Cell position and developmental fate in leech embryogenesis.

The o and p blast cell bandlets of the leech Theromyzon rude, which normally produce two different sets of identifiable cells designated the "O" and "P" fates, respectively, form an equivalence group: in embryos experimentally deprived of their p bandlet, the blast cells of the adjacent o bandlet may "transfate" and take on the P fate. Loss of the p bandlet is not, however, a sufficient condition for transfating of the o bandlet. Rather, loss of the p bandlet allows the o bandlet to shift into ectopic positions, and it is the ultimate position of the o bandlet that mandates which fate--O or P--the blast cells will take on. Therefore, the choice of the pluripotent o blast cells to follow either the O or P developmental pathway depends on their perception of positional cues provided by cells outside the equivalence group rather than on a direct interaction with p blast cell equivalence group members.