Rapid Symbiont Displacement Observed in Beetle Model System
The intricate mutualistic relationships between insects and their bacterial symbionts have evolved over millions of years through coevolution. Despite their long-standing stability, certain insect species occasionally experience the loss or replacement of their original symbionts by new bacteria.
Despite their stability, some insect species experience the loss or replacement of original symbionts by new bacteria.
The mechanisms driving these changes have been largely unexplored due to a lack of accessible experimental systems.
Establishing a Controllable Model for Symbiont Exchange
To bridge this knowledge gap, Ronja Krüsemer and a team from the Max Planck Institute for Chemical Ecology embarked on a mission to establish a controllable model system. Their primary goal was to facilitate the observation of symbiont exchange dynamics in a laboratory setting.
In collaboration with Colin Dale, the researchers introduced the bacterium Sodalis praecaptivus into female grain beetles (Oryzaephilus surinamensis). Sodalis praecaptivus was chosen for its prior successful use in establishing artificial symbiosis within grain weevils, making it a promising candidate for this new model.
Swift Symbiont Displacement and Host Impact
The introduced Sodalis proved highly invasive, colonizing nearly all of the beetles' tissues and organs. Crucially, it was successfully transmitted vertically from mother to offspring. The impact was profound and swift: in the third generation, the original symbiont, Shikimatogenerans silvanidophilus, completely disappeared.
Infection with Sodalis also brought about notable changes in the host beetles. They exhibited lighter cuticles, reduced life expectancy, and lower reproduction rates. Furthermore, their immune systems were activated, as indicated by increased immune gene expression.
Unpacking the Mechanism of Displacement
The mechanism behind this displacement was clear: Sodalis actively invaded the bacteriomes, specialized organs that typically house the original symbionts. Inside these organs, Sodalis altered conditions to the distinct disadvantage of Shikimatogenerans.
Genome erosion in Shikimatogenerans, a consequence of its long-term co-existence with the host and a resulting lack of selection pressure, contributed significantly to its inability to adapt to the new, challenging conditions and ultimately led to its displacement.
A Promising System for Future Research
The newly established Oryzaephilus surinamensis-Sodalis praecaptivus system is now considered a highly promising model. It offers an unprecedented opportunity for investigating symbiont exchange mechanisms, the process of symbiosis establishment, and the broader evolution of mutualisms.
Future studies using this system will delve into how genetic mutations influence the fitness of both hosts and symbionts.
The research indicates that new bacteria can displace original symbionts rapidly, a process that may occur faster than previously believed, providing crucial insights into the dynamics of symbiont exchange.