Anatomy of the trunk mesoderm in tunicates: homology considerations and phylogenetic interpretation |
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Authors: | Thomas Günther Stach |
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Institution: | 1.Freie Universit?t Berlin,Berlin,Germany |
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Abstract: | 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. |
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Keywords: | Chordate heart Coelom Enterocoely Protocoel Hatschek’ s diverticle |
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