Dev Genes Evol (1999) 209:625–628© Springer-Verlag 1999

EXPRESSION NOTE

Tadayoshi Hayata · Hiroki Kuroda · Akira Eisaki
Makoto Asashima

Expression of Xenopus T-box transcription factor,Tbx2 in Xenopus embryo

Received: 24 March 1999 / Accepted: 21 April 1999

Abstract We report here the cloning and expression ofthe Xenopus orthologue of the T-box transcription factor
gene Tbx2 (optomotor-blind in Drosophila). Tbx2 is firstdetected in the ventral mesodermal cells just above theyolk plug at late gastrula. At the neurula stage it isstrongly expressed in the cement gland, dorsal root ganglia, and otic vesicle region. At the tailbud stage strong
Tbx2 expression is observed in the dorsal part of the optic cup and trigeminal ganglia, and it is also expressed in
the branchial arches, heart anlage, nasal pit, proctodeum,
and the region around the pronephros.
Key words Tbx2 · Optomotor-blind · Xenopus · Cement
gland · Dorsal root ganglia
In Drosophila the optomotor-blind (omb) T-box gene isexpressed in the brain region that develops into optic
lobes and, less strongly, in the thoracic part of the ventralganglion. Flies homozygous for omb mutations show defects in optic lobes, reduction in wing size, and increased
abdominal pigmentation (Pflugfelder et al. 1992). omb
homologues, known as Tbx2 in vertebrates, have been
isolated in human, mouse, and chick (Campbell et al.
1995; Chapman et al. 1996; Gibson-Brown et al. 1998a).
In mouse development it is expressed in the otic and optic
vesicles on day 9.5 and in the trigeminal ganglia, facial
Edited by R.P. Elinson
The nucleotide sequence data reported in this paper have been deposited in the DDBJ, EMBL, and GenBank data bases (accessionno. AB 023815).
T. Hayata · H. Kuroda · A. Eisaki · M. Asashima (✉)
Department of Life Sciences (Biology),Graduate School of Arts and Sciences,
University of Tokyo, 3-8-1 Komaba, Meguro-ku,
Tokyo 153-8902, Japan
E-mail: cmasa@komaba.ecc.u-tokyo.ac.jp
Tel.: +81-3-54546632, Fax: +81-3-54544330
M. Asashima
CREST Project, Japan Science and Technology Cooperation,
Tokyo, Japanregions, retina, and limb bud mesenchyme on day 12.5. A
similar pattern of expression is seen in the chick embryo,suggesting that the expression pattern of Tbx2 is generally conserved during evolution. During chick limb specification it has been suggested that Tbx2 may be a direct,
short-range target of sonic hedgehog in the formation of
the zone of polarizing activity (Gibson-Brown et al.
1998b). In Drosophila wing development omb expression
is controlled by decapentaplegic and wingless and is required for distal wing development. Ectopic expression ofomb also induces additional wings (Grimm and Pflugfelder 1996). Recently it was shown that Tbx2 acts as a transcriptional repressor of the melanocyte-specific tyrosinase-related protein 1 promoter (Carreira et al. 1998).
During our screening for T-box genes involved in mesoderm or endoderm formation in Xenopus we have obtained a PCR fragment closely related to Tbx2 and haveisolated a full-length clone from a gastrula stage cDNAlibrary. We thus named this clone Xenopus Tbx2
(XTbx2). XTbx2 has similarities in sequence and expression pattern with other vertebrate Tbx2 genes and Drosophila omb gene.
The XTbx2 cDNA contains a single open reading
frame coding for 688 amino acids and includes a T-box.
The deduced amino acid sequences and alignment withmouse Tbx2 (MTbx2) are shown in Fig. 1. The overallamino acid identity of XTbx2 with MTbx2 is 69%, andXTbx2 has an additional 10 amino acid sequence at theamino-terminus compared with MTbx2. Chick Tbx2(CTbx2) has also this N-terminal sequence (accession no.
AF 033668). In XTbx2, however, since there is a consensus Kozak sequence (Kozak 1984) or NcoI restriction
site at the second methionine (amino acid 11), translationof XTbx2 may be initiated at the second methionine.
There are three alanine-rich regions in MTbx2, residues
50–61, 571–577, and 585–593, but XTbx2 has only thesecond alanine-rich region. MTbx2 also has a glycinerich region (507–517), but XTbx2 lacks this. It is unclear,
however, whether the absence of these regions affects
the transcriptional character of Tbx2. Within the T-domain XTbx2 has strong similarities with Tbx2 homo-626

