Abstract
Bryozoans are aquatic invertebrates that inhabit all types of aquatic ecosystems. They are small animals that form large colonies by asexual budding. Colonies can reach the size of several tens of centimeters, while individual units within a colony are the size of a few millimeters. Each individual within a colony works as a separate zooid and is genetically identical to each other individual within the same colony. Most freshwater species of bryozoans belong to the Phylactolaemata class, while several species that tolerate brackish water belong to the Gymnolaemata class. Tissue samples for this study were collected in the rivers of Adriatic and Danube basin and in the wetland areas in the continental part of Croatia (Europe). Freshwater and brackish taxons of bryozoans were genetically analyzed for the purpose of creating phylogenetic relationships between freshwater and brackish taxons of the Phylactolaemata and Gymnolaemata classes and determining the role of brackish species in colonizing freshwater and marine ecosystems. Phylogenetic relationships inferred on the genes for 18S rRNA, 28S rRNA, COI, and ITS2 region confirmed Phylactolaemata bryozoans as radix bryozoan group. Phylogenetic analysis proved Phylactolaemata bryozoan's close relations with taxons from Phoronida phylum as well as the separation of the Lophopodidae family from other families within the Plumatellida genus. Comparative analysis of existing knowledge about the phylogeny of bryozoans and the expansion of known evolutionary hypotheses is proposed with the model of settlement of marine and freshwater ecosystems by the bryozoans group during their evolutionary past. In this case study, brackish bryozoan taxons represent a link for this ecological phylogenetic hypothesis. Comparison of brackish bryozoan species Lophopus crystallinus and Conopeum seurati confirmed a dual colonization of freshwater ecosystems throughout evolution of this group of animals.
Highlights
Bryozoans are one of the most enigmatic groups in the animal kingdom regarding their insufficiently known evolutionary relationships
Two of collected samples belong to the Gymnolaemata class (Conopeum seurati and Paludicella articulata), while ten of them belong to the Phylactolaemata class (Cristatella mucedo, Fredericella sultana, Lophopus crystallinus, Hyalinella punctata, Plumatella casmiana, Plumatella emarginata, Plumatella fruticosa, Plumatella fungosa, Plumatella geimermassardi, and Plumatella repens)
It is clear that the Lophopodidae family separated independently in the evolutionary history of freshwater taxons from all other members of Plumatellida genus of Phylactolaemata class
Summary
Bryozoans are one of the most enigmatic groups in the animal kingdom regarding their insufficiently known evolutionary relationships. The study of the nucleotide composition of 28S rRNA gene supported the close relationship of Phylactolaemata and Stenolaemata, while the analysis of the 18S rRNA and COI gene classified Phylactolaemata as the radix group within the Bryozoa phylum (Fuchs et al 2009). Monophyletic origin of bryozoans has been proved by molecular approach with the representatives of Phoronida and Brachiopoda (Fuchs et al 2009). Phylogenetic studies show a closer affinity of Phylactolaemata to Phoronida than with other classes of Bryozoa (Mundy et al 1981) and closer evolutionary relatedness between Stenolaemata and Gymnolaemata classes (Waeschenbach et al 2012).
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