PhD thesis
PhD thesis
PhD thesis
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6 <br />
Abstract<br />
Brachiopods are a small phylum with a unique body plan comprising around<br />
370 living species and over 12.000 described fossil species dating back until the<br />
Lower Cambrian. The phylogenetic position of brachiopods is under controversial<br />
discussion. This project led to new insights into the ontogeny of brachiopods,<br />
which are divided into three clades, Rhynchonelliformea, Craniiformea,<br />
and Linguliformea. By use of immunocytochemistry combined with confocal<br />
laserscanning microscopy and 3D reconstruction software I describe the<br />
development of the nervous and muscular system in the rhynchonelliform and<br />
craniiform brachiopod larvae of Terebratalia transversa and Novocrania anomala.<br />
Myogenesis is described for larvae and adults of Joania (Argyrotheca) cordata<br />
and Argyrotheca cistellula and distribution of the transcription factor proteins<br />
DP311, DP312 (Pax3/7) for larvae and juveniles of Terebratalia transversa. The<br />
expression patterns of the developmental homeobox containing genes TtrNot<br />
and TtrCdx in larvae of Terebratalia transversa are described by use of whole<br />
mount in situ hybridization. The main results are: (1) The larval myoanatomy of<br />
rhynchonelliform brachiopod larvae is very similar, despite gross morphological<br />
differences in their outer morphology. (2) The rhynchonelliform and craniiform<br />
brachiopod larvae of Terebratalia transversa and Novocrania anomala show<br />
a serotonergic nervous system comprising eight or four flask-shaped cells<br />
in the apical organ. Such an apical organ with flask-shaped cells might be a<br />
morphological apomorphy of Lophotrochozoa. (3) The expression pattern of<br />
the TtrNot gene in larvae of Terebratalia transversa suggests an ancestral<br />
role of this gene in gastrulation and ectoderm specification in Brachiopoda.<br />
The expression pattern on TtrCdx suggests an ancestral role of this gene in<br />
gastrulation and the formation of posterior ectodermal tissue in Brachiopoda.<br />
The results are discussed in a phylogenetic framework compared to other<br />
lophotrochozoan phyla and implications of the results for the evolution of<br />
Brachiopoda are pointed out.