PhD thesis

Results and discussion
25
Figure 5. Pattern of
BrdU staining in larvae of
Terebratalia transversa.
Overlay of confocal
maximum projections and
light micrographs. Scale
bars equal 50 µm. All stages
show an equal distribution
of proliferating cells,
there are thus no distinct
growth zones identifiable.
(A) Blastula. (B) Early
gastrula with blastopore
(asterisk). (C) Late slightly
elongated gastrula with
blastopore (asterisk). (D)
Early three lobed stage
with the developing apical
lobe (al), mantle lobe (ml),
and pedicle lobe (pe). (E)
Three lobed stage with
apical lobe (al), mantle
lobe (ml), and pedicle lobe
(pl). This stage is at the
onset of setae formation.
(F) Fully developed threelobed
stage with apical
lobe (al), mantle lobe (ml),
and pedicle lobe (pl).
the development of Annelida and Brachiopoda. For annelids, it has been shown
that, although the post-metamorphic segments originate from a posterior growth
zone, the precise location of the growth zone can vary (Seaver et al. 2005;
Brinkmann and Wanninger 2010). However, the rhynchonelliform brachiopods
are regarded derived amongst brachiopod subgroups (Carlson 1995). The
distribution of proliferating cells in Terebratalia transversa can therefore not
completely rule out the possibility that the brachiopod ancestor had a growth
zone. Similar experiments in linguliform and craniiform brachiopods are needed
in order to further assess this issue.
The Annelida-Brachiopoda sister group hypothesis based on the ultrastructure
of the setae has been questioned by Lüter who showed that there is a difference
in the ultrastructure of larval and adult setae in the brachiopods Lingula anatina,
Notosaria nigricans, and Calloria inconspicua, suggesting a convergent
evolution of setae in Annelida and Brachiopoda (Lüter 2000b). An additional