PhD thesis

26 Results and discussion
argument against segmentation in brachiopod larvae is that the segmented
appearance with three larval lobes is not recognizable by the inner bauplan
on the ultrastructural level (Lüter 2000a). This has been shown for Notosaria
nigricans and Calloria inconspicua. In these species, a single coelomic anlage
forms one compartment with all mesodermally derived cells separated only by
cellular membranes. Thus, there is only one mesoderm compartment in these
larvae, which encloses one coelomic cavity (Lüter 2000a). In the segmented
Annelida the coelom forms one pair of coelomic cavities in each segment
(Anderson 1973).
Not and Cdx expression analyses
Results of gene expression patterns of the homeobox genes TtrNot and TtrCdx
are presented in Chapter IV.
In Terebratalia transversa, the ortholog of the homeobox gene Not, TtrNot, is
expressed in the ectoderm from the beginning of gastrulation until completion
of larval development, which is marked by a three-lobed body with larval setae.
Expression starts at gastrulation in two areas lateral to the blastopore and
subsequently extends over the animal pole of the gastrula. With elongation of
the gastrula, expression at the animal pole narrows to a small band, whereas
the areas lateral to the blastopore shift slightly towards the future anterior region
of the larva. Upon formation of the three larval body lobes, TtrNot expressing
cells are present only in the posterior part of the apical lobe. Expression ceases
entirely at the onset of larval setae formation. TtrNot expression is absent in
unfertilized eggs, in embryos prior to gastrulation, and in settled individuals
during and after metamorphosis. Comparison to the expression patterns of Not
genes in other metazoan phyla suggests an ancestral role in gastrulation, germ
layer (ectoderm) specification, and neural patterning, with co-opted functions in
notochord formation in chordates and left/right determination in ambulacrarians
and vertebrates (Chapter IV).
In Terebratalia transversa the ParaHox gene TtrCdx is expressed on the
posterior side of the blastopore and its expression stays in this region until the
three-lobed larva is fully formed. The expression of TtrCdx suggests a function
of this gene during gastrulation and ectoderm patterning in Brachiopoda. The
pattern of Cdx in other metazoans ranges from expression in the mesoderm,
gut, brain, central nervous system to posterior tissues (Fröbius and Seaver
2006). The basal function of Cdx is probably in patterning of posterior tissues.