A new groundbreaking hypothesis counters popular evolutionary thinking that living organisms evolve by adding genes rather than discarding them. The new theory --"Black Queen Hypothesis" --suggests some species are surviving by discarding genes and depending on other species "to play their hand.
According to the 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.
"A common assumption about evolution is that it is directed toward increasing complexity," said Erik Zinser, associate professor of microbiology at the University of Tennessee . "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."
Zinser and colleagues formed their theory after studying photosynthetic bacteria called Prochlorococcus.
"This marine microorganism continued to mystify us because it is the most common photosynthetic organism on Earth, but it is extremely difficult to grow in pure culture," Zinser said. "A major reason for this difficulty is that Prochlorococcus is very sensitive to reactive oxygen species such as hydrogen peroxide and relies on other bacteria to protect them by breaking down these toxic substances for them."
Prochlorococcus had once performed this function itself, but natural selection decided it was too costly, like carrying the Queen of Spades, and discarded this ability. Instead Prochlorococcus benefits from the hard work of others within its community allowing it to concentrate its energies elsewhere—such as multiplying.
The hypothesis offers a new way of looking at complicated, interdependent communities of microorganisms.
"We know that certain microbial activities, such as hydrogen peroxide scavenging, are 'leaky,' meaning their impacts extend beyond the cell and into the environment," Zinser said. "What the hypothesis suggests is that this leakiness can drive a community toward greater interdependence, even if some members are unwitting participants in this process."
This interdependence could lend itself to vulnerabilities. The scientists say the work highlights the importance of biological diversity, because if rare members are lost, "the consequences for the community could be disastrous." This would be analogous to attempting to play Hearts without the Queen of Spades.
Currently, the hypothesis is limited to microorganisms, but Zinser thinks the hypothesis could be extended to larger free-living organisms. All that is needed is a card which no player wants yet is crucial for the game to be played.
Zinser's opinion piece is published 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.
The image at the top of page is a a surface-rendered model of Prochlorococcus, which was visualized by cryo-electron tomography in a near-native state. Its streamlined structure could be advantageous to microorganisms thriving in the low-nutrient conditions characteristic of large regions of the open oceans and thus have consequences for niche differentiation.
Image credit: With thanks to the American Society for Microbiology