An oligochaete homologue of the Brachyury gene is expressed transiently in the third quartette of micromeres

We have isolated a Brachyury homologue (Ttu-Bra) from the oligochaete annelid Tubifex tubifex which displays a direct mode of development. Developmental RT-PCR analysis showed that Ttu-Bra transcripts are present in embryos at stages 9–11, 16 and 17, but undetectable at the remaining embryonic stages. Whole-mount in situ hybridization demonstrated that Ttu-Bra is expressed transiently in the third quartette of micromeres, which are located at the prospective stomodaeum (at stages 9–11). The second burst of Ttu-Bra expression occurs at the posterior end of stage 16 embryo that undergoes body elongation. Ttu-Bra-expressing cells, which are organized in a circle at stage 16, become aggregated at the proctodaeum at stage 17. Consistent with the results of the RT-PCR analysis, there is no sign of Ttu-Bra-expressing cells in embryos that undergo gastrulation during stages 12–15.