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The zinc-finger gene Krox-20 is expressed in two alternating segments, rhombomeres (r) 3 and 5, in the developing mouse hindbrain. This expression pattern is established prior to rhombomere formation in the mouse, but it is not known how the timing of expression relates to cellular events of segmentation, such as lineage restriction. We have cloned Krox-20 sequences from Xenopus and the chick and shown that its alternating expression pattern is conserved in these systems, suggesting that its role in hindbrain development is conserved. Analysis of the early stages of Krox-20 expression in the chick show that both domains of expression precede the restriction of cell lineage to specific rhombomeres, consistent with a role of this gene in early events of hindbrain segmentation. The finding that expression is not coincident with lineage restriction indicates that early expression may not reflect an irreversible commitment of cells to r3 and r5 and/or may be mosaic.
Fig. 1. Conserved structure of the zinc-fingers of Krox-20.
The deduced amino acid sequences of the zinc-fingers of
mouse (M), Xenopus (X) and chick (C) Krox-20 and
mouse Krox-24 are compared. Dashes indicate amino acid
identity with mouse Krox-20, and asterisks the regions that
correspond to the oligonucleotides used to amplify Krox-20
from chick genomic DNA. Xenopus Krox-20 sequences
(Bradley et al. unpublished data), mouse Krox-20
sequences from Chavrier et al. (1988) and mouse Krox-24
sequences from Lemaire et al. (1988).
Fig. 2. Conserved patterns of
Krox-20 expression in mouse,
Xenopus and chick. In situ
hybridisation was carried out
homologous Krox-20 probes
as described (Wilkinson and
Green, 1990). (a) 9.5 day
mouse embryo; (b) stage 28
Xenopus embryo; (c) stage 15
chick embryo, r,
rhombomere. The apparent
signal in the endoderm (e) of
the Xenopus embryo is due
to the refraction of light by
yolky cells, not the
hybridisation of probe.
Anterior is to the right in all
Fig. 3. Onset of Krox-20 expression in the mouse and chick embryo. In situ hybridisation analysis was carried out to
examine the early stages of Krox-20 expression, (a) 8 day mouse embryo; (b) 8.5 day mouse embryo; (c,e) 3 somite chick
embryo; (d,f) 7 somite chick embryo, e and f are higher magnification views of the embryos shown in c and d. The arrows
indicate sites of Krox-20 expression, ne, neural epithelium. Anterior is to the right in all photographs, a and b are from
Wilkinson et al. (1989b). Bar=100/«n.
Fig. 4. Krox-20 expression and rhombomere boundary
formation. The diagram indicates the expression of Krox-20
(shaded) and the time course of rhombomere boundary
formation in the developing chick hindbrain (data from
Vaage, 1969). s, somite stage; r, rhombomere.