Hemichordate phylogeny: a molecular and genomic approach

Abstract

The phylogenetic relationships within Hemichordata are significant for understanding the evolution of the deuterostomes. Hemichordates possess several important morphological structures in common with chordates, and they have been fixtures in hypotheses on chordate origins for over 100 years. However, current evidence points to a sister relationship between echinoderms and hemichordates, indicating that these chordate-like features were likely present in the last common ancestor of these groups. Therefore, Hemichordata should be highly informative for studying deuterostome character evolution. Despite their importance for understanding the evolution of chordate-like morphological and developmental features, relationships within hemichordates have been poorly studied. At present, Hemichordata is divided into two classes, the solitary, free-living enteropneust worms, and the colonial, tube- dwelling Pterobranchia. The objective of this dissertation is to elucidate the evolutionary relationships of Hemichordata using multiple datasets. Chapter 1 provides an introduction to Hemichordata and outlines the objectives for the dissertation research. Chapter 2 presents a molecular phylogeny of hemichordates based on nuclear ribosomal 18S rDNA and two mitochondrial genes. In this chapter, we suggest that deep-sea family Saxipendiidae is nested within Harrimaniidae, and Torquaratoridae is affiliated with Ptychoderidae. Next, in Chapter 3, using a gene genealogy approach, we reveal a previously unknown clade of at least four species of harrimaniid enteropneusts from cold waters, including Antarctica, the North Atlantic around Iceland and Norway, and the deep sea off Oregon. Chapter 4 presents a phylogenomics approach to relationships within Ambulacraria (Hemichordata + Enteropneusta). Our results strongly support reciprocal monophyly of Pterobranchia and Enteropneusta, and Asterozoa (Asteroidea + Ophiuroidea). Lastly, Chapter 5 provides conclusions, synthesis of the preceding chapters, and future directions.