Xylosyltransferases (XylTs) play key roles in the biosynthesis of many different polysaccharides. These enzymes transfer D-xylose from UDP-xylose to substrate acceptors. In this study, we identified 30 XylTs from primary endosymbionts (green algae, red algae, and glaucophytes) and secondary or higher endosymbionts (brown algae, diatoms, Eustigmatophyceae, Pelagophyceae, and Cryptophyta). We performed comparative phylogenetic studies on key XylT subfamilies, and investigated the functional divergence of genes using RNA-Seq. Of the 30 XylTs, one β-1,4-XylT IRX14-related, one β-1,4 XylT IRX10L-related, and one xyloglucan 6-XylT 1-related gene were identified in the Charophyta, showing strong similarities to their land plant descendants. This implied the ancient occurrence of xylan and xyloglucan biosynthetic machineries in Charophyta. The other 27 XylTs were identified as UDP-D-xylose: L-fucose-α-1,3-D-XylT (FucXylT) type that specifically transferred D-xylose to fucose. We propose that FucXylTs originated from the last eukaryotic common ancestor, rather than being plant specific, because they are also distributed in Choanoflagellatea and Echinodermata. Considering the evidence from many aspects, we hypothesize that the FucXylTs likely participated in fucoidan biosynthesis in brown algae. We provide the first insights into the evolutionary history and functional divergence of FucXylT in algal biology.
Keywords: Algal polysaccharides; Functional divergence; Xylosyltransferase evolution.