Cavendish bananas are in crisis. Around the world, a soil fungus known as Tropical Race 4 is killing Cavendish bananas and threatening the global banana trade. We asked Andy Dyer, author of Chasing the Red Queen: The Evolutionary Race Between Agricultural Pests and Poisons, to tell us more about the monoculture of these iconic yellow bananas.
The Cavendish banana was truly fortunate to have been discovered by humans. Without our adoption, this sweet and attractive—but seedless—banana would have disappeared into the jungle long ago because, as a genetic mistake, it was doomed to be an asexual and probably short-lived anomaly. Instead, like winning the big lottery, the Cavendish became the most famous of all bananas, despite having no evolutionary future whatsoever. However, the time has come, the course has been run, and the Cavendish is now likely to disappear, but only to be replaced in the grocery display by another genetic anomaly, another as-yet-unknown seedless banana.
Although humans in love with the long yellow Cavendish banana may mourn its passing, that single banana genotype only exists because we fostered it (by the millions and millions) and because it was unable to reproduce itself by seed. Although all wild bananas naturally make seeds, those seeds make them undesirable as far as humans are concerned. But occasionally the banana plant produces a mutant, a genetic mistake containing three copies of all chromosomes, which renders the new plant unable to produce viable seeds. Humans love these seedless triploids and we have discovered ways to make more of them, as we have done with the seedless watermelon. Sweet and juicy, the triploid fruits have none of those nuisance seeds scattered across their delicious interiors.
Triploid bananas have no real future in the wild. They can only reproduce by cloning, that is, by growing new shoots at the base of the parent plant and those new shoots take over once the parent plant has flowered and produced a stalk of bananas. But the mutant seedless plants are anchored to the spot, in a genetic sense, unlike seed-producing bananas that can disperse their offspring and move across the landscape with each passing generation. Thus, humans have provided the transportation assistance the Cavendish banana needed and we have spread the strange mutant seedless genotype across the globe, making it possibly the most successful dead-end genotype in history.
Unfortunately for the Cavendish banana, an alliance with humans is both the reason for its success and the reason for its downfall. The human propensity for using monocultures for producing food also creates the conditions that will eventually wipe out this most unusual of all things—an organism with only one genotype. For such a creature, success can be sustained only as long as the world around it doesn’t change. But life with humans is life with change, often rapid and unpredictable change, and such change is never favorable for asexual species. Without the recombination of chromosomes through sex, a clonal species cannot produce new genotypes that may be better able to tolerate new stresses in the environment. And unfortunately for humans, there are no natural methods we can use to assist the Cavendish banana to be more sexually active.
The seedless Cavendish banana exists in the very trap that human agriculture now finds itself. In fact, asexual reproduction and genetic uniformity of bananas is the very reason it has fit into the human world so perfectly. Our agricultural system has slowly and intentionally adopted a worldview wherein huge monocultures of genetically identical plants are not just the norm, but a desirable condition. The Gros Michel and now the Cavendish bananas were and are represented around the world by single genotypes. If bananas were corporations, they would be McDonalds and Starbucks; meeting the exact expectations of consumers around the world for uniformity of quality, taste and experience. As long as consumers only want a single flavor, this is a winning business plan. But there are flaws in this worldview and given the right conditions, these flaws will become gaping holes, and it begins with the monoculture.
A monoculture is much like a vacuum and it cannot be tolerated in nature. A monoculture is a vast and uniform resource awaiting exploitation on a first-come-first-served basis, and there are a multitude of herbivores attempting to be first. When any plant-eating organism infiltrates the defenses of human agriculture, it is faced with a seemingly limitless food supply. Under such artificial conditions we always see explosive population growth of the very organisms we try assiduously to keep under control. It is an evolutionary truism: vast expanses of genetically uniform crop monocultures guarantee the emergence of pest species.
Under natural conditions plants are extraordinarily adept at producing defensive chemicals for discouraging herbivory; plants have been fending off herbivores for millennia. Every smell and flavor in plant leaves, flowers, and seeds are important for attracting or dissuading other species. And every natural population of plants has genetic variation for every different trait, particularly for those defensive compounds. Oh, but the banana. The seedless banana has eschewed genetic variation. And the Cavendish banana is being grown in vast genetically uniform monocultures by humans who incessantly propagate only a single genotype rather than embracing genetic variation as a method for preventing the disaster that awaits.
This defiance of nature, both by the banana and by human agricultural practices, is getting harder and harder to sustain. The enemies are gathering at the gates, searching for any weakness in the defenses and we are literally inviting them to find the weak spot. A typical Cavendish banana plantation is now spraying pesticides 25-50 times a year to fend off nature’s would-be consumers of this incredibly unnatural crop. The result is inevitable and predictable. An enemy is a product of evolution and emerges by beating the defense systems that humans have created to defend the defenseless. Whether it is our chemical arsenal selecting for pesticide resistance in an insect or the emergence of a highly chemical-resistant strain of fungi, the result is inevitable. These outcomes are a consequence of the rules of the evolutionary game played by all natural organisms and the rules we refuse to acknowledge in our pursuit of modern agricultural production practices.
Is there a solution? Can the Cavendish banana be saved? Yes, there is, and no, probably not, at least not as the preferred banana genotype worldwide. With 1,000 species of genetically variable bananas to choose from and dozens of seedless varieties already being grown, we will have no particular difficulty finding a replacement for the Cavendish. The nemesis of the Cavendish banana has already been unleashed and, like Frankenstein’s monster, it cannot be recalled.
However, our efforts at this point should be to find many replacements for the Cavendish banana, not just one, and to propagate them in diverse mixtures with no real dependency on any single genotype as in the past. This approach would protect us from the loss of any one or two genotypes to future threats. We should move away from monoculture-style production, such that multiple species of plants are grown on plantations. These changes to banana production would also reduce our reliance on environmentally damaging chemicals and would allow for some protection from naturally-occurring beneficial organisms. In the long run, having experienced two disastrous challenges from nature to our love of bananas, perhaps it is time to follow the rules of the evolutionary game to ensure a safer future for our genetically deficient and nearly defenseless adoptees.
