Horizontally transmitted symbionts are also loyal partners

Because they are retained over several host generations, vertically transmitted symbionts are traditionally thought to be more genetically isolated than horizontally transmitted ones. Using metagenome sequencing, Romero Picazo et al. (2019) provided evidence for high genetic isolation between populations of sulfur- and methane-oxidising symbionts of Bathymodiolus brooksi living at cold seeps. Although both symbiont types are horizontally transferred, nucleotide diversity (π) and ɑ-diversity were found to be low. Furthermore, population isolation (F_ST) and β-diversity were high between mussel individuals. Where 1 indicates complete isolation, F_ST equalled 0.6 and 0.5 for sulfur- and methane-oxidising symbionts, respectively. Host and symbiont F_ST values were not correlated and neither were symbiont F_ST and geographical location. Consequently, these factors do not contribute to the observed genetic isolation.

This research suggests the possibility of marked population isolation in horizontally transmitted symbionts. The authors suggest that genetic isolation is maintained through self-infection of mussel individuals with symbionts. Faced with evidence of high symbiont geneflow in other Bathymodiolus spp. from hydrothermal vents, Romero Picazo et al. (2019) point towards the relatively low concentrations of symbionts in seawater at cold seeps. It seems that the spectrum of genetic isolation in horizontally transmitted symbiont populations is larger than previously thought.

Romero Picazo, D., Dagan, T., Ansorge, R., Petersen, J. N., Dubilier, N. & Kupczok, A. (2019). Horizontally transmitted symbiont populations in deep-sea mussels are genetically isolated. The ISME Journal, 13(12), 2954–2968.