Embryonic expression of a decapentaplegic gene in the oligochaete annelid Tubifex tubifex

We have cloned and characterized the expression of a decapentaplegic homologue (designated Ttu-dpp) from the oligochaete annelid Tubifex tubifex. RT-PCR analysis and in situ hybridization revealed that Ttu-dpp begins to be expressed around the time of the onset of ectodermal germ band (GB) elongation (i.e., the onset of gastrulation). At this time, Ttu-dpp expression is detected in the anteriormost part of the GBs. As development proceeds and the GBs elongate, the domain of Ttu-dpp-expressing cells extends posteriorly. Then Ttu-dpp-expressing cells within the GB are divided into two groups: one group occurs along the ventral midline and coincides with the domain of ventral ganglia; the other is located more dorsally. The latter group of Ttu-dpp-expressing cells subsequently undergoes dorsalward expansion, which results in the formation of a lateral stripe of cells in every segment except the first (i.e., segment I). In embryos that undergo body elongation (that is one of the last morphogenetic movements occurring prior to hatchout), Ttu-dpp expression in the lateral region is confined to setal sacs, which are arranged in the same transverse plane around the periphery of each segment (except segment I).