Affiliation of the parasite Herpodiscus durvillaeae (Phaeophyceae) with the Sphacelariales based on DNA sequence comparisons and morphological observations

The phylogenetic affinities of the brown alga <i>Herpodiscus durvillaeae</i>, an obligate parasite of <i>Durvillaea antarctica</i> (Fucales, Phaeophyceae) endemic to New Zealand, were analysed using combined partial nuclear encoded ribosomal DNA (26S) and plastid encoded RuBisCO (<i>rbc</i>L) gene sequences. Results from phylogenetic analyses place this species within the order Sphacelariales. Molecular data were supported by two morphological features characteristic for the Sphacelariales <i>sensu stricto</i>: the presence of apical cells and the transitory blackening of the cell wall with sodium hypochlorite solution (‘Eau de Javel’). However, the strongly heteromorphic life history distinguishes <i>H. durvillaeae</i> from all other members of the Sphacelariales of which the life cycle is known. This variability in life history provides a new systematic character for the order and should be taken into account in any comprehensive systematic revision of the Sphacelariales.