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Updated: Mar 7, 2026

Fluorescence Live-cell Imaging of the Complete Vegetative Cell Cycle of the Slow-growing Social Bacterium Myxococcus xanthus
Published on: June 20, 2018
Mixed transmission modes and dynamic genome evolution in an obligate animal-bacterial symbiosis
Shelbi L Russell1, Russell B Corbett-Detig2, Colleen M Cavanaugh1
1Department of Organismic and Evolutionary Biology, Harvard University, Biological Laboratories, Cambridge, MA, USA.
Abstract:
Reliable transmission of symbionts between host generations facilitates the evolution of beneficial and pathogenic associations. Although transmission mode is typically characterized as either vertical or horizontal, the prevalence of intermediate transmission modes, and their impact on symbiont genome evolution, are understudied. Here, we use population genomics to explore mixed transmission modes of chemosynthetic bacterial symbionts in the bivalve Solemya velum. Despite strong evidence for symbiont inheritance through host oocytes, whole-genome analyses revealed signatures of frequent horizontal transmission, including discordant mitochondrial-symbiont genealogies, widespread recombination and a dynamic symbiont genome structure consistent with evolutionary patterns of horizontally transmitted associations. Population-level analyses thus provide a tractable means of ascertaining the fidelity of vertical versus horizontal transmission. Our data support the strong influence horizontal transmission can have on symbiont genome evolution, and shed light on the dynamic evolutionary pressures shaping symbiotic bacterial genomes.
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