Ocean currents drive the worldwide colonization of the most widespread marine plant, eelgrass (Zostera marina)

Abstract

Currents are unique drivers of oceanic phylogeography and so determine the distribution of marine coastal species, along with past glaciations and sea level changes. Here, we reconstruct the worldwide colonization history of eelgrass (Zostera marina L.), the most widely distributed marine flowering plant or seagrass from its origin in the Northwest Pacific, based on nuclear and chloroplast genomes. We identified two divergent Pacific clades with evidence for admixture along the East Pacific coast. Multiple west to east (trans-Pacific) colonization events support the key role of the North Pacific Current. Time-calibrated nuclear and chloroplast phylogenies yielded concordant estimates of the arrival of Z. marina in the Atlantic through the Canadian Arctic, suggesting that eelgrass-based ecosystems, hotspots of biodiversity and carbon sequestration, have only been present since ~208 Kya (thousand years ago). Mediterranean populations were founded ~53 Kya while extant distributions along western and eastern Atlantic shores coincide with the end of the Last Glacial Maximum (~20 Kya). The recent colonization and 5- to 7-fold lower genomic diversity of Atlantic compared to the Pacific populations raises concern and opportunity about how Atlantic eelgrass might respond to rapidly warming coastal oceans.

Worldwide population genomics reveal long-term stability of the mitochondrial chromosome composition in a keystone marine plant

“…A long-standing question is therefore whether and how (sub)stoichiometric shifting translates to the next generation thus contributing to the evolution of mitochondrial genetic diversity over time. Here we demonstrate that the mitochondrial chromosome copy number in Z. marina meristematic region remains homogeneous among widely divergent populations, implying a conservation of chromosome stoichiometry in the plant germline over 350,000 years since the last common ancestor of extant populations (Yu et al 2022). Our results thus reveal a novel perspective on plant population genetics.…”

“…The result revealed a similar pattern of the segment copy number within and across populations (Figure 3A). The Z. marina phylogeography indicates deep divergence over 350 kya between the Californian populations and the remaining populations; another deep split between the Atlantic and Pacific populations is estimated over 200 kya (Yu et al 2022; see Figure 3B). The similarity of segment copy number is thus observed across closely and distantly related populations alike.…”

“…To investigate the evolution of chromosome stoichiometry at the population level, we compared the relative mitochondrial chromosome copy number among worldwide Z. marina populations. The analyzed data comprises genome sequences from 163 individual Z. marina clones sampled in 16 locations in the Pacific and Atlantic oceans (Yu et al 2022). The sampled tissues comprised early developmental stages, i.e., the meristematic region and the base of first leaflets.…”

“…Here, we study the mitochondrial chromosome stoichiometry of the marine plant Zostera marina (eelgrass). This species served recently as the focus of investigations on monocotyledonous adaptation to marine habitats and species radiation in the ocean (Olsen et al 2016;Ma et al 2021;Yu et al 2022). Z. marina reproduces both sexually and via vegetative (clonal) reproduction.…”

“…A recent study of Z. marina samples collected in 16 locations worldwide revealed that all extant populations originated in the North-West Pacific and started colonizing Pacific and Atlantic oceans ca. 350kya (Yu et al 2022). The availability of high-resolution genomic information at the population level including a dated phylogeny enabled us to study the evolutionary dynamics of mitogenome composition of a keystone marine plant.…”