Developmental significance of D quadrant micromeres 2d and 4d in the oligochaete annelid Tubifex tubifex.

The annelidTubifex tubifex is a cosmopolitan freshwater oligochaete and a member of the Spiralia, a large group of invertebrate phyla displaying spiral development. Because its developing eggs are easily obtained in the laboratory, this animal has long been used as material for developmental studies, especially spiralian embryology. In spiralian embryos, it has long been known that one blastomere at the four-cell stage, the D cell, and its direct descendants play an important role in axial pattern formation. Various studies have suggested that the D quadrant functions as the organizer of the embryonic axes in molluscs and annelids, and it has recently been demonstrated that the D quadrant micromeres, 2d(11) and 4d, which had been transplanted to an ectopic position in an otherwise intact embryo induce a secondary embryonic axis to give rise to the formation of duplicated heads and/or tails. That 2d and 4d play a pivotal role in Tubifex embryonic development was first suggested from the classic cell-ablation experiments carried out in the early 1920s, and this has been confirmed by the recent cell-ablation/restoration experiments using cell-labeling with lineage tracers. These studies have also shown that in the operated embryos, none of the remaining cells can replace the missing 2d and 4d and that both 2d and 4d are determined as ectodermal and mesodermal precursors, respectively, at the time of their birth. The anteroposterior polarity of these micromeres is also specified at the time of their birth, suggesting that nascent 2d and 4d are specified in their axial properties as well as in cell fate decision.