COMPARATIVE ANATOMY OF SEROTONIN-LIKE IMMUNOREACTIVE NEURONS IN ISOPODS - PUTATIVE HOMOLOGS IN SEVERAL SPECIES

It is now commonly accepted that the arthropod nervous system has evol ved only once, and so homologies between crustacean and insect nervous systems can be meaningfully sought. To do this, we have examined the distribution of serotonin (5-hydroxytryptamine)-like immunoreactive ne urons in the central nervous system (CNS) of four common British isopo ds. Two species of terrestrial woodlouse, Oniscus asellus and Armadill idium vulgare, the littoral sea slater, Ligia oceanica, and the aquati c water hoglouse, Asellus meridianus, all possess approximately 40 pai rs of serotonin-like immunoreactive neurons, distributed throughout th e CNS in a very similar pattern. Interspecific homology is clearly sug gested. Serotonin-like immunoreactive neurons in the first (T1) and fo urth (T4) thoracic ganglia are particularly prominent in each of the f our species studied. Whole-mount immunohistochemistry shows that the p air of T1 neurons have large dorsolateral cell bodies and prominent ne urites that project medially and then anteriorly, whereas the pair of T4 neurons have ventrolateral cell bodies and neurites that bifurcate to form a thin axon projecting anteriorly to terminate in T3 and a thi ck medial axon that projects posteriorly into the abdominal neuromeres of the terminal ganglion. Intracellular cobalt staining of these neur ons reveals more of their arborizations: The T1 neurons send three pro cesses anteriorly, which arborize in the brain and exit from the CNS v ia peripheral nerves, whereas the T4 neurons contribute considerably t o the extensive pattern of serotonin-like immunoreactive fibres in T3- T6 ganglia. The overall pattern of serotonin-like immunoreactive neuro ns in the isopods is similar to that in decapod crustacea, and a numbe r of putative homologies can be assigned. It is more difficult to homo logize the isopod serotonin-like immunoreactive neurons with those in the insect CNS, but some stained brain and thoracic neurons share comm on cell body positions and axon trajectories in isopods, decapods, and insects and may therefore be homologous. (C) 1994 Wiley-Liss, Inc.