The Black Queen Hypothesis!

Bacteria play Hearts! Yes, the same card game that we play as a pastime. While, depending on our skills we win or lose the game; in bacterial world there are some who always Win while some others who always lose by ending up with the black queen!

A recent paper published in mbio describes how organisms in the ocean might be playing Hearts. In the bacterial version of the game as the paper describes, the black queen is the gene responsible for breaking down HOOH, the gene is named katG. HOOH is dangerous for the survival of all bacteria, so it needs to be degraded and katG gene is responsible for the breakdown of HOOH.So, it came as a surprise to many scientists that the most dominant bacterioplankton in the ocean the  “Prochlorococcus” and the “Candidatus Pelagibacter” lacked the katG gene entirely. These two bacteria lack the katG gene and yet manage to survive in the oceans where HOOH is constantly produced via photo oxidation of organic carbon by sunlight. Clearly, the lack of katG gene is costly for any bacteria, then how does the Prochlorococcus manage to not only survive but also dominate in the oceans?

You know how one hates doing boring jobs but its important that they are done, you just have to do it! Unless…someone else does that job for you!

Prochlorococcus is a winner; other bacteria do the dirty job of degrading HOOH for themselves and for Prochlorococcus. It is important to note that katG gene function is leaky i.e other bacteria keep the concentration of HOOH low (sink effect) making the katG gene function dispensable for Prochlorococcus. Thus, other bacteria ‘help’ Prochlorococcus to survive in a HOOH environment. Prochlorococcus does not bear the cost of maintaining the katG gene but earns the benefit of HOOH being degraded.

Morris and co describe the above-mentioned interaction in an evolutionary context and propose the Black Queen hypothesis to describe this interaction,

the black queen refers to a playing card, in this case the queen of spades in the game Hearts. In Hearts the goal is to score as few points as possible. The queen of spades, however, is worth as many points as all other cards combined, and therefore a central goal of the game is to not be the player that ends up with that card. In the context of evolution, the BQH posits that certain genes, or more broadly, biological functions, are analogous to the queen of spades. Such functions are costly and therefore undesirable, leading to a selective advantage for organisms that stop performing them. At the same time, the function must provide an indispensable public good, necessitating its retention by at least a subset of the individuals in the community—after all, one cannot play Hearts without a queen of spades. The detoxification of HOOH fulfills both of these criteria, and therefore the BQH predicts that this function will be performed by helpers that comprise only a fraction of the community.

Prochlorococcus is successful in its survival and dominance in oceans since the benefits of losing the katG gene outweighs the cost of losing that gene, according to the BQH.

BQH thus provides a unique perspective of looking at interactions between different organisms in a given natural environment. Not only does it successfully explain genome reduction in free-living organisms but also the occurrence of essential yet rare functions!


The paper has created quite a stir and has grabbed a lot of media attention, links to few  …

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