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Xrx1, a novel Xenopus homeobox gene expressed during eye and pineal gland development.
Casarosa S
,
Andreazzoli M
,
Simeone A
,
Barsacchi G
.
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We have isolated a novel Xenopus homeobox gene, Xrx1, belonging to the paired-like class of homeobox genes. Xrx1 is expressed in the anterior neural plate, and subsequently in the neural structures of the developing eye (neural retina and pigmented epithelium), and in other forebrain structures deriving from the anterior neural plate: in the pineal gland, throughout its development, in the diencephalon floor and in the hypophysis. Its rostral limit of expression corresponds to the chiasmatic ridge, which some authors consider as the anteriormost limit of the neural tube: thus, Xrx1 may represent one of the most anteriorly expressed homeobox genes reported to date. Moreover, its expression in organs implicated in the establishment of circadian rhythms, may suggest for Xrx1 a role in the genetic control of this function. Finally, analysis of Xrx1 expression in embryos subjected to various treatments, or microinjected with different dorsalizing agents (noggin, Xwnt-8), suggests that vertical inductive signals leading to head morphogenesis are required to activate Xrx1.
Fig. 2. Developmental expression of Xrx1 by whole-mount in situ hybridization.
In panels (C,E) anterior is to the left and dorsal to the top. In panels (C-E) Xrx1 expression in the pineal gland is marked with an arrowhead. (A,B) Xrx1 expression of stage 12.5 and stage 15 neurulae. Frontal views show the expression in the anterior neural plate and ridge. In
(B) the dotted line defines the anterior limit of the neural ridge.
(C) Lateral view of a stage 24 embryo showing Xrx1 expression in the optic vesicle and pineal gland.
(D) Dorsal view of a stage 28 embryo. Anterior is to the bottom. The fainter staining between the eyes is the diencephalon floor.
(E) Lateral view of a stage 34 embryo showing that the lens (L) does not express Xrx1.
Abbreviations:
cg, cement gland;
L, lens.
Fig. 3. Whole-mount in situ hybridization showing Xrx1 asymmetrical expression in the eye. In panels
(A-C) the arrowheads indicate the ventral region of stronger Xrx1 expression.
(A) Frontal view of a stage 17/18 embryo. The optic vesicles are still frontal and they show a stronger Xrx1 expression in the ventral region.
(B) At stage 22 the vesicles have rotated laterally, and the region of stronger Xrx1 expression is ventro-nasal.
(C) Dorsal view of (B), showing the gradient of Xrx1 expression along the D-V axis of the eye.
Fig. 4. Developmental expression of Xrx1 by radioactive in situ hybridization on sections.
Panels (A-D), dark fields;
(A'-D') respective bright fields.
(A) Transversal section of a stage 20 embryo, showing Xrx1 expression in the optic vesicles and in the floor of the forebrain.
(B) Transversal section of a stage 28 embryo. The expression in the optic cups covers both the neural retina (nr) and the pigmented epithelium (pe). The arrow indicates the expression in the hypophysis.
(C) Longitudinal section of a stage 34 embryo. The arrow shows the rostral limit of Xrx1 expression in the chiasmatic ridge.
(D) Horizontal section of a stage 35/36 embryo, showing the lack of Xrx1 expression in the lens (L).
Abbreviations:
di, diencephalon;
ep, epiphysis (pineal gland);
f, forebrain;
h, hypophysis;
L, lens;
m, mesencephalon;
nr, neural retina;
ov, optic vesicle;
pe, pigmented epithelium;
ph, pharynx;
t, telencephalon.
Fig. 5. Expression pattern of Xrx1 in embryo sections, after whole-mount in situ hybridization.
(A) Transversal section of a stage 32 embryo. Expression in the pineal gland is indicated by the arrowhead.
(B) Longitudinal section of a stage 35/36 embryo, showing Xrx1 stronger expression in the regions that border the optic fissure (arrow). Abbreviations: as in Fig. 4; e, eye.
Fig. 6. Expression of Xrx1 by whole-mount in situ hybridization in embryos subjected to various treatments.
(A) Albino exogastrula at stage 24 showing the lack of Xrx1 expression.
(B) Stage 23 albino embryo treated with a 30 min pulse of l0E-6 M RA at stage 10+.
(C) Stage 34 pigmented embryo treated with 0.3 M LiCl for 10 min at 32-cell stage. The circular cement gland is indicated by the arrowheads.
(D) Dorsal view of a stage 17 albino embryo that was injected with 100 pg of Xwnt-8 synthetic RNA in a vegetal position at two cell stage. Dotted lines indicate the dorsal midlines of the two axes; hybridization signals are detected in the anterior neural plate of both axes.
(E) Frontal view of a stage 23 pigmented embryo that was injected with 100 pg of Xwnt-8 synthetic RNA in a ventral-vegetal position at 4 cell stage, showing three eyes with Xrx1 hybridization signal (arrowheads) and two cement glands (arrows).
(F) Dorsal view of a stage 23 pigmented embryo that was injected with 100 pg of noggin synthetic RNA in a ventral-vegetal position at 4 cell stage. Xrx1 expression is detected in the primary but not in the secondary axis (indicated by arrowheads).
