Experimental assessment of environmental versus genetic influences on <i>Macrocystis</i> morphology

Abstract The two most common ecomorphs of giant kelp, Macrocystis pyrifera and Macrocystis integrifolia , exhibit almost no geographic overlap across their range in the temperate waters of North and South America, with few exceptions such as Stillwater Cove, CA, USA. However, in laboratory settings they are able to interbreed. The mechanism for the morphological variation is unknown, and whether these features are genetically fixed or malleable under different environmental conditions is unclear. Our study, for the first time, directly compared the growth, development, and morphological transformations of the two Macrocystis ecomorphs in a common garden experiment. We experimentally tested the influence of local environment on giant kelp morphology by rearing lab‐cultured embryonic sporophytes from spores released by M. pyrifera and M. integrifolia sporophylls collected from multiple individuals at Stillwater Cove, CA. The spores were cultured in three treatments: “pyrifera‐morph” only, “integrifolia‐morph” only, and mixed (50:50 “pyrifera‐morph”:“integrifolia‐morph” spores). We outplanted the resultant embryonic sporophytes to concrete blocks installed at 7.5–9 m depth in the ocean and monitored the development of the sporophytes over 5 months. Our findings of distinct differences in morphology between the pyrifera‐treatment and integrifolia‐treatment individuals at multiple stages of development including reproductive adults indicates that the morphological differences between the two ecomorphs are genetically determined rather than environmentally induced. We found that primary stipe length and number of branches can be used as diagnostic traits for distinguishing the ecomorphs prior to the stage when adult sporophyte morphology can be definitively characterized. Additionally, no morphological hybrids were observed in the mixed‐treatment, and ultimately the mixed‐treatment individuals were more often categorized as “integrifolia‐morph”‐like than “pyrifera‐morph”‐like.