CONTENT - International Society of Zoological Sciences

S11 ICZ2008 - Abstracts
New insights on the nematogenesis process give new elements
to understand the acquisition of the nematocyte
Elsa Denker, Nicolas Rabet, Eric Bapteste and Michael Manuel
UPMC Univ Paris 06, UMR 7138 CNRS UPMC MNHN IRD, Case 05,
7 quai St Bernard, F-75005 Paris, France.
Nematocytes are a synapomorphy of Cnidaria probably including
Myxozoa (Jimenez-Guri et al., 2007). The fundamental importance of
this cell in living Cnidaria or Myxozoa suggests that its acquisition
probably conditioned the evolutionary success of the whole group. In
order to understand the evolution of this particular cell, we
investigated nematogenesis in a new model, the tentacle bulb of
Clytia (Hydrozoa), a promising experimental system (Denker et al.,
2008). Firstly, we investigated the expression of homologues of
bilaterian neural genes. Together with data from the literature,
coming from development or neurophysiology, we confirmed that the
cnidocyte is a very specialised neural cell. Then, we investigated the
nematocyst, the typical extrusive organelle of the nematocyte. We
made a comparison of extrusive organelles occurring in various
lineages using data from the literature. In all cases except the
Myxozoa, we were led to exclude primary homology with the
cnidarian nematocyst on the basis of deep ultrastructural differences,
thus excluding endosymbiotic origin of these organelles contrary to
previous proposals (Okamura & Canning, 2003). In order to
understand particularities of nematocysts, we also investigated the
capacity of PGA (poly gamma-glutamate) synthesis, a polymer
accumulating in the nematocyst and known only in Cnidaria among
Eukaryotes.
We found that cnidarian genomes harbour one homologue of a
bacterial gene known to be implicated in PGA synthesis and
accumulated indications that this gene was horizontally transferred
before the diversification of Cnidaria.
We propose a scenario in which the ancestral nematocyte was a
neuro-sensory cell, which acquired an extrusome with particular
efficiency.
Jimenez-Guri E, et al. 2007. Science 317(5834): 116-118.
Denker E, et al. 2008. Develop Biol 315(1): 99-113.
Okamura B, Canning EU. 2003. TREE18(12): 633-639.
Evolutionary conservation of the glycoprotein hormone alpha
and ß subunit gene precursors
Sandra Dos Santos, Claire Bardet, Damien Habert and Bruno Quérat
UMR5166 USM501 CNRS-MNHN RDDM BP32, 57 rue Cuvier,
75231 Paris cedex 05, France
The pituitary gonadotropins and thyrotropin are heterodimers
composed of a common alpha subunit (SU) and a specific βSU. Two
genes were recently identified in vertebrates and in some protostome
genomes that present structural similarities with the alpha and β SUs
and were thus named glycoprotein alpha2 (GPA2) and β5 (GPB5).
Urochordates, the sister group of vertebrates and more basal
deuterostomes have no pituitary gland and no acknowledgeable
pituitary hormone genes. But they do have GPA2 and GPB5 related
genes that might thus represent the molecular precursors of the
alpha and β glycoprotein hormone SUs. In this study, we show that
GPA2 and GPB5 are organized in inverse tandem in basal
deuterostomes, an organization that is not conserved in vertebrate
genomes. However, some of the genes found in the vicinity of the
tandem in basal deuterostomes are also present close to vertebrate
glycoprotein hormone β SU genes. GPA2 and GPB5 are expressed
in the gonads of the cephalochordate amphioxus, but also in
intestine and other non-endocrine tissues. We also have identified a
glycoprotein hormone receptor-like gene in the amphioxus genome.
The localization of its expression is in progress. Our results bring
new arguments for parental relationships between GPA2 and GPB5
and the vertebrate glycoprotein hormone SUs but further studies are
necessary in order to assess whether a possible GPA2/GPB5
heterodimer could represent the functional glycoprotein hormone
ancestral form.
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Evolution of the Neural Crest: New Insights From Urochordates
William R. Jeffery
Department of Biology, University of Maryland, College Park, MD.
20742 USA
New insights from ascidians on the chordate ancestry of the neural
crest (NC) will be discussed. Ascidians have neural crest-like cells
(NCLC), defined by migration from the dorsal midline, expression of
some vertebrate NC markers, and development into body pigment
cells. These characters suggest that primordial NC cells were
already present in the common ancestor of the vertebrates and
urochordates, which have been recently inferred as sister groups.
The primitive role of NCLC may have been pigment cell dispersal
and development. Later, additional functions may have appeared in
the vertebrate lineage, resulting in the evolution of definitive NC cells.
I will also discuss new evidence showing that the ascidian
homologues of vertebrate NC specifier genes, including foxD3, cMyc,
and twist-like, and NC effector genes, such as those encoding
rhoABC and cadherins, are expressed in Ciona intestinalis NCLC. In
contrast, the ascidian homologues of vertebrate neural plate border
specifier genes, such as msxb, pax3/7, and dlx3/5, are not
expressed in Ciona NCLC. These results suggest that the definitive
vertebrate NC was elaborated to include many additional functions
by co-option of neural plate border genes into a pre-existing genetic
cascade responsible for NCLC development.
Evolution of segmentation: Regulation of spider segmentation
by Wnt8
Alistair P. McGregor, Matthias Pechmann, Evelyn E. Schwager,
Natália M. Feitosa, Sarah Kruck, Manuel Aranda and Wim G. M.
Damen
University of Cologne, Institute for Genetics, Evolutionary Genetics,
Zülpicher Straße 47, D-50674 Köln, Germany
One outstanding question in animal evolution is whether the last
common ancestor of the three segmented phyla, chordates, annelids
and arthropods, was itself segmented. In vertebrates, somitogenesis
is regulated by FGF, Wnt and Delta/Notch signaling. Our previous
analysis of Delta/Notch signaling in spiders suggested that minimally
this aspect of the regulation of vertebrate somitogenesis and
arthropod segmentation is similar. Our latest data show that another
important regulator of vertebrate somitogenesis, Wnt8, is also
required for spider segmentation. Knockdown of Wnt8 expression in
Achaearanea tepidariorum resulted in failure to properly establish a
posterior growth zone and caused a breakdown of posterior
segmentation. Our results suggest Wnt8 has a similar role in
vertebrates and spiders in establishing and maintaining a posterior
segmentation zone and may be part of an ancient regulatory network
for segmentation that could have been present in the common
ancestor of segmented animals.