Making very similar embryos with divergent genomes: conservation of regulatory mechanisms of Otx between the ascidians Halocynthia roretzi and Ciona intestinalis
Izumi Oda-Ishii
1
2
Vincent Bertrand
0
Isao Matsuo
1
Patrick Lemaire
0
Hidetoshi Saiga
)
2
0
LGPD, IBDM, Case 907, Campus de Luminy
,
F-13288 Marseille Cedex 09
,
France
1
Head Organizer Project, Vertebrate Body Plan Group, RIKEN Center for Developmental Biology
,
2-2-3 Minatojima Minamimachi, Chuou-Ku, Kobe, Hyougo 650-0047
,
Japan
2
Department of Biological Sciences, Graduate School of Science, Tokyo Metropolitan University
,
1-1 Minamiohsawa, Hachiohji, Tokyo 192-0397
,
Japan
of regulatory mechanisms of Otx between the ascidians
-
Ascidian embryos develop with a fixed cell lineage into
simple tadpoles. Their lineage is almost perfectly
conserved, even between the evolutionarily distant species
Halocynthia roretzi and Ciona intestinalis, which show
no detectable sequence conservation in the non-coding
t regions of studied orthologous genes. To address how a
en common developmental program can be maintained
m without detectable cis-regulatory sequence conservation,
p
lo we compared in both species the regulation of Otx, a gene
ev with a shared complex expression pattern. We found that
e in Halocynthia, the regulatory logic is based on the use of
D very simple cell line-specific regulatory modules, the
activities of which are conserved, in most cases, in the Ciona
In recent years, the ascidian larva has often been regarded as
an organism close to the ancestral form of chordates, and has
served as an interesting and informative model system for
understanding the genesis of a simple chordate body plan
(Satoh et al., 1996; Satoh et al., 2003). Halocynthia roretzi and
Ciona intestinalis are representative species of the two orders,
the Pleurogona and the Enterogona, which constitute the class
Ascidiacea. Although these two classes probably diverged deep
in the history of ascidian evolution, their embryos show
remarkable similarity, with almost perfect conservation of the
lineages up to the early gastrula stages. In addition, the recent
cloning of a large number of genes in both species has shown
a remarkable conservation of their embryonic expression
profiles. This applies in particular to homeobox genes such as
the Otx, Pax and Hox genes, which are expressed along the
anteroposterior axis in the larval central nervous system (CNS)
with distinct expression domains; these expression domains are
also very similar to their vertebrate orthologs (Wada et al.,
1998).
The conservation of expression domains between ascidian
species, and with vertebrates, raises the possibility of the
conservation of regulatory logics within the chordate lineage.
However, cross-species analysis of the activity of the
embryo. The activity of each of these enhancer modules
relies on the conservation of a few repeated crucial binding
sites for transcriptional activators, without obvious
constraints on their precise number, order or orientation,
or on the surrounding sequences. We propose that a
combination of simplicity and degeneracy allows the
conservation of the regulatory logic, despite drastic
sequence divergence. The regulation of Otx in the anterior
endoderm by Lhx and Fox factors may even be conserved
with vertebrates.
regulatory regions of Ci-Hox3 in the mouse suggested lack of
conservation (Locascio et al., 1999). Even between ascidians,
two observations have been made that suggest a possible
divergence of regulatory networks in spite of strikingly similar
embryonic development. First, existing cDNA/EST and
genomic data suggest a very poor sequence conservation
between Halocynthia and Ciona. For example, the coding
sequences of Halocynthia and Ciona Brachyury, a T-box gene
specifically expressed in the notochord of both species, are
remarkably different (Marcellini et al., 2003). Second, a
previous report has suggested that Brachyury, may be regulated
by very different mechanisms in Halocynthia and Ciona
(Takahashi et al., 1999).
To readdress the question of the conservation of the
regulatory logic among ascidians, and with vertebrates, we
chose Otx as a model, as it is one of the most phylogenetically
conserved developmental genes. Otx/otd genes have been
isolated from various animal species, including cnidaria,
Drosophila, ascidians and vertebrates (Bally-Cuif et al., 1995;
Finkelstein and Perrimon, 1991; Hudson and Lemaire, 2001;
Li et al., 1994; Pannese et al., 1995; Simeone et al., 1993;
Smith et al., 1999; Wada et al., 1996). They have the same
expression domain, in the anterior part of embryos, suggesting
the evolutionary conservation of essential roles in the
formation and patterning of anterior embryonic territories.
Consistently, Drosophila, mouse or ascidian embryos mutated
or knocked down for Otx/otd genes exhibit defects in head
structures, such as deletion or differentiation deficiency in the
anterior central nervous system (CNS) (Acampora et al., 1996;
Acampora et al., 1995; Ang et al., 1996; Finkelstein and
Perrimon, 1991; Matsuo et al., 1995; Satou et al., 2001, Wada
et al., 2004).
In mouse embryos, the e (...truncated)