A molecular investigation of Mazzaella (Gigartinales, Rhodophyta) morphologically intermediate between Mazzaella linearis and M-Splendens

The red algal genus Mazzaella presents an interesting challenge to systematists and ecologists. The two species M. linearis and M. splendens are easily distinguished in their ideal form; however, in areas of intermediate wave exposure individuals form a seemingly continuous grade between the two species. On the basis of ecological investigations, it was concluded that the two forms are distinct at the species level and that plants of intermediate morphology can be best assigned to M. splendens. Field-identified 'classic' M. linearis and M. splendens (17 individuals each), as well as individuals of intermediate morphology (20 individuals), were collected. Sequences of the internal transcribed spacer (ITS) of the ribosomal cistron were determined for all of these individuals, as well as for related Mazzaella spp. Molecular phylogenetic analyses confirmed a close relationship between M. linearis and M. splendens, and identified their closest relative, M. sanguinea. Pairwise distance comparisons within M. splendens (including intermediate plants; 0-4 differences) and M. linearis (0-2 differences) were less than variation between species (8-12 differences), and both solidly clustered as monophyletic (M. sanguinea as outgroup; 0-2 differences among four isolates) confirming the results of the ecological investigations. Two further aspects of the earlier ecological studies are also discussed: the conclusion that stipe length is a reliable character for distinguishing between these two species; and, a suggestion that M. splendens displays a slightly heteromorphic alternation of generations, the plants of intermediate morphology being tetrasporophytes (preferring the moderately wave-exposed habitats) whereas the 'classic' M. splendens are gametophytic with a preference for wave-sheltered habitats.