SEROTONERGIC NERVOUS SYSTEM OF TWO MACROSTOMID SPECIES - RECENT OR ANCIENT DIVERGENCE

摘要

The serotonergic elements df the nervous system and the morphology of the copulatory stylet and gland cells in the pharyngeal gland ring were studied in two closely related turbellarians, Macrostomum hystricinum marinum and M. pusillum. Both species belong to the monophyletic Macrostomum hystricinum species group (clade), in which they are members of two separate subclades. The morphological differences observed in widely distributed meiobenthic turbellarians may reflect an ancient speciation process. Our results support the hypothesis of loss of paratomy in the Macrostomida. Immunoreactivity to serotonin (5-HT) was shown by means of a whole-mount immunofluorescence technique. Comparison of the 5-HT-positive elements of the nervous system revealed the same basic pattern, but definite differences in the organization of the longitudinal nerve cords, the commissures, the pharyngeal nervous system, and the neuropile of the brain. Our data suggest the existence of two subclades with distinct morphology. The serotonergic elements of the nervous system support the possibility of an early separation of the two species. The single pharyngeal gland cell type observed in M. pusillum, on the other hand, allows arguments for either an early or late separation of the subclades. The M. hystricinum clade may be particularly useful for further testing congruence between morphological differences and molecular sequence analysis. Furthermore, our data suggest that asexual reproduction through paratomy is correlated with the presence of numerous serially arranged serotonergic neurons along the main nerve cords. The concentration and reduction of serotonergic neurons at the postpharyngeal commissure in M. h. marinum and M. pusillum may be correlated with the loss of asexual reproduction and the loss of regeneration of the posterior end of the animals. [References: 49]