Pared down genomes are the norm in symbiotic microbes, but how do non-symbionts get away with cutting out functions it would appear that they need? The authors of an Opinion piece in mBio this week explain their ideas about the matter. They say microbes that shed necessary functions may well be getting others to do the hard work for them, an adaptation that can encourage microorganisms to live in cooperative communities.
The Black Queen Hypothesis, as they call it, puts forth the idea that eliminating one function and getting another organism to carry out that process instead confers a selective advantage. In these cases, it would make evolutionary sense for a microbe to lose a burdensome gene for a function it doesn't have to perform for itself. The authors, Richard Lenski and J. Jeffrey Morris of Michigan State University, and Erik Zinser of the University of Tennessee, named the hypothesis for the queen of spades in the game Hearts, in which the usual strategy is to avoid taking this card. The organism stuck with the job of carrying out the process for another organism is, according to their theory, holding "the queen of spades", a card that is a burden for the holder, but necessary for the cardgame.
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