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While doing some background reading about the biology of giant pumpkins, I came across a paper in an obscure publication called the International Journal of Non-Linear Mechanics. The authors of the study, in an attempt to figure out why gigantic fruits and vegetables have such different shapes from ordinary-sized ones, compressed whole pumpkins in materials testing machines to evaluate their elasticity, and constructed models simulating the distribution of pumpkins’ internal stresses.

I, too, have internal stresses — as well as external ones like looming thesis deadlines — so I didn’t spend too much time on the models, focusing instead on the photos of growers alongside their prizewinning giants. One man held up a sweet potato twice the size of his head; a woman peeked out from between the leaves of a cabbage like a fairy. The text explained how giant fruits that hang from trees tend to become elongated, while giant fruits that sit on the ground tend to grow wide and flat. For pumpkins, growing in this pancake shape is apparently adaptive, decreasing the probability of collapsing under its own weight, rather than strictly a result of gravitational forces.

Non-linear mechanics (or any kind of mechanics) is not my field, so I can’t speak to the validity of the study’s conclusions, but seeing it made me think to myself, “I sure am sure somebody is doing that.” Of course, if nobody had, it would not have occurred to me that someone should. But since someone did think of it, it feels right and inevitable that they should follow that line of inspiration to its culmination, as a true artist would. And when the subject in question is giant pumpkins, it feels safe to say that questions of hubris and ethics don’t enter into it as much as with many other questions of technology and engineering. If we learn something about how living organisms cope with gravity, great! — and if not, a giant pumpkin is its own reward.

I was reading about giant pumpkins in the first place because I was thinking about the general theme of how much of human endeavor, including science, is driven by what we learn from things we do for no real explicable reason. Competitive giant-fruit-growing uses principles from agriculture: select the variants with the characteristics closest to what you’re after, and figure out the conditions under which they thrive. But the resulting glorious, gigantic fruits have no agricultural value, being mostly composed of water and practically inedible. The same is true of breeding fancy chickens, in which people select for increasingly extraordinary ornamentation. In both of these cases, scientists have realized that the products of these efforts can teach us a great deal about fundamental biological phenomena. In the case of chickens, we can attempt to detect the genetic mechanisms underlying seemingly useless sexually attractive traits by looking at the ones that are so exaggerated as a result of breeding. In the case of pumpkins, we can learn about the factors limiting plant growth, and the fundamental physical and chemical constraints that living organisms are always pushing against in their endlessly creative attempts to survive.

By selecting for gigantism, pumpkin-growers have been creating extreme test cases that plant physiologists can examine for insights into how more ordinary-sized plants work. After all, selective breeding is evolution in action, whereby organisms with particular traits propagate more than others. It’s just that we know exactly why those individuals are the fittest (it’s because they have the traits we’re interested in), so we can observe how traits change in a more direct fashion than if they were subject to all the whims of history.

The genus Cucurbita consists of 12-14 squash species domesticated at different times throughout North and South America. Cucurbita pepo, the species that encompasses most of the pumpkin and a number of other familiar squash varieties (including zucchini), is thought to be the first food plant to be domesticated in the New World. (The first domesticated non-food plant in these regions is probably the bottle gourd Lagenaria siceraria, a hard-shelled relative of squashes used as containers and flotation devices in Africa and Asia.) The wild relatives of squashes were already present in the New World when humans arrived, with a distribution from Canada to Mesoamerica. Although all the C. pepo varieties can inter-hybridize, the species appears to be composed of at least two subspecies which were domesticated independently from two different wild squashes, one in eastern North America and one possibly in southern Mexico.

The reason for all these qualifiers is that untangling plant evolution is a messy process, requiring multiple lines of evidence: archaeological (where do we find which seeds in relation to human settlements?), morphological (what visible traits do different species have in common?), genetic (how many chromosomes do different species have? Which species can hybridize with which other ones?), and molecular phylogenetic. Molecular phylogeny attempts to reconstruct evolutionary relationships between organisms by evaluating inherited differences in DNA sequences. The classic analogy is to monks copying manuscripts and then copying other people’s copies. By looking at which errors are present in which texts, you can reconstruct who copied from whom. The same is true for errors made during copying DNA, which are perpetuated between generations and therefore can be used to reconstruct who shares which common ancestors. The first challenge is in identifying the correct stretch of DNA to analyze for differentially accumulated errors. It can’t be a region in which errors in the sequence result in defects in the organism, because then the sequence will be too similar in all non-defective organisms you look at. But it also can’t be something that’s too different between organisms, or you won’t be able to deduce the pattern of relatedness from it.

Beyond this, there are a number of theoretical, practical, and computational challenges involved, which is why many studies using many different techniques are often required to resolve even simple questions about pumpkin cultivation. For example, from molecular phylogenies we learn that the giant pumpkins of today are not really pumpkins. The record-breaking gourds are usually from the species Cucurbita maxima, which includes cultivars like Hubbard squash — which was domesticated on a different occasion, in a different place (Argentina) and from a different wild relative than C. pepo. Breeders working with this species were able to select for fruit more massive than anyone working with any other species (the latest record-holder, from 2014, weighed over 2300 pounds).

Specifically, the hyper-gigantic pumpkins entered into international competitions today are all part of one lineage, called the Atlantic Giant; the family farm where this strain originates can still be visited in Windsor, Nova Scotia. A recent analysis of the physiology of Atlantic Giants attempted to understand the physiological basis of their giant size. Is this lineage better at photosynthesis, harvesting energy to turn into tissue, than other lineages, or is it better able to turn its energy into specifically fruit mass? Is a fruit’s gigantism driven mainly by having more cells, or by having larger ones? What limits are ordinary fruit subject to that Atlantic Giants are not?

