Abstract
Phaeostrophion irregulare S. & G. is distributed along the Pacific coast of NW America, and has been placed in the family Punctariaceae, order Dictyosiphonales (or Ectocarpales s.l.), based on morphological resemblances. Culture studies have indicated a direct life history, with erect thalli forming unilocular and/or plurilocular sporangia. However, the occurrence of a perennial prostrate system (holdfast), presence on the holdfast of marginal meristematic cells, and lack of pyrenoids in the chloroplasts, suggest that this taxonomic assignment is questionable. Our TEM observations confirm the lack of pyrenoids in the chloroplasts. The occurrence of reductive divisions in the unilocular sporangia was confirmed by comparing the DNA content in each nucleus using a fluorescent stain, but both unilocular and plurilocular sporangia‐bearing erect thalli showed diploidy. Molecular phylogenetic analysis using rbcL (chloroplast) and 18S rDNA (nuclear) gene sequences reveals that Phaeostrophion is not included in the clade of Ecotocarpales s.l., but diverged relatively early in the evolution of Phaeophyceae, forming a clade with Sphacelariales and Syringodermatales, which share the apical/marginal meristematic mode of growth. Although we propose a new family Phaeostrophionaceae to accommodate Phaeostrophion, we suspend judgment of the appropriate taxonomic treatment at the ordinal level.
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