Phylogenetic Re-Evaluation of the Laurencia Complex (Rhodophyta) with a Description of L. Succulenta Sp. Nov. from Korea

Abstract

Laurencia succulenta sp. nov. (Rhodophyta) is described from Korea. This species exhibits vegetative and reproductive structures typical of the genus, but is distinct from similar species in its epiphytic habitat and the fleshy, robust, thick and subcompressed thalli with basically distichous branching. In addition, it is readily distinguished from the most similar species, such as L. nipponica Yamada and L. okamurae Yamada, by the cystocarps with a somewhat protuberant ostiole. In a phylogenetic analysis of 47 species of the Laurencia Lamouroux complex from various localities around the world based on 49 morphological characters, four major clades (Laurencia, Chondrophycus palisadus (Yamada) Nam group, C. cartilagineus (Yamada) Garbary et Harper group and Osmundea Stackhouse assemblage), each of which forms a monophyletic group, were recognized. Among these, the Laurencia clade is basal to the overall assemblage, and is defined by the vegetative axis with four rather than two pericentral cells. The Osmundea clade is supported by autapomorphic characters for the genus, features associated with spermatangial formation of the filament type and tetrasporangial production from epidermal cells. By contrast, Chondrophycus, a genus characterized by a combination of features (vegetative axis with two pericentral cells, trichoblast-type spermatangial development and tetrasporangial production from pericentral cells), is paraphyletic, and the species were separated into two well-supported clades, the C. palisadus group and C. cartilagineus group. These clades are distinguished from each other by the position of the first pericentral cell relative to the trichoblast, the presence or absence of fertility at the second pericentral cells and number of sterile pericentral cells in the tetrasporangial axis, the pattern of formation of spermatangial branches on trichoblasts, post-fertilization feature associated with the formation time of the auxiliary cell, and, probably, the number of pericentral cells in the procarp-bearing segment. Of these features, the side position of the first pericentral cell in the latter group (a synapomorphy for the C. cartilagineus group plus Osmundea) suggests that the C. cartilagineus group is more closely related to Osmundea than to the C. palisadus group. This cladistic analysis indicates that Chondrophycus is not monophyletic, suggesting that the C. palisadus group should be separated from Chondrophycus at the genus level. Based on this result, Palisada (Yamada) stat. nov. is proposed for the group, together with an emendation of the generic delineation of Chondrophycus, and relevant nomenclatural changes for several Chondrophycus species are also included. In addition, Corynecladia J. Agardh is reinstated for the type species L. clavata Sonder.