Marine population-genetic inferences reveal stronger oceanic structure in protists than Archaeplastida (plants) and Metazoa (animals).

Abstract

While the biogeographic patterns of animals and plants are well defined, the distribution of the microbial eukaryotic world remains contentious. Classically, protists were assumed to be cosmopolitan, dispersing everywhere with little geographic constraint. We tested this hypothesis by compiling 88 marine cytochrome oxidase subunit I metabarcoding studies to perform a global population-genetic analysis. We uncovered a fundamental biogeographic divide: most animal (Metazoa) and plant (Archaeplastida) phyla have haplotype diversity that is strictly concentrated within specific ocean basins, while most protist phyla share identical haplotypes across multiple, distant oceans. The combination of large geographic ranges with moderate among-ocean genetic differentiation in protists is inconsistent with strict historical cosmopolitanism and is compatible with high connectivity and range expansion dynamics, including, but not limited to, recent colonization events. Our findings provide a unified population-genetic perspective on marine eukaryotic biogeography and refine current views of microbial dispersal at ocean-basin scales.