DNA BARCODING AND GENETIC DIVERGENCE IN THE GIANT KELP MACROCYSTIS (LAMINARIALES)1

The brown alga Macrocystis C. Agardh is widely distributed throughout the cold temperate waters of the Northern and Southern hemispheres, forming ecologically diverse and productive kelp forests. The taxonomy of this alga has been under constant discussion. Since the first description, species have been mostly described by holdfast and blade morphology; however, the importance of these taxonomic characters has been questioned. Based on a morphological study, the genus has recently been synonymized into a single species, M. pyrifera (L.) C. Agardh, but additional genetic evidence is still lacking. Using the DNA-barcoding gene (COI), we examined the taxonomy of Macrocystis collected from 19 sites worldwide, covering the distribution of the four ecomorphs (M. pyrifera, M. angustifolia, M integrifolia, and M. laevis). Our molecular data strongly support the recognition of a single species; therefore, the genus should contain only one species, M. pyrifera, the oldest name. Results also reveal shared haplotypes in several distant sites around the Southern Hemisphere and very low variability among samples. Additionally, samples of the ecomorphs M. integrifolia and M. pyrifera from a sympatric population in California had the same haplotype. The revised taxonomy changes questions of Macrocystis distribution from interspecific dispersal and evolutionary questions to intraspecific ecological questions on the maintenance of Macrocystis in certain environments that produce particular morphologies.