N. Azpiazu et M. Frasch, TINMAN AND BAGPIPE - 2 HOMEO BOX GENES THAT DETERMINE CELL FATES IN THE DORSAL MESODERM OF DROSOPHILA, Genes & development, 7(7B), 1993, pp. 1325-1340
Whereas the mechanisms of early Drosophila mesoderm formation have bee
n studied in much detail, the subsequent processes determining regiona
l identities within the mesoderm remain largely unknown. Here, we desc
ribe two homeo box genes, tinman (tin) and bagpipe (bap), which spatia
lly subdivide the mesoderm and determine cell fates in the dorsal meso
derm. These two genes are components of a cascade of genetic interacti
ons that result in the spatial restriction of tin mRNA to the dorsal m
esoderm and in the activation of bap in segmental clusters of cells in
this region. A subset of cells from those clusters segregate to form
visceral mesoderm that differentiates into gut musculature. This indic
ates that the visceral mesoderm is derived from metamerically repeated
primordia. In embryos mutant for bap, visceral mesoderm formation is
strongly disrupted. Most cells of the visceral mesoderm fail to differ
entiate properly, and a portion of them are transformed into body wall
musculature and gonadal mesoderm. In tin mutant embryos, bap expressi
on is not activated in the dorsal mesoderm. Probably as a consequence,
neither visceral mesoderm nor midgut musculature are formed in these
mutants, and the absence of visceral mesoderm results in strong disrup
tions of endoderm migration and midgut morphogenesis. In addition to v
isceral mesoderm development, tin is required for the formation of the
heart from dorsal mesoderm and for the specification of founder cells
for particular body wall muscles.