Distribution and evolution of serotonin-like immunoreactive cells in Thaliacea (Tunicata).

Thaliacea are marine planktonic animals within the taxon Tunicata. The species-poor taxon is characterized by diverse life cycles, with sexually (blastozooid) and asexually (oozooid) reproducing generations, that usually evolved polymorph phenotypes. While recent molecular phylogenetic studies indicate that tunicates might be closest living relatives to free-living craniates, additional insights into the evolution of the nervous systems of planktonic tunicates are overdue. To test the hypothesis that polymorphism in the different stages correlates with different distribution patterns of neurotransmitters, we comparatively conducted immunostaining experiments utilizing antibodies against serotonin and tyrosinated-α-tubulin, and confocal laser scanning microscopy in planktonic Thaliacea. We considerably increase the available data on serotonin-like immunoreactive (serotonin-lir) in Thaliacea and found that the pattern of distribution of serotonin-lir cells clearly differs between oozooid and blastozooid stages. Thereby, we reveal that the distribution of serotonin-lir cells in different tissues of thaliaceans can be considered as independent characters. Further, we test the potential phylogenetic signal present in serotonin-lir in thaliaceans, propose a phylogenetic mapping of conceptualized characters, and analyze the evolution of serotonin-lir cells. Comparing our results with data from previous studies indicated that it was necessary to reevaluate already described distribution patterns of serotonin-lir. Due to the complex life cycles of Salpida and Doliolida, erroneous descriptions of the localization of serotonin-lir occur in thaliacean literature. To facilitate the evaluation of data on serotonin-lir in thaliaceans for future readers, we suggest depicting whole-mount stainings of complete animals. [ABSTRACT FROM AUTHOR]