PHENOTYPIC PLASTICITY RECONCILES INCONGRUOUS MOLECULAR AND MORPHOLOGICAL TAXONOMIES: THE GIANT KELP, MACROCYSTIS (LAMINARIALES, PHAEOPHYCEAE), IS A MONOSPECIFIC GENUS¹

Abstract

The giant kelp genus Macrocystis C. Agardh (Laminariales, Phaeophyceae) is one of the world's most ecologically and economically important seaweed taxa, yet its taxonomy remains uncertain. Although the genus currently contains four accepted species based on variable holdfast and blade morphology [M. pyrifera (L.) C. Agardh, M. integrifolia Bory, M. angustifolia Bory, and M. laevis C. H. Hay], numerous recent studies on Macrocystis interfertility, genetic relatedness, and morphological plasticity all suggest that the genus is monospecific. We reviewed this evidence and present an explanation for the extreme phenotypic plasticity that results in morphological variability within Macrocystis, driven by the effects of environmental factors on early development of macroscopic sporophytes. We propose that the genus be collapsed back to a single species, with nomenclatural priority given to M. pyrifera.

The biogeography of kelps (Laminariales, Phaeophyceae): a global analysis with new insights from recent advances in molecular phylogenetics

“…The geographical information for the current study was also extracted from Algaebase, again with slight additions or errors known to the author corrected. In particular, the genus Macrocystis is accepted to be monospeciWc (Coyer et al 2001;Graham et al 2007b, Demes et al 2009, and the revision of Northeast Table 1 List of kelp species recognised in this analysis (for species authorities, see www.Algaebase.org) a The family placement of Aureophycus is not deWnite, but it appears most closely related to the Alariaceae: Kawai et al (2008) abyssalis, agardhii, appressirhiza, bullata, complanata, cordata, digitata, ephemera, farlowii, hyperborea, inclinatorhiza, longipes, multiplicata, nigripes, ochroleuca, pallida, philippinensis, platymeris, rodriguezii, ruprechtii, setchellii, sinclairii, solidungula, yezoensis Macrocystis pyrifera…”

“…This species complex includes Wve currently recognised species in the North Atlantic, and many in the North PaciWc. It is possible that further studies including molecular techiques, will reduce this number of recognised species, as has been the case recently with global Macrocystis (Coyer et al 2001;Demes et al 2009) and Northeast PaciWc Alaria (Lane et al 2007).…”

Genetic structure of the giant kelp Macrocystis pyrifera along the southeastern Pacific

“…The presence of upwelling in this area is thought to be responsible for a biogeographic break in brown macroalgae (Santelices 1980. Although there is a clear phylogeographic break at 33°S for M. pyrifera, this break may be related to some environmental adaptation of Macrocystis 'ecomorphs' (Graham et al 2007, Demes et al 2009). In northern-central Chile, 2 different ecomorphs are present (Macaya & Zuccarello 2010), haplotype Mpyr2 whose southern distribution limit is at 33°S represents samples of the M. integrifolia ecomorph, while the ecomorph M. pyrifera has its northern distributional limit at 33°S.…”

Morphological variability of intertidal Eisenia arborea (Laminariales, Ochrophyta) at Punta Eugenia, Baja California Sur

“…Influence of water action in an intertidal population of Eisenia arborea was evaluated near the lighthouse (El era (Demes et al 2009). Also morphological change has been documented in Laminaria setchellii (Klinger and De Wreede 1988) and other kelps species (Gerard andMann 1979, Koehl 1986).…”