A Novel Bacterial Speciation Process Observed in a Symbiotic Marine Population

Bacteria undergo asexual reproduction, but genetic exchange is common. Current models posit that speciation of sexual bacteria is a gradual and long process, and genetic exchanges decrease but never cease between nascent species. Here, we show that in a panmictic bacterial lineage found in symbiosis with invertebrate hosts across multi-marine ecosystems, a series of speciation events occurred unusually fast with between-species genetic exchange approximating a complete halt. The speciation process is too rapid to sort out ancestral polymorphisms, giving rise to between-species phylogenetic incongruence at a genome-wide scale. This novel bacterial speciation process leads to incomplete lineage sorting, which to date has only been observed in eukaryotes but is 100-1000 times faster. Our finding allows for a more unified understanding of the speciation process across prokaryotes and eukaryotes.