Fig. 1 Comparison of amino acid sequences between Xenopus Tbx2 and mouse Tbx2. Vertical lines identical residues; double-underlined, T-domain; highlighted, three alanine-rich regions and a glycine-rich region

627

Fig. 2A–F Expression pattern of Xenopus Tbx2 by whole-mountin situ hybridization. A Posteriodorsal view of late gastrula stageembryo (stage 12). Dorsal is left. Arrow staining of ventral mesodermal cells. B Lateral view of neurula stage embryo (stage 19).
C Dorsal view of the same embryo as in B. D Ventral view of thesame embryo as in B. E Lateral view of early tailbud stage embryo (stage 31). F Enlarged view of the same embryo as in E. Embryos that hybridized a sense probe show no background signals.

Full-length XTbx2 antisense probe was prepared by digestingpXTbx2 with BglII and transcribing with T7 RNA polymerase.
XTbx2 sense probe was prepared by digesting pXTbx2 with XhoIand transcribing with T3 RNA polymerase. ba Branchial arches;
cg cement gland; drg dorsal root ganglia; e eye; h heart; np nasal
pit; pn pronephros; pr proctodeum; ov otic vesicle; trg trigeminalganglia

628logues, showing 98% identity with mouse and 86% identity with Drosophila (data not shown). These high similarities suggest that the DNA-binding specificity of Tbx2
is conserved during evolution.
To determine the spatial expression pattern of XTbx2
whole-mount in situ hybridization was performed (Fig.
2A–F). As shown in Fig. 2A, low levels of XTbx2 expression were first detected in ventral mesodermal cells
just above the yolk plug at the late gastrula stage (stage12), which is different from MTbx2, which is not expressed in mouse embryos during gastrulation (Chapmanet al. 1996). The expression in this domain was continued in future proctodeum throughout this developmental
stage (Fig. 2D, E). From the neurula stage XTbx2 expression was detected in the cement gland, otic vesicles, anddorsal root ganglia (Fig. 2B, C). By the tailbud stage theventral part of cement gland is positive (Fig. 2E, F).
From the tailbud stage (stage 31) strong XTbx2 expression was observed in the dorsal part of optic cup and trigeminal ganglia, and there was weak expression in nasal
pit, branchial arches, heart anlage, and the region aroundthe pronephros (Fig. 2E, F).
Expression of Tbx2 in Xenopus was generally similar
to that in other vertebrates and Drosophila. However,XTbx2 expression in the cement gland is unique to Xenopus because there is no homologous organ in other vertebrates. Tbx2 expression in the proctodeum is also uniqueto Xenopus. Tbx2 expression in the optic cup is homologous to that of omb in the optic lobes and of chick andmouse Tbx2. This region of expression is conserved between Drosophila and vertebrates, suggesting an important role for Tbx2 in eye formation. It is interesting tonote that XTbx2 is expressed at the most anterior region(cement gland) and in the posterior region (proctodeum)in Xenopus embryo. omb is involved in Drosophila wingdevelopment, and chick Tbx2 is involved in limb bud
specification, but its function in other organs is not yetknown. Further analysis should focus on its role in organogenesis, especially that of the nervous system and kidney.
Acknowledgements We are very grateful to Dr. Y. Etoh (Ajinomoto) for giving us human recombinant activin A. This work was
supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan and by
CREST (Core Research for Evolutional Science and Technology)of Japan Science and Technology Corporation.

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