Key characters uniting hemichordates and chordates: homologies or homoplasies?

Abstract

Four chordate characters -dorsal hollow nerve cord, notochord, gill slits, and endostyle -are compared morphologically, molecularly, and functionally with similar structures in hemichordates to assess their putative homologies. The dorsal hollow nerve cord and enteropneust neurocord are probably homoplasies. The neurocord (= collar cord) may be an autapomorphy of Enteropneusta that innervates a unique pair of muscles, the perihemal coelomic muscles. Despite the apparent lack of organ-level homology, chordates and enteropneusts share a common pattern of neurulation that preserves a "contact innervation" between neuro-and myo-epithelia, which may be the primitive deuterostome pattern of neuromuscular innervation. The chordate notochord and hemichordate stomochord are probably homoplasies. Other potential notochord antecedents in hemichordates are examined, but no clear homolog is identified. The comparative morphology of notochords suggests that the "stack-of-coins" developmental stage, retained into adulthood only by cephalochordates, is the plesiomorphic notochord form. Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton, as occur in adults of Cephalodiscus spp., developmentally in some enteropneusts, and in many urochordates. Functional morphology, I 125 -binding experiments, and genetic data suggest that endostylar function may reside in the entire pharyngeal lining of Enteropneusta and is not restricted to a specialized midline structure as in chordates. A cladistic analysis of Deuterostomia, based partly on homologs discussed in this paper, indicates a sister-taxon relationship between Urochordata and Vertebrata, with Cephalochordata as the plesiomorphic clade.Résumé : Nous comparons quatre caractères des chordés -la corde nerveuse dorsale creuse, la notochorde, les fentes branchiales, l'endostyle -des points de vue morphologique, moléculaire et fonctionnel aux structures similaires chez les hémichordés afin d'évaluer les homologies putatives. La corde nerveuse dorsale creuse et la neurocorde des entéropneustes sont probablement des homoplasies. La neurocorde (= corde du collier) peut être une autapomorphie des entéropneustes qui innerve une paire spéciale de muscles, les muscles du coelome périhémal. Malgré l'absence apparente d'homologies entre les organes, les chordés et les entéropneustes possèdent en commun un même système de neurulation qui conserve une « innervation par contact » entre les épithéliums nerveux et musculaires, ce qui peut être un arrangement primitif de l'innervation neuromusculaire chez les deutérostomiens. La notochorde des chordés et la stomochorde des hémichordés sont probablement des homoplasies. Nous avons examiné d'autres antécédents potentiels de la notochorde chez les hémichordés, sans toutefois trouver d'homologie claire. La morphologie comparée des notochordes laisse croire que le stade de développement « en empilement de pièces de monnaie », retenu à l'état adulte seulement chez les cé...

Additional molecular support for the new chordate phylogeny

“…Inverting the phylogenetic position of tunicates and cephalochordates within monophyletic chordates highlights the prevalence of morphological simplification with characters that are likely ancestral for chordates, such as metameric segmentation, being lost secondarily in the tunicate lineage. On the other hand, the loss of preoral kidney and the presence of multiciliated epithelial cells might in fact constitute morphological synapomorphies for olfactores (Ruppert, 2005). The new chordate phylogeny further portrays tunicates as highly derived chordates with specialized lifestyles and developmental modes, whereas cephalochordates might have retained more ancestral chordate characteristics.…”

Molecular genetic insights into deuterostome evolution from the direct-developing hemichordateSaccoglossus kowalevskii

“…Given the uncertainty over the relationships of the major groups within the hemichordates, the possible homology of this extension to the pterobranch stalk, and even more controversially, the chordate post-anal tail, remains unresolved Swalla & Smith 2008 1996). Most of these are now rather weakly supported by both morphological and molecular datasets (Peterson et al 1999;Nieuwenhuys 2002;Lowe et al 2003Lowe et al , 2006Ruppert 2005). However, only gill slits, as primary ciliated pouches, may represent an ancestral feature of the deuterostomes (Ogasawara et al 1999;Okai et al 2000;Tagawa et al 2001;Cameron 2002;Rychel et al 2006;Rychel & Swalla 2007) and possibly the post-anal tail (Lowe et al 2003).…”

The eyes of Tullimonstrum reveal a vertebrate affinity

“…Where topological data in fossils are equivocal, other homology criteria, normally subordinate to topology assume greater importance [9][10][11] . Here we apply the criterion of the intrinsic properties of body parts (also referred to as 'special qualities'' 10 or 'correspondence of composition' 11 ) allowing us to resolve the phylogenetic placement of Tullimonstrum.…”