Specification and positioning of the Drosophila heart is mutually dependent on positional and region-specific information. W.K. Lockwood 1, R. Bodmer 2. 1) Dept. of Anatomy & Cell Biology; 2) Dept. of Biology, University of Michigan, Ann Arbor.
wingless (wg) and decapentaplegic (dpp) encode secreted signaling molecules expressed orthogonally to each other in the embryo, providing positional information along two axes independently. The re-use of wg and dpp in patterning events throughout the Drosophila embryo raises the question of how the same positional signals can uniquely specify many different cell types. This problem has two aspects, as wg and dpp are required not only for patterning multiple regions of the embryo, but within each of these regions, are required for the specification of multiple cell types. We considered each of these aspects of wg and dpp signaling with respect to the specification and positioning of the Drosophila heart.
Complexity within a region is generally thought to occur as the result of morphogenetic gradients or sequential relay systems. wg and dpp are expressed in dynamic patterns in the embryo, suggesting a third alternative. We altered the patterns of wg and dpp within the mesoderm, and show that the elaboration of the tinman (tin) expression pattern is determined by both the early, primary patterns of wg and dpp, as well as their later, secondary patterns. Wildtype and ectopic heart specification also occurs only in regions exposed to both the primary and the secondary wg/dpp intersects.
Complexity between regions could be explained by the pattern of region-specific genes. For example, the mesoderm-specificity of tin is sufficient, at least in principle, to ensure that the cellular response to wg and dpp is mesoderm-specific. In this case it should be possible to generate ectopic heart not only by altering wg and dpp within the tin-expressing mesoderm, but also reciprocally, by altering tin within the wg- and dpp-expressing ectoderm. We show that the mis-expression of tin in thoracic imaginal primordia, ectodermal regions exposed to wg and dpp, also results in the ectopic specification of heart.