Mapping micromere fate in the clitellate annelid Tubifex tubifex

In spiralians, the relationship between early cleavage patterns and the establishment of the juvenile body plan remains one of the central questions. In the oligochaete Tubifex tubifex, the micromeres other than 2d and 4d are bilaterally symmetrically arranged in an arc-shaped assemblage that surrounds the anterior side of the large 2d micromere. The bilateral configuration of the micromeres in Tubifex is distinct from the radially symmetric micromere arrangement in other spiralians. To address the possibility that the bilaterally symmetric arrangement of the micromeres contributes to establishing bilateral symmetry in Tubifex embryos, we mapped micromere fate using DiI as a lineage tracer. The developmental fates of all the micromeres except 2b, 2d, 3b, and 4d were examined. The results showed that the micromeres in T. tubifex embryos lineage-specifically contribute to the head epidermis, cerebral ganglion, buccal cavity, pharynx, oesophagus, and yolk sac ectoderm, comprising the dorsal temporary epithelium and ventral/ventrolateral epidermis, of the juvenile. The micromeres, except for 1c, 1d, and 2c, contributed to a single structure (tissue or organ) of the juvenile. T. tubifex shares most of these micromere-specific structures with other annelids, such as Helobdella, Capitella, and Platynereis. There are two exceptions. One is the temporary epithelium derived from 1c and 1d, which is clitellate-specific; the other is the ventral epidermis derived from 3c and 3d, which is species-specific. The present results showed that the micromeres located to the left of the midline at the 22-cell stage generate the left side of the embryo in terms of tissues and organs, and those located to the right of the midline give rise to the right side of the embryo based on the position of individual micromeres relative to the (future) midline. The pattern of micromere contribution to the left and right sides of the Tubifex embryo is almost identical to that in other spiralians. These results imply that in T. tubifex, the bilateral juvenile body plan is set up by the early cleavage pattern.