Abstract
We have traced the developmental origins of various CNS neurons and glial cells of a leech to 10 clonally related groups of cells, the bilaterally paired M, N, O, P, and Q kinship groups. Each kinship group is descended from one of 10 identifiable blastomeres of the early embryo, the teloblasts. Of the approximately 200 neurons in each side of a segmental ganglion, 130 to 160 are in the ipsilateral N, 20 to 50 in the O, 8 to 12 in the P, 6 to 9 in the Q, and 3 to 6 in the M kinship group. A given identified neuron or glial cell was invariably found to belong to a particular kinship group, indicating that in leech development neuronal lineage is highly stereotyped. But cells of related function and morphology do not necessarily belong to the same neuronal kinship group: of the mechanosensory neurons, the T and N neurons belong to the N, the Pv neuron belongs to the P and the PD neuron belongs to the O kinship group. Similarly, glial cells arise from all four ectodermal teloblasts. Conversely, neurons within a kinship group are not obviously related in structure or function: the N kinship group includes sensory, motor, and effector neurons and interneurons: the O and P kinship groups each include sensory neurons and interneurons; both the P and Q groups contain representatives of three distinct morphological classes of interneurons. Consequently, in early development, the determinants of neuronal identity in the leech CNS are not segregated in any obvious thematic way in the cleavages that give rise to the five bilateral pairs of teloblasts. Rather, the neural kinship groups may be merely the evolutionary vestige of a primordial distributed nervous system, each quadrant of which was derived from one teloblast.