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Fig. 9. Model for the patterning of the A. gambiae dorsal ectoderm.
(A) The diagrams represent cross-sections of early mosquito (bottom)
and fruit fly (top) embryos. The sog expression pattern (blue) is
restricted to ventral regions of mosquitoes, but is broadly distributed in
lateral regions of D. melanogaster. The ventralization of the
sog pattern in A. gambiae might cause the indicated
expansion of Dpp signaling and pMad expression (red). There is sequential
expression of Sog and Tld in both the fruitfly and mosquito embryo. However,
the two patterns are shifted towards ventral regions in the mosquito embryo.
(B) Quality of Dorsal binding sites in the Anopheles sog
enhancer (average score of 6.7) as compared with those in the
Drosophila enhancer (average score of 10.1). The score range covered
by the box contains 50% of all data points (the second and third quartiles of
distribution). The bottom and top marks correspond to maximal and minimal
score values, respectively (see Papatsenko
and Levine, 2005). The Roman numerals beneath the plots indicate
each of the three major patterning thresholds. For example, the htl
and sna enhancers are type 1 enhancers that are activated only by
high levels of the Dorsal gradient. (C) Tld is responsible for
generating a peak of Dpp signaling at the dorsal midline, resulting in a spike
of pMad activity in the Drosophila dorsal ectoderm (left panels). By
contrast, the altered patterns of tld and sog expression in
A. gambiae embryos are expected to generate two peaks of Dpp
signaling activity, resulting in the broad plateau of pMad staining in the
dorsal ectoderm (right panels). The subdivision of the dorsal ectoderm into
distinct amnion and serosa lineages can be explained on the basis of the
expanded pMad staining pattern, and the recruitment of the repressor Ttk into
the Dpp signaling network. The asterisks indicate specific regulatory linkages
that are lost in D. melanogaster. Only one of these linkages is
required for the expression of ttk, or some other serosa-specific
repressor in A. gambiae.
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