**4. Conclusions**

In this study, we highlighted an expansion and particular distribution of the ray-finned fish ST8Sia repertoire owing to several duplications and loss events of *st8sia* genes, and we refined their evolutionary history. Our analyses of the molecular evolution in ST8Sia sequences and in key functional motifs (i.e., motif L and PSTD) let us suggest that the polysialyltransferases might evolved new enzymatic activities and/or specificities in the course of Vertebrate evolution. Their expression profiles in Salmonid tissues differ from those observed in mammals and further point to a subfunctionalization of these poly-α2,8-sialylatransferases. Altogether, we have laid the foundation for further studies towards understanding of the remarkable differences between α2,8-linked polySia chains found in mammals and fish.

**Supplementary Materials:** Supplementary materials can be found at http://www.mdpi.com/1422-0067/21/2/513/s1.

**Author Contributions:** Conceptualization, data curation, and writing—review and editing: S.P.G., A.R., D.P., and A.H.-L.; Funding acquisition: S.P.G., A.R., and A.H.-L.; Investigation and methodology: M.T.V., M.D., J.M.R., M.N., V.C., S.P.G., A.R., D.P., and A.H.-L.; Supervision: S.P.G., A.R., and A.H.-L.; Writing—original draft: M.T.V., S.P.G., A.R., D.P., and A.H.-L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the financial support of the CNRS, the University of Lille (FST), the program PHC Procope 2019 (project 42533RC), and the German Academic Exchange Service (DAAD) for financial support (PN:57446225).

**Acknowledgments:** The authors are very grateful to Brigitte Schöpel, Christian Plinski, Torsten Viergutz for the Laboratory support, and to Olga Plechakova for her assistance with the GT-database. We thank Ralf Bochert (Landesforschungsanstalt Mecklenburg-Vorpommern, Germany) for providing maraena whitefish. The authors acknowledge the Research Federation FRABio (Univ. Lille, CNRS, FR3688, Biochimie Structurale et Fonctionnelle des assemblages Biomoléculaires) for providing the scientific and technical environment conducive to achieving this work, the Ministère de l'enseignement supérieur, de la Recherche et de l'innovation, and the Région Hauts de France for providing research fellowships to M.N. and M.D., and the contribution of the COST Action CA18103-INNOGLY supported by the European Cooperation in Science and Technology (COST). This paper is dedicated to the memory of Roland Schauer.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **Abbreviations**

