Light and electron microscopy, molecular phylogeny and mannitol content of Dictyotopsis propagulifera (Dictyotales, Heterokontophyta) isolated from mangrove mud in Perak, Malaysia

Dictyotopsis propagulifera is an enigmatic species of brown algae, first described by Wilhelm Troll in 1931. In the present study, we successfully established a unialgal culture strain of D. propagulifera collected from mangrove sediments in Perak, Malaysia, directly across the Malacca Strait from the type locality in Indonesia. Our observations over 10 years of culture confirm vegetative propagule formation as originally described by Troll. However, sexual reproduction typical of Dictyotales (e.g. flagellate sperm and non-flagellate eggs) was not confirmed. Detailed ultrastructural analysis revealed that D. propagulifera shares many cellular characteristics with Dictyota, including chloroplasts with the three-layered thylakoid lamellae, well-developed plasmodesmata forming pit fields and the presence of a single apical meristematic cell. In addition, biochemical analysis confirmed the presence of the polyol mannitol as main photosynthetic product and organic osmolyte, which is characteristic for brown algae. Phylogenomic analysis using both plastid and mitochondrial genomes, along with multigene analysis, further supports the monophyly of D. propagulifera and Dictyota, following the earlier divergence of Dictyopteris. These findings not only provide an updated description of D. propagulifera based on modern microscopy techniques, but also propose potential evolutionary pathways responsible for its developmental patterns and asexual reproductive strategy.