Weevil Knievel: Daring to destroy endosymbionts that are no longer useful

I teach a fourth year undergraduate course where I introduce students to a wide range of bizarre and interesting endosymbiotic relationships. I have often wondered how transient these relationships are and have assumed that one partner or the other might continuously be trying to get the upper hand in the relationship. A recent research paper explores this question in greater detail in insects (h/t to Tristan Long for passing along the article).
Vigneron and co-workers explored the relationship between the weevil Sitophilus and its endosymbiont Sodalis pierantonius . They show that young adult weevils have very high numbers of the endosymbiont in their guts in order to generate the large amounts of tyrosine and phenylalanine required to make dihydroxyphenylalanine (DOPA) which is in turn required to make the cuticle essential for their exoskeleton. Once the cuticle is completed, these high DOPA levels lead to the active elimination of the endosymbionts in gut tissues. The endosymbionts are recycled using a combination of autophagy and programmed cell death. In contrast, the endosymbiont populations in reproductive tissues remain unharmed.
Using a combination of fluorescent in situ hybridization and scanning and transmission electron microscopy the authors clearly show that the weevils can effectively modulate the size and number of bacteriomes (structures that house the endosymbionts) throughout development. The images in the paper are amazing! The authors propose that such co-ordinated and targeted endosymbiont destruction avoids inflammation and the induction of the immune system and that this recycling may allow the weevil to recover some of the metabolites and energy invested in the earlier stages of the relationship. This is a really cool example of co-evolution! At first glance it seems that the weevil has the upper hand in this relationship, but it’s also important to remember that the endosymbiont has still managed to ensure its transmission to the next generation in germ-line tissues which is no small feat.

Citation: Vigneron et al., 2014. Insects Recycle Endosymbionts when the Benefit Is Over. Current Biology. 24: 2267-2273.

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