CONTENT - International Society of Zoological Sciences

S10 ICZ2008 - Abstracts
Myogenesis in the articulate brachiopods Argyrotheca cordata
(Risso, 1826), Argyrotheca cistellula (searles-wood, 1841) and
Terebratalia transversa (Sowerby, 1846)
Andreas Altenburger and Andreas Wanninger
University of Copenhagen, Institute of Biology, Research Group for
Comparative Zoology, Universitetsparken 15, Building 3, 2100
Copenhagen, Denmark
We investigated development and muscle formation in larvae of the
articulate brachiopods Argyrotheca cordata, A. cistellula and
Terebratalia transversa using immunocytochemistry combined with
confocal laser scanning microscopy. Full grown larvae are threelobed
and express two pairs of bristles. Muscle development in the
three species investigated shows a number of similarities. As such,
the first anlagen of the musculature develop in the bristle pouches
and the pedicle lobe. Late-stage larvae show a network of
longitudinal muscles running from the apical to the pedicle lobe as
well as transversal muscles situated in the apical lobe. Strong
muscles attach to both the bristles pouches and the pedicle lobe. We
found only few similarities between the larval myoanatomy of
brachiopods and the hitherto investigated representatives of the two
other lophophorate phyla, Phoronida and Ectoprocta. This may be
due to an early evolutionary split of the ontogenetic pathways of
Brachiopoda, Phoronida and Ectoprocta.
The evolutionary origin of the bilaterian CNS
Detlev Arendt
EMBL Heidelberg, Germany
Animal nervous systems are composed of neuron types specialized
for functions as diverse as light perception or hormone secretion.
While some animals such as cnidarians have few neuron types,
human neuron types count in hundreds. Understanding the evolution
of neuron types is key to understanding the evolution of animal
nervous systems
The recent advent of molecular fingerprints for defining cell types
sets the stage the study of neuron type evolution. Each neuron type
displays a unique profile of expressed genes, which encode
transcription factors, microRNAs and differentiation gene batteries.
Since many of these genes are deeply conserved in animal evolution,
this allows the identification of homologous cell types over large
evolutionary distances. Molecular fingerprint comparisons also allow
identifying, within a given species, sister cell types that are related by
evolutionary diversification.
In my lab, we are currently generating an expression profiling atlas
for all cell types of the developing Platynereis nervous system by a
novel technique, Wholemount In Silico Expression Profiling. This
technique provides a close-to-complete inventory of neuron type
molecular fingerprints in the Platynereis brain. We use this to identify
homologous cell types between annelids and other bilaterians such
as insects and vertebrates. This approach will be exemplified for the
mushroom bodies, the associative centre of the annelid brain, and
for the various types of photoreceptor cells that harbour the
Platynereis brain. I will discuss the implications of these data for our
understaning of the origin of the bilaterian brain.
Evolution and development of the ascidian neural gland
complex
Hélène Auger, Shungo Kano, Laurent Legendre and Jean-Stéphane
Joly
INRA MSNC Group, DEPSN, Institut A. Fessard, CNRS, 1 Avenue
de la Terrasse, 91198 Gif-sur-Yvette, France.
The cerebral ganglion of adult ascidians is apposed to the neural
gland complex. This later has three parts: a ciliated funnel, a ciliated
duct and one neural gland. I will first report our progress on the study
of the ciliated funnel epithelium.
S11 - Evolution and Development
- 38 -
We have looked for genes expressed in this cell type, proposed to be
related to some of the vertebrate pituitary endocrine cells. These
experiments are based on in situ hybridization experiments, and on
micro-array experiments on fluorescent epithelia micro-dissected
from GFP expressing transgenic lines. The neural gland is another
organ whose homology with vertebrate organs is still debated. It is a
spongious sac with a folded epithelium. It has been suggested that it
is involved in osmoregulation, with the ciliated duct generating an
unidirectional influx of seawater into the neural gland. In Deyts et al.
(2006), we reported that the ascidian neuropeptide G-proteincoupled
receptors, the vertebrate homologues of which are involved
in homeostasis, are expressed in the neural gland complex. More
recently, ISH on thick sections revealed that vertebrate markers
specifically found in the choroid plexus, such as Transthyretin or
Gelsolin, are also expressed in the ascidian neural gland. These
results highlighted similarities between the ascidian neural gland and
the vertebrate circumventricular organs. Circumventricular organs
are located along the midlines of vertebrate brain ventricles. They
appear to be associated with multiple functions, at first as
transducers of information between the blood, neurons and the
cerebrospinal fluid. Among them, the vertebrate choroid plexus, a
neural circumventricular organ, produces large quantities of
cerebrospinal fluid.
These findings, together with electrophysiological experiments and
cell lineage analysis through metamorphosis should allow us to
determine more precisely the ontological origin and morphogenesis
of the neural gland and its function in ascidians. This leads us to
raise hypotheses on the origin in chordates of neuroendocrine and
endocrine cell types associated with nervous system.
The segmented ancestor of protostomes and the origin of
parasegmental patterning
Guillaume Balavoine
Centre de Génétique Moléculaire du CNRS, avenue de la Terrasse,
91198, Gif-sur-Yvette, France
The comparison of the genetic processes of segment formation in
three distant groups of bilaterian metazoans (arthropods, insects and
annelids) has opened a heated debate that considerably impacts on
our conceptions of metazoan evolution: was the last common
ancestor of protostome and deuterostome animals a metameric
organism? Processes of segment formation can be divided in three
steps: the proliferation of axial tissues, the production of a periodic
segmentation signal along this axis and the patterning of the subparts
of individual segments. While the ?cascade? of metamere
patterning genes of the fruitfly and the known mechanisms of somite
formation in vertebrates have little in common, intriguing similarities
have been uncovered in other model animals. Gene expression and
interference data suggest that the Notch pathway may be involved in
synchronising a periodic segmental signal in some arthropods
(spiders, centipedes) in a similar way as in vertebrates. Also, a
number of genes have been shown to present similar ?segment
polarity? patterns in the marine annelid Platynereis dumerilii and
arthropods. Pharmacological treatments suggest that the hedgehog
pathway is involved in regulating the individual segmental patterns in
Platynereis in the same way as in arthropods. The spatial
relationships between these annelid ?segment polarity? patterns are
remarkably similar to their arthropod homologues and leave few
doubts on the segmented architecture of the protostome ancestor
body. Additionally, they suggest that annelid segments are
homologous with arthropod parasegments. A scenario for the origin
of arthropod parasegmental patterning is proposed.