Phylogenetic position of Petrospongium rugosum (Ectocarpales, Phaeophyceae): insights from the protein-coding plastid rbcL and psaA gene sequences 1 Ga Youn CHO and Sung Min BOO * Department of Biology ,Chungnam National University , Daejon 305-764,Korea (Received 19 May 2005, accepted 31 J uly 2005) Abstract — The spongy, crustose brown alga Petrospongium rugosum (Okamura) Setchell et Gardner occurs in Korea, Japan, Australia, New Zealand,and along the Pacificcoast of North America. Although the species has been classified in the Chordariaceae of the Ectocarpales sensu lato or the family Leathesiaceae of the Chordariales sensu stricto, the relationship of the species to other brown algal lineages is less studied in terms of the plastid ultrastructure and molecular phylogeny. We examined the morphology of P . rugosum and also determined protein-coding psaAand rbcL sequences from four samples of the species from different locations ,comparing them with homologous positions of newly sequenced putative relatives (Leathesia difformis and Spermatochnus paradoxus) and withpublished sequences of other brown algae. The species occurs in the upper intertidal zone on the Korean south coast from November to J une. Thalli aremarkedly rugose and are comprised of haplostichous filaments ,arranged into cortical and medullary layers . Unilocular sporangiaariselaterally on the lower cells of corticallayers. A large pedunculatepyrenoid,with a cap, is present in the parietal discoid plastids . The specimens from four different locations were almost identical in rbcLand psaA sequences, and were monophyletic. All phylogeneticanalyses of both genes reveal that P . rugosum is clearly separated from Leathesia and other members of the Chordariaceae. The sister relationship of the species to Ectocarpus was not supported by bootstrap or Bayesian analyses. Petrospongium rugosum / brown algae / molecular phylogeny / psaA / rbcL / pyrenoid / taxonomy / ultrastructure Résumé — Position phylogénétique de Petrospongium rugosum (Ectocarpales, Phaeophyceae) : contributions des séquences des gènes plastidiaux rbcL et psaA. L’algue brune crustacée et spongieuse Petrospongium rugosum (Okamura) Setchell et Gardner est présente en Corée,au Japon, en Australie, en Nouvelle-Zélande et le long de lacôte Pacifique de l’Amérique du Nord. Bien que cette espèce ait été placée dans les Chordariaceae,au sein des Ectocarpales sensu lato, ou dans la famille des Leathesiaceae, dans les Chordariales sensu stricto, ses relations avec les autres lignées d’algues brunes a été peu étudiée en termes d’ultrastructure plastidiale et de phylogénie moléculaire. Nous avons examiné la morphologie de P . rugosum et déterminé les séquences des gènes psaA et rbcLà partir de quatre récoltes provenant de différentes localités . Nous avons comparé les séquences aveccelles , homologues , d’espèces supposées proches et nouvellement Cryptogamie,Algol., 2006, 27 (1): 3-15 © 2006 Adac. T ous droits réservés 1. The authors are very pleased to dedicate this paper toDr. R. Delépine for his contribution to brown algae. * Correspondence and reprints: smboo@cnu.ac.kr Communicating editor:Frederik Leliaert