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Phaeostrophion irregulare, which is distributed along the Pacific coast of northwestern America, has been placed in the family Punctariaceae, order Dictyosiphonales (or Ectocarpales sensu lato), because of anatomical resemblances. On the basis of a previous culture study, the species was assumed to have a direct type of life history, with erect thalli forming unilocular or plurilocular zooidangia. However, the occurrence of a perennial prostrate system (holdfast), the presence of marginal meristematic cells on the holdfast and the lack of pyrenoids in chloroplasts suggest that this taxonomic assignment is questionable. Transmission electron microscopy observation confirmed the absence of pyrenoids in the chloroplasts. Molecular phylogenetic studies using rbcL (chloroplast) and partial 18S and 26S ribosomal RNA (nuclear) gene sequences revealed that Phaeostrophion is not included in the clade of Ectocarpales s.l., but forms a clade with Sphacelariales which diverged relatively early in the evoluti... |
