Once upon a time, there was a banana named Big Mike (Gros Michel was its French name). Popular with people who eat fruit and with farmers who made a living off of providing that fruit to them, Big Mike was planted over and over. This particular banana tree was cloned — farmers grew genetically identical versions of it on many Latin American plantations, a practice known as monoculture.
Alas, along came a dastardly fungus: the disease known as Fusarium wilt. It destroyed Big Mike, who had next-to-no protection from disease. And with all that cloning, if Fusarium wilt was able to invade one banana tree, it could make a whole field of trees die.
When they hit upon a popular, quick-growing, or pest-resistant plant, farmers often prefer to grow cloned individuals (plants whose genetic codes are copies of each other). That way, they and their consumers can expect consistent crops — plants which have very similar growing requirements and look and taste just like one another. But Big Mike was wiped out in the 1950s and 1960s, when Fusarium wilt spread.
That is why today, most banana lovers eat not Big Mike, but the Cavendish banana. Fusarium wilt does not destroy Cavendish bananas, so farmers replaced Big Mike with it, again cloning their chosen banana, partly to protect it from Mike’s fungus.
But might cloning itself be the problem? Is there a benefit to being surrounded by genetically different individuals? These questions were posed by Evolutionary Biologist Amanda Gibson and one of her students. They looked at many studies of “the monoculture effect,” to see whether planting one individual over and over makes a crop more susceptible to disease.
A number of scientists had studied this, often concluding that adding genetic diversity to a host population helps keep it healthy. Gibson and her student did a “meta-analysis,” comparing the work of many of these scientists to see whether this conclusion holds up across all the studies that have addressed this problem.
At the time Math4Science interviewed Gibson, the meta-analysis was not yet finished, so the results may change. But certain conclusions were emerging: genetic diversity does seem to provide some protection from genetic diseases.
In agricultural fields, the results seem to be strongest: rejecting monocultures in favor of biologically diverse crops (with genetically different individuals) makes those crops healthier.
Why do agriculturally cloned crops seem to be more vulnerable to disease than do monocultures which occur in nature? Gibson hypothesizes that it’s because the parasites attacking genetically identical crops have had many years to adapt to the clones farmers plant. “We’ve given pathogens time to get really good at going after these crops.”
The Cavendish banana, for instance, started out really resistant to disease. But “now, these parasites are excellent at attacking it — they’ve had decades to adapt.”
The always-changing relationships between different species and the parasites that attack them are the focus of Gibson’s work. Perhaps she first became interested in biological diversity as a child, growing up near New Jersey’s Great Swamp National Wildlife Refuge. Or perhaps it was the books she read: “I used to read all these things about fossils and dinosaurs and the La Brea tar pits and I thought that was really cool.”
There’s a lot of talk about how to get more women into STEM fields these days. For Gibson, the possibility of becoming a scientist was always there. “I grew up with a mom who did science and it was never questioned.”
Mandy’s mother, a biologist, helped develop drugs to treat heart disease, obesity, and diabetes. She also worked for the National Institutes of Health. Visiting her lab “was really cool.” And Mandy was proud when her mom visited her at school. “She’d be holding some formaldehyded heart and corner boys in the classroom who were scared of it.”
Gibson’s curiosity about biology and different species grew as she headed off to Amherst College. During Christmas break, she read The Song of the Dodo, by David Quammen. His descriptions of the diversity of life on islands and the experiments of researchers exploring that fascinated her. She appreciated the way the scientists Quammen covered moved from abstract hypotheses to experiments one could use “to understand an island in Indonesia and an island in the Caribbean and compare them to a patch of land in the Amazon.”
Genetic diversity played a role in Gibson’s doctoral work at Indiana University. She explored the differences between females in a species of New Zealand snails who reproduced sexually (creating genetically diverse offspring) and those who reproduced asexually (clonally). Which were more vulnerable to parasites?
Gibson collected snails in New Zealand lakes, using a net. She separated them from the other stuff the net brought up, using sieves. And then she transported them to a lab north of Christchurch.
She then dissected the females. Snails infected with parasites were filled with “pearly cysts.” She froze tissue from their heads, which would later be used to determine which snails were diploid (as are humans), reproducing sexually, and which were triploid, reproducing asexually.
This was done with flow cytometry. Cells from the snails’ heads would be pumped through a chamber. Gibson labeled their nuclei with a fluorescent dye, shot them through a tube, and then pointed lasers at them. “The laser excites the fluorescence of the labeled DNA, which could then be quantified by the flow cytometer.” There’s 50% more DNA in the nuclei of triploid snails’ cells, so the nuclei that produced more fluorescence were from asexual snails.
Gibson and other researchers have found that parasites sometimes attack sexually reproducing females less than they do those who reproduce asexually. But the relationships between parasites and their hosts in nature are complicated. Gibson and her colleagues found that this relationship varies a lot in space and time.
When does genetic variation matter for disease resistance and when might it not matter? How can farmers take advantage of the answers to these questions without making the food they grow so varied that people don’t want to buy it? The work of evolutionary biologists like Gibson helps us begin to understand the answers to questions like these and may keep bananas in our diets for years to come.