It’s the great squash Charlie Brown! Doesn’t have quite the same ring, but it’s the more accurate description of this fall-time thing. You probably think of them as “Giant Pumpkins” but, sorry to squash your beliefs – these giant fruits are actually squash of a different species! Cucurbita pepo sounds like a witchy spell, but it’s actually the scientific name for these *squash.* The things we call Pumpkins and carve up into jack-o-lanterns are squash too, but of a different species – Cucurbita pepo.
You know fall’s coming when Cold Spring Harbor Laboratory puts out its cucurbita maxima (thanks Dr. Zach Lippman)! And you know some science behind their large size is coming when you see a weird grad student in a lab coat cape out taking pictures with it… You might think it comes from that gene editing technique called CRISPR which I talked about in my last 2 Bri*fings, but the large size of these squash is the result of the original GMO technique – selective breeding. So let’s dive in – and I hope you enjoy this reading!
New parents are often bemoaning how fast their babies grow – well human babies have nothing on giant squash – those guys can grow up to 50 pounds a day! (I really hate that the U.S. doesn’t use the metric system – 50lb is a little under 23kg.) But farmers – instead of groaning about their babies growing up – celebrate each pound – and keep their own kind of baby journals in the form of lab books of a sort filled with growth curves and stuff.
The records for giant “pumpkins” have TRIPLED since the 1980s – and we’re not talking about going from 1 to 3 pounds. We’re talking thousands of pounds – the world record for heaviest pumpkin is currently 1,190.49 kg (2,624.6 lb) – it was grown by Belgian Mathias Willemijns in 2016. You might think this dramatic increase has to do with genetic engineering tools like CRISPR – but it’s actually the result of old-fashioned selective breeding, along with clever farming! Scientists grow the pumpkins and then once the weighing’s over they purposefully crack them open and take out the seeds – not to eat, but to save for next year!
CSHL is home to one of the 2019 McArthur “genius awardees” and my genetics prof from a few years ago, Zachary Lippman – he reached fame through CRISPR work – he does a lot of genetic tweaking of tomatoes – but he still grows his giant pumpkins with selective breeding and TLC. Apparently CRISPR is really hard with cucurbits because they’re hard to transform (stick stuff into their cells) – and you have to stick the CRISPR machinery and the guide DNA in there to make changes to the DNA with CRISPR so it hasn’t been a good option – but in the future, who knows! Still, Lippman loves giant pumpkins and has been growing them from a young age.
The reason the giant squash look so “squashed” has to do with gravity but not just directly – it’s not just that the top of it’s “hard to hold up” – the cells at the bottom of the pumpkin do get pushed on more – but this makes it easier and more beneficial for them to split – so you actually get increased cell growth at the bottom, leading to that “hamburger-ish” shape
And speaking of food – these squash apparently aren’t very tasty because they’re like 90% water – they drink ~100 gallons a day! and so much of their weight depends on their drinking that in the hours between cutting off the vine & sticking on the scale they’re at risk of shrinking! They lose like a whole pound of water weight an hour, so farmers wrap them up in wet towels and stick their stems in water (that’d require one giant vase – so they tape a bag of water to the stem instead).
The untastiness probably helps protect them from things like groundhogs, but microorganisms like bacteria and fungi don’t have taste buds and are happy to dine – if they can get in – if the squash skin cracks, bacteria & fungi and other microorganisms can sneak in and start rotting the squash from the inside out. But not all microorganisms are bad – fungi that live in the soil and on the roots (mycorrhizal fungi) can take nitrogen out of the air and “Fix” it – trapping it in the form of ammonia, which the pumpkins can use as a nitrogen source for building things like DNA and proteins.
What’s the secret to the size? Plants use energy from sunlight to turn CO2 and water into sugar through a process called photosynthesis. And that happens in the leaves – so that’s your sugar “source” but the big thing the farmers are interested in is the fruit – the sugar has to get from the leaves to the fruit (a sugar “sink”), so it needs some “plumbing.” Plants have 2 types of “pipes” – 2 forms of vasculature – the xylem moves water and nutrients from the soil through the plant & the phloem moves sugar and other solutes (dissolved things) around.
If you have something like grapes where there are a lot of little fruits the plant has to feed, the resources have to get divvied up. But with giant pumpkins you just have 1 big fruit you have to send sugar to – and these giants can move a couple of pounds of sugar into the fruit each day. How?
The giant varieties have been selectively bred so that they make more phloem – farmers weren’t knowingly selecting for this phloeminess – they just kept & used seeds from big squash – but when scientists looked at the difference between giant pumpkin breeds and non-giant ones they found more phloem – links to some cool articles at bottom
But, they have similar photosynthetic capability as non-giants – the selective breeding (at least for the varieties they looked at) has only been selecting for shifts in “carbon partitioning” not increased ability for photosynthesis – the giants don’t make more sugar (proportionally to their size), they just make more pipes to the fruit so they can move more of it there. Another thing they found – the giant ones also had a thinner and more elastic outer fruit wall (exocarp), which likely helps them expand without cracking.
A little on the history: “Squash” is a name given to melons, cucumbers, and gourds of the Cucurbitaceae family, so you can call them all cucurbits. As to the species part, there are multiple ways to define a species, but one of the common ones is that a species is a term scientists use to describe organisms that can’t naturally reproduce with one another.
The field pumpkin is Cucurbita pepo. From the normal pumpkins you carve to those tiny “munchkin” pumpkins that you see on display, those are all pepo – it’s kinda like how great Danes & chihuahuas are all the same species – they’ve just been bred for different things. Cucurbita pepo traces its origins to Mexico and the common ancestor of zucchini & spaghetti squash.
One of the first recorded reverences about giant pumpkins is by Henry David Thoreau, who, in 1857 in Concord, Massachusetts planted seeds of Potiron Jaune Gros de Paris (fat yellow Paris pumpkin) and one grew to almost 125 pounds (huge at the time but tiny by today’s standards). That fat yellow Paris pumpkin was a squash of the same “Mammoth group” of squash as today’s big prizewinners
Competitive growing can be traced back to the 1900s. One of the early pros was a machinist & farmer named William Warnock – his 403 pound “pumpkin” (achieved at the 1904 St. Louis World’s Fair) held the world record for over seventy years. Modern competitive growing really went “extreme” with a Canadian farmer named Howard Dill. After years of crossing pumpkin varieties he got his first world record in 1981 – 493.5 pounds. The 1000 pound barrier was broken in 1996.
Size isn’t the only trait squash can be bred for – commercial seed suppliers have bred for things like wartiness and color.
This post is part of my weekly “broadcasts from the bench” for The International Union of Biochemistry and Molecular Biology Be sure to follow @the_IUBMB if you’re interested in biochemistry! They’re a really great international organization for biochemistry.⠀
link to article on phloem differences: http://bit.ly/36kVcN3
link to interview with author Jessica Savage: http://bit.ly/2q7OfOU
more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0⠀