PhD thesis

General conclusions and perspectives for future research
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General conclusions and perspectives for future research
The results presented herein are the first developmental gene expression
studies in Brachiopoda, as well as the first detailed comparative description of
myogenesis and neurogenesis in brachiopod larvae based on antibody staining,
confocal laserscanning microscopy, and 3D reconstruction software. This study
shows that microanatomical data can yield new insights into the evolution and
development of lesser known metazoan phyla such as Brachiopoda. It provides
the first evidence of an apical organ in brachiopod larvae that comprises
serotonergic flask-shaped cells, similar to those found in ectoprocts and
spiralians. This result strongly suggests that such an apical organ constitutes a
morphological apomorphy of Lophotrochozoa.
Gene expression analyses of TtrNot imply an ancestral role of this gene in
gastrulation and ectoderm specification in Brachiopoda. The function of Not in
notochord formation in chordates and left/right determination in ambulacrarians
and vertebrates might thus be co-opted in these deuterostome clades. Analysis
of the TtrCdx gene expression suggests an ancestral role in gastrulation and the
formation of posterior tissues in Brachiopoda as well as in Bilateria in general.
Further studies should extend the database of brachiopod morphogenesis
and gene expression patterns to more organ systems as well as to the third
brachiopod subtaxon, Linguliformea. This would allow for a full representation
of the phylum Brachiopoda with its three clades Craniiformea, Linguliformea,
and Rhynchonelliformea and should allow significant inferences concerning
gene function and organ system evolution within this lophophorate phylum.
Such data would allow insights into the evolution of organ systems, and body
plans in Brachiopoda. Additionally, investigation of gene expression patterns in
Brachiopoda is needed in order to compare the function of genes, co-option,
and ancestral gene functions among Brachiopoda and other animal phyla. An
expressed sequence tags or genome-based approach would be the best choice
in order to obtain the sequences of the whole range of developmental genes.
Preferably, this should be done for one representative of each brachiopod clade.
Morphological and molecular data together would facilitate the reconstruction of
the evolution of organ systems in Brachiopoda once the phylogenetic position
of Brachiopoda and its sister groups has been settled.