Developmental trajectory of Caenorhabditis elegans nervous system governs its structural organization
Fig 6
Developmental histories of neurons show a bifurcation into early and late branches, with a predominance of motor neurons in the latter.
(A) Bulk of the sensory and interneurons appear early, i.e., during the embryonic stage, while a large fraction of motor neurons differentiate much later (L2 or L3) during development. (B) Planar projections of a three-dimensional representation of the developmental history of the entire somatic nervous system of C. elegans. Different colors and symbols have been used to denote distinct neuron types (viz., sensory, motor and interneurons). The projection on the top surface shows the lineage tree with branching lines connecting the single cell zygote (shown at rung 0) to each of the differentiated neurons located on their corresponding rungs. At higher rungs (>11) we see that the differentiated cells are tightly clustered into two bundles of branches with a predominance of motor neurons (also seen in the chrono-dendrogram projection shown at the right face of the base). We note the absence of segregated clusters comprising exclusively the same functional type of neurons (viz., sensory, motor or inter), suggesting that the progenitor cell can give rise to neurons of different types. This in turn implies that commitment to a particular neuron function occurs quite late in the sequence of cell divisions. The projection along the base (left face) shows trajectories representing the developmental history of each final differentiated neuron, indicating the time of each cell division starting from the zygote along with the corresponding rung. For the first few rungs, cell divisions across different lineages appear to be synchronized and occur at regular time intervals, which is manifested as an almost linear relation between time of division and rung. However, between rungs 6-9, we observe a bifurcation of the trajectories into two clusters widely separated in time. One of these comprises cells which differentiate in the embryonic stage (termed as the “early branch”) while the other consists of cells that differentiate much later (“late branch”). This is manifested in a bimodal distribution of birth times for neurons occurring in rungs ≥ 10. In contrast to the regularly spaced cell divisions in the early branch, the trajectories belonging to the late branch are widely dispersed, with relatively little correlation between birth time of neurons and their rungs.