Life history and molecular phylogenetic relationships ofAsterocladon interjectumsp. nov. (Phaeophyceae)

A small prostrate filamentous brown alga, isolated from the warm temperate Pacific coast of Japan, was cultured to investigate its life history, morphology, ultrastructure and molecular phylogenetic position. The isolate had an isomorphic, diphasic life history with anisogamous sexual reproduction. Gametogenesis was regulated by temperature and occurred only at temperatures ≤14°C. At higher temperatures, gametophytes reproduced by means of asexual zoids. All swarmers from plurilocular and unilocular zoidangia lacked an eyespot. Both sporophytes and gametophytes had oligostichous filamentous axes, lateral hairs with a basal sheath and no erect thalli. Three to eight oblong chloroplasts, each with a protruding pyrenoid, aggregated to form a stellate chloroplast configuration. All these morphological features suggested that this alga belongs to the genus Asterocladon. Plurilocular gametangia produced in an intercalary or terminal position and zoids lacking eyespots, are, however, unlike those of Asterocladon lobatum, formerly the only member of the genus. Therefore, we described the alga as a new taxon, Asterocladon interjectum sp. nov. Molecular phylogenetic analyses based on the large RUBISCO subunit (rbcL) and partial SSU nuclear ribosomal RNA gene sequences showed that A. interjectum, A. lobatum and Asteronema rhodochortonoides together formed a monophyletic clade, sister to the Ectocarpales sensu lato. Because the molecular phylogenetic analyses showed Asteronema to be paraphyletic, but monophyly of Asterocladon spp. and Asteronema rhodochortonoides was supported by the absence of cytoplasmic invaginations in pyrenoids, Asteronema rhodochortonoides was transferred to Asterocladon and an emended generic circumscription of Asterocladon was included.