endoderm suggest that pharyngeal slits are homologous across deuterostomes, and hence likely present in the deuterostome ancestor. The slits and the cartilaginous skeleton that lies between are strikingly similar in hemichordates and cephalochordates. Like vertebrate cartilage, the hemichordate and cephalochordate structures contain fibrillar collagen; however, they are entirely or partially secreted by the pharyngeal endoderm, rather than neural crest-derived chondrocytes that secrete pharyngeal cartilage in vertebrates. To further study evolution of deuterostome and chordate characteristics, I constructed a phylogeny of hemichordates using molecular markers to identify whether the ancestral form of hemichordates was solitary or colonial. In this phylogeny, colonial pterobranchs are not ancestral as traditionally placed. Instead, they are sister to the solitary, direct-developing harrimaniids. This relationship suggests that the deuterostome ancestor was a solitary, filter-feeding worm. In addition to pharyngeal slits, some anterior structures in hemichordates may be homologous to chordate features. Anterior regeneration following amputation can be used to examine the development and homology of these structures. First, however, the basic cellular properties of hemichordate regeneration must be established. To this end, I examined the process of regeneration in the hemichordate Ptychodera flava. The process involved wound healing, followed by programmed cell death primarily in the endoderm, and proliferation of putative stem cells in the mesenchyme and cells in the ectoderm and endoderm as new anterior structures are elaborated. This study of Plychodera flava regeneration provides a foundation for further development of a potentially powerful new stem cell and regeneration model in a close relative to vertebrates.