Black Queen Hypothesis: Some Species Survive by Discarding Genes

Posted on April 7, 2012

A new hypothesis on evolution, the Black Queen Hypothesis, suggests some living organisms evolve by discarding genes and letting other species play their hand. The hypothesis - posed by University of Tennessee, Knoxville, associate professor Erik Zinser and his colleagues - gets its name from the game of Hearts. In Hearts, the goal is to avoid "winning" the Queen of Spades (the Black Queen), which is worth a lot of points. Players allow others to take this high-point card while they enjoy low-score tallies. The scientists says this same premise applies in evolution.

According to the Black Queen Hypothesis, evolution pushes microorganisms to lose essential functions when there is another species around to perform them. This idea counters popular evolutionary thinking that living organisms evolve by adding genes rather than discarding them.

Zinser says, "A common assumption about evolution is that it is directed toward increasing complexity. But we know from analysis of microbial genomes that some lineages trend toward decreasing complexity, exhibiting a net loss of genes relative to their ancestor."

The authors formed their theory after studying a photosynthetic bacteria called Prochlorococcus. Prochlorococcus is the most common photosynthetic organism on Earth, but it is very difficult to grow in pure culture. NPR says the oxygen in one out of every five breaths you take comes from Prochlorococcus.

Zinser says the major reason why Prochlorococcus is hard to grow in the lab is because it is very sensitive to reactive oxygen species such as hydrogen peroxide. He says Prochlorococcus "relies on other bacteria to protect them by breaking down these toxic substances for them."

Prochlorococcus once performed this function itself, but natural selection decided it was too costly, like carrying the Queen of Spades, and discarded the ability. Prochlorococcus now benefits from the hard work of others within its community, allowing it to concentrate its energies elsewhere, such as on multiplying.

This interdependence could also lend itself to vulnerabilities. Getting other species to do your work for you is clever, but if the other species disappears the dependent species is left totally vulnerable. The researchers say the work highlights the importance of biological diversity. The removal of a member of an interdependent group of species could be disastrous for the entire community.

The Black Queen Hypothesis is currently limited to microorganisms, but Zinser thinks the hypothesis could be extended to larger free-living organisms. He says, "All that is needed is a card which no player wants yet is crucial for the game to be played."

Zinser's opinion piece about his hypothesis is published here in mBio, the online open-access journal of the American Society for Microbiology. Jeffrey Morris and Richard Lenski of Michigan State University are co-authors. Morris was Zinser's doctoral student at UT.