Molecular phylogeny of the brown algal genera Akkesiphycus and Halosiphon (Laminariales), resulting in the circumscription of the new families Akkesiphycaceae and Halosiphonaceae

Abstract

Phylogenetic relationships of Akkesiphycus lubricum with the Laminariales and related taxa were assessed by molecular phylogenetic analyses based on the Rubisco large subunit gene (rbcL), internal transcribed spacer regions (ITS1 and ITS2), and 18S ribosomal DNA (18s rDNA) sequence data. Among the three data sets, rbcL gene sequences gave the best resolution for indicating familial and ordinal relationships within the Laminariales and related taxa. Using rbcL data, Akkesiphycus showed the closest phylogenetic relationship to the Pseudochordaceae (Laminariales) and this was also supported by ITS and 18S rDNA data. The clade of Akkesiphycus and the Pseudochordaceae formed a sister group to the Alariaceae/Laminariaceae/Lessoniaceae group and the Chordaceae of the Laminariales in maximum parsimony and neighbour-joining analyses. Morphologically, Akkesiphycus differs significantly from other Laminariales in having gametophytes with anisogamous planogametes. However, considering that some laminarialean species have residual flagella in eggs and the close phylogenetic relationship with the Pseudochordaceae, as demonstrated in the present study, we propose to establish a new family, the Akkesiphycaceae Kawai & Sasaki fam. nov., to accommodate A. lubricum; we place it in the order Laminariales. The Akkesiphycaceae is distinct in having foliose sporophytes with multicellular paraphyses and monomorphic dioecious gametophytes that form anisogametes. As a result of inclusion of the Akkesiphycaceae, the ordinal definition of the Laminariales has to be emended to include anisogamy. On the other hand, the distant phylogenetic relationship between Halosiphon and Chordaceae, suggested previously by ITS and 18S rDNA data, was confirmed by rbcL data, and hence, a second new family, the Halosiphonaceae Kawai & Sasaki fam. nov. is described, although its ordinal assignment needs further discussion. The family Halosiphonaceae is characterized by having simple, cord-shaped sporophytes with multicellular assimilators and monoecious oogamous gametophytes.