The nematodes C. elegans and P. pacificus, havingdiverged more than 100 million years ago, now inhabit different habitats and display distinct patterns of behavior, including odor-mediated paralysis and predation in P. pacificus. To understand how the nervous systems of these two species have diverged, we undertook a detailed examination of the neuroanatomy of the chemosensory system of P. pacificus. Using several independent features such as cell body position, axon projections and lipophilic dye uptake, we have assigned homologies between the amphid neurons, their first-layer interneurons, and several internal receptor neurons of P. pacificusand C. elegans. We found that neuronal count and soma position are highly conserved. However,the morphological elaborations of several amphid cilia are different between these two nematodes and show less diversity among each other in P. pacificus, most notably in the absence of 'winged' cilia morphology. To assess potential divergences in gene expression patterns, we considered three phylogenetically conserved genes that encode for regulatory and signaling factors. We found that
Ppa-che-1p::rfp (zinc finger TF) animals expressed the reporter in two neuron pairs in the head region of the worm, with one of the two pairs of amphid neurons being the ASE. Interestingly, the P. pacificusAS5(ASE) axons do not cross each other at the dorsal midline, as they do in C. elegans, but instead terminate at the dorsal midline, suggesting the left and right ASE neurons are electrically coupled in P. pacificus. Furthermore, the differences in the existence of molecular regulators (
lsy-6) and effectors (gcy genes) of C. elegans ASE laterality suggest that the ASE neurons of P. pacificus may not be functionally lateralized. In contrast, the expression of the
Ppa-odr-3p::rfp (G-protein subunit) and
Ppa-odr-7p::rfp (nuclear hormone receptor) appear to have switched expression patterns in the P. pacificussuch that they are expressed inthe AD3(AWA) and AS7(AWC) counterparts, respectively.When combined with neural circuitry analysis (see abstract by S. Cook et al), these findings indicate that the major substrate for evolutionary divergence does not lie in changes in neuronal cell number or neuronal process targeting, but rather in sensory perception, cilia morphology, and synaptic partner choice within inherent constraints in neuronal processes.