EVOLUTION OF REPORDUCTIVE BEHAVIOR AND PATTERNING OF SEROTONERGIC NEURONS IN FREE-LIVING NEMATODE RELATIVES OF C. ELEGANS

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C. Loer, L. Rivard, E. Geltz, G. Riedesel, A. Stone
Dept Biology, Univ San Diego, San Diego, CA, USA

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As a first step toward understanding the evolution of behavior and neuronal function, we have compared the identified serotonin-IR neurons of C. elegans (C.e.) to those of 17 species of free-living nematodes representing 12 different genera. We found many neurons that are likely homologs of those in C.e. such as the NSMs in the pharyngeal nervous system; we also found sex-specific neurons such as the egg-laying HSNs, VCs, and male CAs, CPs and ray neurons which play roles in mating. The patterns of these cells vary in number and position in the species examined. Although related nematodes tend to have similar patterns of neurons, there is sufficient variability between clades that an ancestral pattern is not obvious. In females/hermaphrodites, the vulval region is typically innervated by serotonin-IR processes. Nearby somata typically include both HSNs and VCs, although some species appear to have only HSNs or only VCs. At least one group (3 species of Oschieus) has no apparent serotonergic HSNs, VCs or vulval innervation. In C. elegans, HSN is born in the tail and migrates to the central body during embryogenesis; likely HSN homologs in the species we examined are found in a variety of locations in the body. In most species, HSNs are found near a central vulva, like in C. elegans. In 2 species with posterior vulvae, HSN homologs are found in the tail. One of these (M. longespiculosa DF5017) actually has 2 pairs of HSN-like cells in the tail. Mutations in the C.e. ham-1 gene can cause PHB to develop like its sister cell HSN¹. In 3 species with central vulvae, HSNs are found about halfway between the tail and vulva. In P. oxycerca (EUK106), likely HSN somas are found in the head, perhaps indicating a longer migration than in C.e. Interestingly, over-activation of a canonical Wnt pathway in C.e. can cause HSNs to migrate more anteriorly than normal². In the C.e. male, CP neurons innervate male diagonal muscles and are involved in mail tale curling behavior during mating. CP neurons are products of the repeated Pn.a lineage that adds neurons to the ventral nerve cord postembryonically; six CPs are serotonergic. The males of all species we examined had serotonergic CP-like neurons although the number ranged from four to ten. Many species also had likely CA homologs paired with their sister CP cells. Most species showed from one to three pairs of ray sensory neurons with serotonin; C.e. normally has three pairs. Finally, in a number of species we examined, we have also shown that exogenous serotonin affects reproductive behaviors similarly to C. elegans, causing male tail curling and egg laying. Refs: ¹Desai et al. (1988) Nature 336:638. ²Forrester et al. (2004) Genetics 168:1951.