Evolution and Biogeography of Laminarialean Kelps

Abstract

This review covers the evolution of Laminariales and recent phylogeographic studies focusing on Chorda and Undaria. In Laminariales, the phylogenetic relationships between basal families (i.e., Akkesiphycaceae, Pseudochordaceae, and Chordaceae) with simple thallus structures, and derived families (i.e., Alariaceae, Laminariaceae, and Lessoniaceae) with large elaborate sporophytes, have remained unclear. Derived Laminariales have been suggested to consist of three major clades roughly corresponding to Alariaceae, Agaraceae (=Costariaceae), and Laminariaceae/Lessoniaceae. Recently, a novel species Aureophycus aleuticus, basal to all derived Laminariales, was found in the Bering Sea, and was shown to be basal to all derived Laminariales. Geographically, the majority of derived families in the Laminariales only occur in the Northern Hemisphere, and the Laminariales are therefore considered to have originated in the Northwestern Pacific and spread to the other regions including the Atlantic and the Southern Hemisphere. The limited distributional range of the basal families Akkesiphycaceae and Pseudochordaceae in northeastern Asia, and that of Aureophycaceae in the Bering Sea, supports the notion that the Laminariales originated in the Northwestern Pacific and evolved to giant taxa such as Macrocystis in the course of dispersal to the Northeastern Pacific, perhaps through the Aleutian Archipelago. Chordaceae has wide distributional ranges both in the Atlantic and Pacific oceans, but of the four clearly recognized species of Chorda, C. asiatica, C. rigida, and C. kikonaiensis are found only in the Northwestern Pacific, whereas C. filum is also found in the North Atlantic. In addition, the genetic diversity within C. asiatica is greater than that of C. filum, and it is suggested that Chorda also originated and diverged in the Pacific, then spread into the Atlantic. In the genus Undaria (Alariaceae), three species, U. pinnatifida, U. undarioides, and U. peterseniana have been traditionally recognized based on morphological characters, and U. crenata was recently described. Based on the genetic studies, U. crenata was considered to be conspecific with U. pinnatifida or U. peterseniana, although the three species were shown to be genetically rather close, at present we suggest retaining the species-level taxonomy. Based on genetic analyses, we discuss the likely geographic origin and dispersal pathways for nonindigenous populations of U. pinnatifida.