Environmental gradients influence geographic differentiation and low genetic diversity of morphologically similar Ulva species in the Northwest Pacific

Species classified in the genus Ulva are important foundational marine primary producers distributed worldwide. These species are particularly abundant and diverse through the northwest Pacific (NWP) where they experience marked latitudinal gradients of environmental heterogeneity. It is unclear, however, to what extent such dynamic conditions can modulate phenotypic and genetic patterns in these organisms, potentially reflecting the influence of historical and contemporary biotic and abiotic factors. Here, we assessed inter- and intra-specific genetic patterns of Ulva species through the NWP using plastid rbc L and tuf A gene sequences. Although we initially targeted Ulva australis based on morphological identification, we recovered eight Ulva genetic entities masked by morphological similarities. Except for the Ulva linza–procera–prolifera and U. lactuca–reticulata complexes, six of these genetic entities were recovered as individual species (i.e., U. australis , U. ohnoi , U. californica , U. compressa , U. lacinulata , and U. arasakii ), and showed biogeographic patterns likely explained by clines in sea surface temperature and ocean current dispersal. At intra-specific level, all the genetic entities showed low genetic variation and divergence based on rbc L (0–0.3 %) and tuf A (0–0.9 %) data. Our results provide insights regarding intra- and inter-specific genetic patterns characterizing morphologically similar Ulva species through the NWP. However, further studies are needed to understand the mechanisms underpinning such patterns and the associated ecological and evolutionary implications.