A new hypothesis posed by a University of Tennessee, Knoxville, associate
professor and colleagues could be a game changer in the evolution arena. The
hypothesis suggests some species are surviving by discarding genes and
depending on other species to play their hand.
The Black Queen Hypothesis got 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. Subsequently, players allow others to
take the high-point card while they enjoy low-score tallies.
This same premise applies in evolution, the scientists say.
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. “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’s opinion piece is published in mBio.
Jeffrey Morris and Richard Lenski of Michigan
State University
are coauthors. Morris was Zinser’s doctoral student at UT.
The authors 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.
