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Fig. 9. Current model of cardioblast development and the roles of Dorsocross,
tinman and pannier. Dorsocross is involved in multiple steps
of early cardiogenesis. First (stage 10), positional information transmitted
by Dpp and Wg signals is integrated within dorsal cells of the mesodermal A
domain, in which the Doc genes are activated. In parallel, tin is
activated by Dpp in the entire dorsal mesoderm. Second (stage 11), the
cardiogenic mesoderm becomes delineated with the Doc- and
tin-dependent activation of pnr, which itself is required
for maintained expression of tin and Doc. During this stage, cells
start to align at the dorsal mesodermal margin, while ectodermal pnr
maintains dpp expression along the dorsal leading edge. The combined
action of Doc, tin and pnr selects a pool of progenitors for
all cells of the dorsal vessel, including those of the lymph gland, with the
exception of the Eve-pericardial progenitors, which are Doc independent. Third
(early stage 12), refined co-expression of Doc, tin and pnr
leads to the activation of specific target genes in presumptive cardioblasts
or their progenitor cells, thereby promoting cardioblast specification. Owing
to additional localized inputs (particularly on svp, which
additionally requires extrinsic Hh signals)
(Ponzielli et al., 2002 ),
cardioblast fate diversifications are also initiated during this process.
Other regulators, including mid and hand, are turned on in
all cardioblasts. During the second half of stage 12, tin and Doc are
re-activated in complementary subsets of cardioblasts through the actions of
mid and svp, respectively, as indicated (broken arrow
indicates permissive role of svp on Doc; S. Zaffran, I.R. and M.F.,
unpublished). Tin and Doc, either alone or in combination with uniformly
expressed cardioblast factors, then activate various regulatory and
differentiation genes in the respective subtypes of cardioblasts. In the heart
portion, which is defined by abd-A expression within cardioblasts,
Doc-positive cardioblasts form ostial cells marked by wg expression,
whereas the Tin-positive cells form `regular' myocardial cells marked by
Sulfonylurea receptor (Sur) expression.
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D-lacZ, stained with
anti-ß-Galactosidase (dorsal view). ß-Galactosidase signals mark all
cardioblasts of the dorsal vessel of wild-type embryos (A), but are absent in
homozygous Df(3L)DocA embryos (B). (C-H) Lateral views of stage 14
embryos fluorescently labeled for markers, as indicated by the color code.
Control embryos (left column) and homozygous Df(3L)DocA mutant
embryos (right column) carry c381-GAL4 and UAS-Doc2 used for
amnioserosa rescue. (C,D) Detection of mid RNA and Tin protein as
markers for cardioblasts (CB; mid-positive and Tin+) and
Tin+ pericardial cells (PC; mid-negative). The number of
Tin+ cells is severely reduced and mid-expressing
cardioblasts are almost absent in mutants. mid RNA is still
detectable in ectodermally derived tissues. (E,F) Detection of Tin and Eve
proteins. Tin+ cardioblasts (Tin-CB, red) and pericardial cells
(Tin-PC, red) are largely lost in the DocA mutant. Most of the
remaining Tin+ cells are Eve-pericardial cells that express both
markers (Eve-PC, yellow). Eve+ dorsal muscles (DA1, green) are also
present in DocA mutants. (G,H) Embryos carrying one copy of AE127
(svp-lacZ) stained for ß-Galactosidase and Mef2 to identify
Svp-cardioblasts (Svp-CB), which are absent in the DocA mutant
(asterisk). (I) Dorsal vessel of Df(3L)DocA svp-lacZ heterozygous
embryo at stage 16 stained as in G showing normal number of cardioblasts. (J)
Defective dorsal vessel of Df(3L)DocA svp-lacZ/Df(3L)29A6
transheterozygous embryo stained as in G and I. The reduced Doc gene copy
number has caused the loss of numerous cardioblasts, especially of the Svp-CB
type. Asterisks indicate positions of Tin-CB loss. (K-M) Dorsal trunk mesoderm
of early to mid stage 12 embryos stained for Mef2 protein (red) and
mid RNA (green). (K) mid expression initiates in small
clusters along the dorsal edge of the mesoderm (arrowhead) in the wild-type
(K) but not in the DocA mutant embryo (L). (M) Mesodermal expression
of Doc2 via tinD-GAL4 rescues cardiac mid
expression in a homozygous Df(3L)DocA mutant background.
