Tunicates and not cephalochordates are the closest living relatives of vertebrates

Tunicates or urochordates (appendicularians, salps and sea squirts), cephalochordates (lancelets) and vertebrates (including lamprey and hagfish) constitute the three extant groups of chordate animals. Traditionally, cephalochordates are considered as the closest living relatives of vertebrates, with tunicates representing the earliest chordate lineage. This view is mainly justified by overall morphological similarities and an apparently increased complexity in cephalochordates and vertebrates relative to tunicates. Despite their critical importance for understanding the origins of vertebrates, phylogenetic studies of chordate relationships have provided equivocal results. Taking advantage of the genome sequencing of the appendicularian Oikopleura dioica, we assembled a phylogenomic data set of 146 nuclear genes (33,800 unambiguously aligned amino acids) from 14 deuterostomes and 24 other slowly evolving species as an outgroup. Here we show that phylogenetic analyses of this data set provide compelling evidence that tunicates, and not cephalochordates, represent the closest living relatives of vertebrates. Chordate monophyly remains uncertain because cephalochordates, albeit with a non-significant statistical support, surprisingly grouped with echinoderms, a hypothesis that needs to be tested with additional data. This new phylogenetic scheme prompts a reappraisal of both morphological and palaeontological data and has important implications for the interpretation of developmental and genomic studies in which tunicates and cephalochordates are used as model animals.

Common and divergent pathways in alternative developmental processes of ascidians

Colonial ascidians offer opportunities to investigate how developmental events are integrated to generate the animal form, since they can develop similar individuals (oozooids from eggs, blastozooids from pluripotent somatic cells) through very different reproductive processes, i.e. embryogenesis and blastogenesis. Moreover, thanks to their key phylogenetic position, they can help in the understanding of the molecular mechanisms of morphogenesis and their evolution in chordates. We review organogenesis of the ascidian neural complex comparing embryos and buds in terms of topology, developmental mechanisms and terminology. We propose a new interpretation of bud territories, and reconsider nervous system development based on recent results suggesting that ascidians have vertebrate placodal and neural-crest-like cells. Comparing embryonic and blastogenic development in Botryllus schlosseri, we propose that the bud has territories with a placodal potentiality, suggesting that chordate ancestors possessed neurogenic placodes, and that the genetic pathways regulating neurogenic placode formation were co-opted for new developmental processes, such as blastogenesis.

Evolution and development of the chordates: collagen and pharyngeal cartilage

Chordates evolved a unique body plan within deuterostomes and are considered to share five morphological characters, a muscular postanal tail, a notochord, a dorsal neural tube, an endostyle, and pharyngeal gill slits. The phylum Chordata typically includes three subphyla, Cephalochordata, Vertebrata, and Tunicata, the last showing a chordate body plan only as a larva. Hemichordates, in contrast, have pharyngeal gill slits, an endostyle, and a postanal tail but appear to lack a notochord and dorsal neural tube. Because hemichordates are the sister group of echinoderms, the morphological features shared with the chordates must have been present in the deuterostome ancestor. No extant echinoderms share any of the chordate features, so presumably they have lost these structures evolutionarily. We review the development of chordate characters in hemichordates and present new data characterizing the pharyngeal gill slits and their cartilaginous gill bars. We show that hemichordate gill bars contain collagen and proteoglycans but are acellular. Hemichordates and cephalochordates, or lancelets, show strong similarities in their gill bars, suggesting that an acellular cartilage may have preceded cellular cartilage in deuterostomes. Our evidence suggests that the deuterostome ancestor was a benthic worm with gill slits and acellular gill cartilages.

Historical introduction, overview, and reproductive biology of the protochordates

This issue of the Canadian Journal of Zoology exhaustively reviews most major aspects of protochordate biology by specialists in their fields. Protochordates are members of two deuterostome phyla that are exclusively marine. The Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with echinoderms and appear to be closely related, but they also have many chordate-like features. The invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates with both solitary and colonial forms. The cephalochordates are all free-swimming, but the tunicates include both sessile and free-swimming forms. Here I explore the history of research on protochordates, show how views on their relationships have changed with time, and review some of their reproductive and structural traits not included in other contributions to this special issue.