Host-shift speciation proceeded with gene flow in algae covering shells

Host shifts represent the advancement of a novel niche and often lead to speciation in symbionts. However, its mechanisms are not well understood. Here, we focused on the alga Pseudocladophora conchopheria growing on the shells of intertidal snails. Previous surveys have shown that the alga has host specificity—only attaching to the shell of Lunella correensis—but we discovered the alga attaches to the shells of multiple sympatric snails. A genome-wide SNP analysis (MIG-seq) was performed to determine whether host-associated speciation occurred in the algae. As a result, there was no, or limited, gene flow among the algae from different hosts, and some algae were genetically differentiated among hosts. As well, the demographic estimate revealed that speciation with gene flow occurred between the algae from different hosts. Therefore, these results support the idea that host-shift speciation gradually proceeded with gene flow in the algae, providing insight into the early evolution of host shifts.