Developmental trajectory of Caenorhabditis elegans nervous system governs its structural organization
Fig 1
Birth time cohort membership and neurite lengths of neurons govern their connectivity.
(A-B) Matrices representing synaptic (A) and gap-junctional (B) connections that exist between neurons, grouped into three classes [indicated by blue broken lines] according to their process lengths ℓ measured relative to the worm body length L, viz., short (ℓ ≤ L/3), medium (L/3 < ℓ ≤ 2L/3) and long (ℓ > 2L/3), and ordered within each class according to birth time. Increasing birth time is indicated by arrows, with red lines marked th (time of hatching) separating neurons which differentiate in the embryonic stage from those born later. Matrix entries correspond to the existence of a connection, with its color representing the distance (measured in mm) between cell bodies of the corresponding neurons (see legend). We observe that there is evidence of birth time assortative mixing, with neurons born early(later) having a higher probability of connecting with other early(late) born neurons, which is particularly marked in the case of neurons having short processes. The gap junction matrix shows a large number of entries adjacent to the diagonal which correspond to connections between paired neurons [see Fig 5(A)]. (C) Distribution of distances d between cell bodies of pairs of neurons distinguished in terms of their respective process lengths (S: short, M: medium, L: long), which are connected by synapses (top) and gap junctions (bottom). As synaptic connections are directed, there are nine possible combinations of pairs of the classes (S/M/L) to which the pre- and post-synaptic neurons belong (e.g., SL refers to a synapse from a neuron with a short process to a long process length neuron). On the other hand, as gap junctions are undirected, only six possible combinations need be considered. We note the bimodal distributions of d when at least one of the two neurons connected by synapse or gap junction has a long (or medium) process. (D) The mean distance 〈d〉 between cell bodies of neurons connected by synapses (left) and gap junctions (right) are grouped according to their process lengths (L/M/S) [indicated by blue broken lines] and further subdivided into those born early (i.e., embryonic stage) and those born late (i.e., L1, L2 or L3 stages) [separated by red lines]. Distances are expressed in mm (see legend for the color code). We note that pre-synaptic neurons with long processes tend to connect with post-synaptic neurons having short processes which are located far from them, corresponding to the higher peak in the bimodal distribution for LS in top panel of (C). Note that we have considered in this analysis the subset of 225 neurons for which information about process length is available.