The answers seems to lie in the amount of phloem, the tissue that carries sugars throughout a plant. Atlantic Giants simply have more of it leading to their fruits compared to Hubbard squash. Combined with the nearly 1.5 times as long that it takes for Atlantic Giants to mature, it means that these plants have the ability to pump much more sugar to grow their expanding fruits. The fruits have a more tender skin than other squashes to accommodate their expanding volume.

When someone weirdly obsessed with phloem cells meets someone weirdly obsessed with growing the world’s largest pumpkin, magic happens.

Determining this took an incredible amount of highly fiddly technical work. Knowing what daily life in the lab is like, I am willing to bet that at every turn, there were machines that inexplicably stopped working, analysis software that needed license updates, specific types of vials that were on backorder at the lab supply company. The researchers had to examine collect samples from an existing Atlantic Giant patch, grow their own greenhouse samples; measure photosynthetic rates by clipping devices that measure gas exchange onto leaves; measure and weigh leaves, flowers, fruits, and shoots; cross-section flowers into thin slices and stain them to visualize the structure of the tissue so it could be measured; determine the size and number of pores in phloem tissue using a technique involving spattering it with gold, following the transport of compounds through the phloem using fluorescent dyes and lasers; and send samples to an isotope analysis laboratory after freezing them in liquid nitrogen.

What these researchers were not able to determine was what, in turn, constrains the amount of phloem built. The Atlantic Giants seemed to have the same photosynthetic capacity as other squash – what other resources must run out before it becomes physically impossible for the pumpkin to grow any bigger? Pumpkin-growing records are being broken every year. We have plenty of material to work with.

So, what does it take to raise a truly gigantic pumpkin? As the experts at the charmingly named bigpumpkins.com say, “Good seeds, good soil, and good luck!” That pretty much sums up all of evolution, whether natural or human-driven. To survive and thrive, an organism must have the right genes (seeds), the right environment for those genes to reach their potential (soil), and so much luck. Natural selection proceeds because individuals with better-adapted traits leave behind more offspring, but any time there’s a single individual with a particularly well-adapted new variant, chances are that individual will die just by chance before the trait has any possibility of taking hold in the population. When humans are involved, every individual organism’s luck is a little bit better, but it’s still only luck.

Likewise, “nature versus nurture” and “genetics versus environment” is not an easily made distinction. Atlantic Giant seeds, given poor soil, will not grow that big, while there are some individuals of C. pepo that have reached considerable size according to the books. But beyond that, for many organisms, even how much a particular trait is affected by environmental conditions is itself a variable trait, encoded by variation elsewhere in the genome. Some pumpkins varieties produce larger fruit when placed in a better environment; some produce more fruit; some hardly change at all. One of the notable things about the Atlantic Giant lineage is how fast people have been able to increase their size over the past 100 years, which suggests that maybe one of the things people have been selecting for is increased responsiveness to cultivation.

A lot of what scientists do depends on the principle that we don’t yet know what obtaining this knowledge might be useful for, but every detail along the way must be obsessively refined until you’ve got it exactly right. The same is generally true for individuals following their passions (like giant fruit-growing!). Nearly everything useful in the world comes from a convergence of people following their own obsessions until they’ve achieved maximum expertise. You don’t have to know exactly where you’re going with what you’re doing, but you do have to take yourself seriously along the way. When someone weirdly obsessed with phloem cells meets someone weirdly obsessed with growing the world’s largest pumpkin, magic happens. We might not know what it’s good for yet, but we’re getting somewhere.


References 

ERICKSON, D. L., SMITH, B. D., CLARKE, A. C., SANDWEISS, D. H. & TUROSS, N. 2005. An Asian origin for a 10,000-year-old domesticated plant in the Americas. Proceedings of the National Academy of Sciences of the United States of America, 102, 18315-18320.

HU, D. L., RICHARDS, P. & ALEXEEV, A. 2011. The growth of giant pumpkins: How extreme weight influences shape. International Journal of Non-Linear Mechanics, 46, 637-647.

SANJUR, O. I., PIPERNO, D. R., ANDRES, T. C. & WESSEL-BEAVER, L. 2002. Phylogenetic relationships among domesticated and wild species of Cucurbita (Cucurbitaceae) inferred from a mitochondrial gene: Implications for crop plant evolution and areas of origin. Proceedings of the National Academy of Sciences of the United States of America, 99, 535-540.

SAVAGE, J. A., HAINES, D. F. & HOLBROOK, N. M. 2015. The making of giant pumpkins: how selective breeding changed the phloem of Cucurbita maxima from source to sink. Plant Cell and Environment, 38, 1543-1554.

SISKO, M., IVANCIC, A. & BOHANEC, B. 2003. Genome size analysis in the genus Cucurbita and its use for determination of interspecific hybrids obtained using the embryo-rescue technique. Plant Science, 165, 663-669.

SMITH, B. D. 2005. Reassessing Coxcatlan Cave and the early history of domesticated plants in Mesoamerica. Proceedings of the National Academy of Sciences of the United States of America, 102, 9438-9445.

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Sasha Mushegian is a doctoral student in biology, investigating the ecology of the sprightly water flea. Her interests include scholarly journals and fruit.

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