Developmental trajectory of Caenorhabditis elegans nervous system governs its structural organization
Fig 7
Neurons functioning as connectors between different network modules lead in development.
(A) Schematic representation of the network of neurons belonging to the somatic nervous system of Caenorhabditis elegans, indicating the role of each neuron (indicated by the node size, see legend) in the mesoscopic structural organization of the network. This organization is manifest in the partitioning of the entire network into six structural modules [34] which are characterized by relatively dense connections among neurons in each module compared to the connections between neurons belonging to different modules (node color representing the identity of a module to which a neuron belongs). Within each module, neurons can be further distinguished into those which have significantly higher number of connections to neurons within their own module (hubs) and those which do not (non-hubs). According to their intra- and inter-modular connectivity, every neuron is then classified into one of seven possible categories (see Methods), viz. R1: ultra-peripheral (non-hub nodes with all their connections confined to their own module), R2: peripheral (non-hub nodes with most of their connections occurring within their module), R3: satellite connectors (non-hub nodes having with many connections to other modules), R4: kin-less (non-hub nodes with connections distributed uniformly among all modules), R5: provincial hubs (hub nodes with a large majority of connections within their module), R6: connector hubs (hub nodes with many connections to other modules) and R7: global hubs (hub nodes with connections distributed uniformly among all modules). One representative neuron from each of the categories is separately indicated with a label identifying them by name (note that there are no neurons in the C. elegans somatic nervous system which belong to categories R4 or R7). Neurons which function as connectors, e.g., RIAL (R6) and RIFL (R3), are seen to have links to neurons belonging to many different modules (as indicated by the node color of their network neighborings) while neurons belonging to other categories are connected predominantly to neurons within their own modules (indicated by their network neighborhood being almost homogeneous in terms of node color). Neighbors of labeled neurons are either shown clustered around them (for VA07, PVM, RIFL and DD02) or indicated by a lighter shade of node color (for RIAL). (B) Distributions of differentiation times of neurons belonging to the different network functional role categories indicate that the development of those functioning as connectors and/or hubs (i.e., R3, R5 and R6) lead the other classes of neurons in the embryonic, as well as, L1 stages. In particular, more than 90% of satellite connectors, provincial hubs and connector hubs have appeared before hatching, while for the peripheral categories (R1 and R2), 70% or less of their members would have differentiated by that time.