PhD thesis

Results and discussion
23
of four classically defined subgroups of the Pax family transcription factors
(Balczarek et al. 1997). Pax3/7 shares its expression among distantly related
insects and shows several patterns including pair-rule, segment polarity,
and neural patterning (Davis et al. 2005). In crustaceans Pax3/7 genes are
expressed in iterated stripes (Davis et al. 2005). In myriapods and chelicerates
Pax3/7 gene expression exhibits iterated stripes that form early in the posteriormost
part of the germ band (Davis et al. 2005). In the tardigrade Hypsibius
dujardini, the Pax3/7 proteins localize in a segmentally iterated pattern in the
ectoderm, after establishment of endomesoderm segmentation, but before the
visible segmentation of the ectoderm (Gabriel and Goldstein 2007). Pax3/7 is
also localized within the developing head region of the tardigrade embryo, but
no pair-rule pattern is visible during any stage of embryogenesis (Gabriel and
Goldstein 2007). Tardigrades, together with arthropods and onychophorans
belong to Panarthropoda (Halanych 2004).The expression pattern of Pax3/7 in
H. dujardini suggests that the pair-rule function of Pax3/7 may have arisen near
the base of Arthropoda.
In the annelid Platynereis dumerilii Pax3/7 proteins are found in the peripheral
nervous system (Kerner et al. 2009). In larvae of the brachiopod Terebratalia
transversa DP311 and DP312 show identical staining patterns. Pax 3/7 starts
to be present in four cells of the apical lobe in the late elongated gastrula (Fig.
4B). The cells containing Pax3/7 products are later distributed in a ring on the
apical lobe of early three-lobed larvae without setae (Fig. 4C). Fully established
larvae show a loose distribution of cells that contain Pax3/7 products in their
apical lobe (Fig. 4D, E). In juveniles Pax3/7 containing cells are mainly found
in the growing lophophore (Fig. 4F). The presence of Pax3/7 gene products
in the apical lobe indicates a function of those genes during neurogenesis in
T. transversa. However, further experiments are necessary in order to assess
whether the staining specifically shows Pax3/7 protein products, since the
antibodies used were developed against the Pax3/7 sequences of Drosophila
melanogaster. Ideally, cloning of the sequences of the Pax3/7 homologs of
Terebratalia transversa should be carried out, followed by mapping of the
epitopes of DP311 and DP312 on peptide arrays with the known peptide
sequences of T. transversa and other metazoans (Harlow and Lane 1999). The
final proof would then be in situ hybridizations with the specific corresponding
probes. In addition, a double staining with serotonin would be necessary in
order to prove that the cells containing Pax3/7 gene products are co-localized
with the nervous system.