The evolutionary past and the uncertain future of foundational species

How do species evolve into foundational roles in ecosystems, and what is the prognosis for them in a changing ocean? Seagrasses, or marine angiosperms, have examples of foundational species that have become widespread throughout the coastal ocean. When flowering angiosperms first entered the ocean, an event dated to 140 million years ago and repeated several times (1, 2), they likely developed or retained a set of traits from flowering plants that would aid their success, including the ability to pollinate in the water (hydrophily) and clonal reproduction. Although only a minor number of species across the flowering plants, seagrasses have had success across the major ocean basins, with ∼60 species recognized (1). As seagrasses evolved, the species Zostera marina became widespread in the Northern Hemisphere, colonizing the Pacific and eventually the Atlantic, and becoming the most geographically widespread marine angiosperm. Genome duplication and gene loss events likely accompanied it in this new habitat (3). Significant evolutionary events for eelgrass may have occurred during the Pleistocene when the repeated advances and retreats of glaciers removed and added habitat and altered the environment. Through a large-scale and multiauthor effort across the Pacific and Atlantic Oceans, Duffy et al. (4), in PNAS, detect an existing signature of historical events, including these Pleistocene episodes. In their paper entitled “A Pleistocene legacy structures variation in modern seagrass ecosystems,” they find persistent genetic and phenotypic features of seagrass that reverberate through the seagrass ecosystem today, affecting plant shape and density, and the interaction of eelgrass with other key members of the ecosystem. Marine angiosperms, including Z. marina, fix and store immense amounts of carbon, referred to as “blue carbon. ” Their widespread abundance and high productivity yield impressive estimates of carbon storage, from 48 Tg per y to 112 Tg per y (5). Marine angiosperms provide critical habitat for myriad species in ocean environments because their blades and their roots modify the environment and provide a surface for other species to thrive. The eelgrass Z. marina is a “foundational” species, and Duffy et al. (4) describe how its morphological features differ among ocean basins. Eelgrass in Pacific Ocean locales has a higher stature and lives in sparser aggregations, while the Atlantic Ocean form is shorter and often in more-dense “meadows” (Fig. 1). The shape of eelgrass has effects up the food chain, including the taxa that grow on the blades (periphyton) and the invertebrates that use the plants for habitat. Thus, the growth form characteristic of each ocean basin has ecosystem-level effects. Growth form, often considered highly plastic in plants, also has a genetic and geographic basis in eelgrass. While morphological differences among eelgrasses might be ascribed only to the environmental features of each ocean basin, the authors show that these morphological distinctions were set historically through the genome and likely limit the response of the plants to changing environments. Further, the longer evolutionary history of the Pacific Ocean has imparted greater time for mutations and resulted in genetic isolation by distance in Pacific Ocean Fig. 1. A representation of the patterns of genetics and phenotype in the eelgrass Z. marina as described by Duffy et al. (4). Pacific Ocean eelgrass populations are more genetically disparate and have a more elongate phenotype. Atlantic Ocean eelgrasses are shorter statured and differ in the density of their blades. As younger populations, they are genetically more similar. The darker shading in the Pacific Ocean represents older populations with a greater influence of evolutionary history on present-day phenotype compared with the Atlantic populations. The dashed line represents a colonization event.