Anatomy of the trunk mesoderm in tunicates: homology considerations and phylogenetic interpretation

The tadpole stage of tunicates has played a pivotal role in understanding chordate evolution. While the organization of the mesoderm has been given high importance in comparative anatomical studies of Bilateria, this morphological character remains largely unexplored in tunicate tadpoles. For larvae of the phlebobranch ascidian Ciona intestinalis, the presence of two mesodermal pockets had been claimed, raising the possibility that paired coelomes are present in the larval ascidian. Using computer assisted 3D-reconstructions based on complete series of 1 μm-sections analyzed by light microscopy complemented by TEM-investigation of selected regions a comparative anatomical study of tadpole stages from four major tunicate clades, Aplousobranchiata, Phlebobranchiata, Stolidobranchiata, and Appendicularia is presented. In the aplousobranch Clavelina lepadiformis numerous mesodermal cells are found throughout the entire trunk plus the unpaired ventral rudiment of the pericardium. In the phlebobranch Ascidia interrupta, massive mesodermal components occur in the posterior trunk, whereas more anteriorly situated mesoderm consists of loose streaks of cells or isolated cells. This is also the case in the stolidobranch ascidians Herdmania momus and Styela plicata. In the stolidobranch Molgula occidentalis and the appendicularian Oikopleura dioica the anterior trunk is entirely devoid of mesodermal cells. TEM-investigation revealed that all mesodermal structures in the trunk of tunicate tadpoles were mesenchymal with the exception of a ventral portion of the mesoderm in C. lepadiformis, which probably corresponds to the developing pericardium, and the differentiated pericardium of the juvenile O. dioica. Thus no evidence for paired coelomic cavities in Tunicata was found. Outgroup comparison suggests that the reduction of paired coelomic cavities is an apomorphic trait of Tunicata. Within Tunicata a stepwise evolutionary reduction of the anterior larval mesenchyme is documented.     

Minireview: On the homology of the protocoel in Cephalochordata and 'lower' Deuterostomia

Hypotheses regarding the homology of the protocoel in planktonic deuterostome larvae and mesodermal structures in ontogenetic stages of cephalochordates are evaluated. The prevalent ‘classical’ hypothesis describes the protocoel as being homologous with the diverticula of Hatschek, which, on the left side, develop into the preoral pit, subsequently into Hatschek’s pit and groove (in part). This hypothesis is based mainly on the position of Hatschek’s diverticula anterior to the rest of the mesoderm during their enterocoelic origin. It is shown here that during development the mesodermal segment that develops into Hatschek’s nephridium is the most anterior one prior to formation of Hatschek’s diverticula, and this segment assumes an anteriormost position after differentiation of Hatschek’s diverticula. Additional similarities between this segment and protocoels are: (i) presence of endomesodermal cells with podocytic extensions, (ii) excretory function, (iii) relatively early ontogenetic origin, (iv) probable lack of association with nervous structures, (v) probable ectodermal origin of a portion of the canal, and (vi) position relative to the mouth opening. Therefore, homology between the protocoel and the segment that becomes Hatschek’s nephridium is proposed. It is concluded that a glandular structure homologous to the diverticula of Hatschek and anterior to the protocoel/Hatschek’s nephridium is a synapomorphy of notochordates or chordates.     

Analysis of the embryonic development of Pogonophora in connection with the problems of phylogenetics

The development of the mesoderm in the Pogonophora being a point of argument, some stages of their ontogenesis are analyzed. The cleavage of Siboglinum caulleryi is considered as a modified spiral cleavage with the demonstration of the prospective significance of blastomeres. All mesoderm in Pogonophora is formed in enterocoelic mode from the anterior quadrant B. The spiral cleavage of pogonophores is compared to that of the Polychaeta and other animals. Some aspects of the formation and structure of the telosoma in the larva and adults are analyzed with a discussion of the nature of its segmentation. Some general problems of the evolution of the spiral cleavage are considered. The division of the Coelomata into 5 superphyla is confirmed, the Pogonophora being one of them.