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Fig. 7. Schematic drawing of the effects of Gb'hb RNAi during
embryogenesis. All embryos are shown from a ventral view. (A) Wild-type and
Gb'hb RNAi embryos at early germband stage (38-42 hAEL). In
Gb'hb RNAi, early germbands are shortened and flattened. Reduction of
Gb'Antp expression domain indicates that the prospective thoracic
region is reduced in Gb'hb RNAi embryos. (B) Wild-type and
Gb'hb RNAi embryos at the beginning of germband elongation. Vertical
lines on the embryos represent the anterior segmental stripes of
Gb'wg. The stripes affected by Gb'hb RNAi segments are
indicated by red arrowheads in the wild-type embryo, with the deleted region
(posterior T1 to anterior T2) shown by a red bracket. In Gb'hb RNAi,
the Gb'wg stripes indicated by dotted lines are not eliminated but
irregularly shaped. Gb'abdA is expressed in a limited domain in the
posterior region of the wild-type embryo, while its ectopic expression is
induced in the anterior region of the Gb'hb RNAi embryos.
Accordingly, the segment identities of Gb'hb RNAi embryos change in
the prospective mandible to T3 segments (labeled by parenthesized
abbreviations). (C) Wild-type and Gb'hb RNAi embryos at stage 9. The
Gb'hb RNAi phenotype is a combination of the transformation of the
gnathal and thoracic regions (the maxillary to T3 segments) towards the
abdominal identity and the reduction of the number of segments in the abdomen.
The vestiges of T3 legs (arrowhead) are observed in most RNAi embryos. The
phenotype would also represent the deletion of one segment, as inferred from
the modification of Gb'wg stripe pattern resulting from the fusion of
the T1 and T2 segments, though it was not obvious from the inspection of
embryos in later stages because of segmentation disturbances. (D) Comparison
of the hb functions in Gryllus and Drosophila. Red
bars indicate deletions in the affected regions, while yellow bars indicate
transformation into abdominal identity (or more posterior identity in the A1
segment of Drosophila). For Drosophila, the case of the
class V alleles (Lehmann and
Nüsslein-Volhard, 1987) is shown. In Gryllus, only
three abdominal segments (dark blue arrow) are formed in severe cases of
Gb'hb RNAi depletion, probably owing to the defect in the posterior
growth accompanied by segmentation. (E) Models for regulatory networks of
hb and other segment-patterning genes in the anterior region of
Drosophila and Gryllus. In Drosophila, bcd
activates hb and Kr in the anterior region of the embryo.
hb and bcd activate eve (stripe 2), while
Kr represses it. In Gryllus, hb is activated by cad
and regulates the Kr and eve expression in the thoracic
region, directly or indirectly. Gryllus hb and probably Kr
repress abdA as in Drosophila (see text).
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