Applied and Environmental Science

Staying Together or Seeing Other People?

Scientists have long thought that the bacterial symbionts inside the gills of the clam Solemya velum are vertically transmitted from parent to offspring because they are also found in the ovaries and embryos of their hosts.  The genome of symbionts usually becomes smaller when it is vertically transmitted, but when the bacterium’s genome was sequenced, it was close in size to its free-living relatives.  To find out why, Russell, Corbett-Detig, and Cavanaugh of Harvard University sequenced the genomes of symbiont and host mitochondria from 61 S. velum clams collected along the east coast of the United States and found many signs that pointed to horizontal transmission.  These included the facts that the evolutionary histories of the partners were decoupled within geographical sites and that both recombination and horizontal gene transfer were found in symbiont genomes.  The phenomena suggest that symbiont populations in S. velum have been mixing and have also been exposed to the environment. The symbionts are therefore likely to have a mixed mode of transmission: vertically passed to host offspring via host eggs and horizontally exchanged through environmental exposure.

The original article can be found here.

Citation: Russell, S. L., Corbett-Detig, R. B., & Cavanaugh, C. M. (2017). Mixed transmission modes and dynamic genome evolution in an obligate animal–bacterial symbiosis. The ISME Journal 11, 1359-1371.