Coevolution

by Jordan Pennells

Examples of coevolution are not only pervasive between species in nature, but are intimately linked to the complex evolution of plants and animals themselves. Mitochondria, commonly proclaimed as the ‘powerhouse of the cell’, was likely once a \nbacterial entity that infected a primordial eukaryotic cell. While the bacteria found a safe habitat and took advantage of available nutrients in the cell, the eukaryotic host harnessed the bacterial cell’s energy production capabilities, resulting in one of the earliest forms of coevolution (known as The Endosymbiotic Theory). This explains why mitochondria have their own genome sequence separate to the nuclear DNA of the cell, as they were once an organism of their own. This is true for plants and their photosynthetic chloroplast too.

biology-unit-2-notes-the-endosymbiotic-theory-5-638

The evolution of eukaryotic cell, Michael Tribe. https://g.co/kgs/PLC7vX

While coevolution has dramatically shaped the evolution of life in general, it also shapes the evolution of species interaction, either in an antagonistic or symbiotic fashion.

Mutualistic Coevolution: Leaf Cutter Ants

Mutualism is a type of coevolution that creates a system where two or more organisms benefit from each other; a fascinating example of this is the leaf cutter ant community. You may be aware of the iconic image of these ants carrying thousands of bright green leaf segments back to their nest, presumably for the colony for food or building purposes. But the reality is that they’re in a symbiotic relationship with the fungus Leucoagaricus, providing leaf biomass for them to digest, which in turn generates sugars that the ants consume. The fungus is provided safe refuge in the ant’s ‘fungal garden’, in return for producing the food source for their hosts.

Atta Cephalotes

Leaf cutter ants tending to their fungal garden. Photo: Don Parsons, Dept. Bacteriology, University of Wisconsin-Madison

This scenario is mutually beneficial for both parties, which strongly favours the coevolution of this mutualistic system, to the extent that leaf cutter ants have lost the ability to digest other food commonly consumed by other ant species.

However, nature is rarely this harmonic. The pathogenic fungus Escovopsis takes advantage of this fungal banquet, infiltrating this mutualistic community for their selfish benefit. As such, this system also provides an example of antagonistic coevolution.

But this complex system doesn’t end there. Like any successful farmer, these ants use pesticides to promote the cultivation of their ‘crops’. In another example of coevolution, colonies of the bacteria Streptomyces grow on the ant’s bodies, producing antifungal chemicals that inhibit the growth of the pathogenic Escovopsis fungus. Additional players in this complex system can be seen in the following diagram, representing the coevolutionary interactions of the leaf cutter ant colonies.

leaf cutter ants

Figure: Microbiol. Mol. Biol. Rev.December 2011 vol. 75 no. 4583-609

As this example highlights, coevolution isn’t restricted to symbiotic relationships, but also develops antagonistic relationships – especially between predators and their prey.

Predator-Prey Coevolution: Venomous Snakes

Besides developing improved antivenoms, research into snake venom is being undertaken to develop novel pharmaceutical drugs to treat conditions ranging from hypertension, type 2 diabetes and chronic pain (Capoten®, Prialt® and Exenatide®, respectively). Producing venomous saliva is a trait that is strongly shaped by evolution, for a couple of reasons:

  1. Each toxic component in venom is a single gene product (protein). As opposed to more complex traits that involves a whole range of underlying genes, for example the snake’s speed of movement, the venom trait constitutes a direct gene to protein relationship. A change in a single nucleotide/amino acid can result in a dramatically increased toxicity of the venom
  2. Venom is in the centre of a fierce predator-prey arms race. The reason some snake venom is capable of killing adult humans, an animal greater than 10x the body mass of their typical prey, is because it has been continually evolving in potency to overcome the continually evolved defences of its prey. Snake prey evolve to minimise the toxicity of the venom by altering the targets of the venom molecules. However, while venom can evolve any solution that increases its potency, the prey must evolve a specific mechanism that responds to the snake venom’s action. In this regard, prey lag behind in this evolutionary arms race.

Interestingly, the mechanism of venom evolution is closely linked to the prey they feed on, whether that be birds, rodents or other reptiles. In a study researching the causes of antivenom ineffectiveness, it was found that snake populations (saw-scaled vipers) in different regions evolve variations in their toxin’s mechanism of action based on their different prey, even for the same snake species.

Host-Pathogen Coevolution: Humans vs Infectious Disease

One of the most paradoxical examples of human coevolution involves the relationship between malaria and sickle cell anaemia. Throughout history, humans have been in a powerful evolutionary arms race with our arch nemeses: infectious disease pathogens. Population statistician Carl Haub has proposed that malaria could have killed up to 50% of the estimated 108 billion people that have ever lived, inflicting deep evolutionary scars in our genetic makeup. The genetic disease sickle cell anaemia (SCA) is a direct result of the steps that the human evolution took to mitigate the effects of malaria. In tropical African regions where malaria was most prevalent, it was discovered that the number of SCA disease cases was disproportionally higher than in the rest of the world.

Sickle_cell

McGraw Hill Companies, Inc.

While malaria is a potently lethal disease for young children under 5, and people with two copies of the SCA gene mutation had a reduced life capacity and developmental issues, people carrying a single copy of the SCA mutation (heterozygous) had enough healthy red blood cells to maintain sufficient oxygen carrying capacity, but also had enough malaria-resistant sickle red blood cells to prevent malaria virus taking over their body. Even though SCA had detrimental disease effects, the resistance it provided against the lethal malaria virus ensured its coevolution in response to the assault of this pathogen.

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