The wonders of genetics: a seedless melon

August 16, 2014 • 12:27 pm

I haz a big melon:

It’s a selfie!

And it’s seedless!!

There’s nothing better on a hot summer day than digging into the sweet, scarlet, crunchy meat of a chilled watermelon. And it’s even better when the watermelon is seedless: no spitting and less mess. We didn’t have seedless watermelons when I was a kid, so it’s still a great treat for me to eat one. And it’s even better when they’re like the one above, which I got at the produce store this morning. It must weigh at least 12 pounds, and cost only $3.59. I’ll get at least seven evening treats out of that puppy.

Seedless watermelons are in fact a triumph of genetics, for you can really design them de novo. You just double the chromosome number of a regular watermelon, so that instead of two sets of chromosomes it has four. The methods, outlined in this article, are simple: they involve treating watermelon seedlings with colchicine, a chemical derived from crocuses that inhibits cell division. By so doing, you can produce watermelon plants that have cells with 44 instead of the normal 22 chromosomes. These plants are called “tetraploids,” for they have four instead of the normal two sets of chromosomes (“diploids”).

When those cells produce flowers, the pollen (or eggs) in those flowers will have ½  X  44, or 22 chromosomes, while the pollen or eggs of a normal diploid watermelon will have ½ X 22 = 11 chromosomes. If you cross pollen of the latter to eggs from an engineered tetraploid melon, the zygote, or fertilized seed, will have 11 + 22 = 33 chromosomes. That seed can grow into a normal plant: a watermelon with three sets of chromosomes, called a triploid.

But when that triploid melon itself tries to make seeds, it has to do so through the process of meiosis, the same way we make sperm and eggs. That process begins with the members of chromosome sets pairing together in the cell before they split up for the reduction division. And if you have three representatives of each chromosome instead of the normal two, they don’t pair properly. When that happens, a normal female egg cell can’t form, and you get tiny, aborted seeds. In other words, triploid melons are sterile. They are the mules of fruit. (This is exactly how modern bananas are produced as well: they are also triploids, engineered to get rid of the seeds of their wild ancestors. When you cut across a banana and see those black specks in the middle, those are the aborted banana seeds.)

Ergo, we have a tasty triploid melon that lacks seeds: just what we want. The only problem is this: how do we grow more of them if they are sterile? Farmers get the triploid seeds from horticultural companies that produce them by the colchicine + crossing process. (Farmers can’t be bothered with all that technical stuff.)

The process is also described in a “Fun facts” FAQ from the National Watermelon Promotion Board. Their description starts out okay:

Where do seedless watermelons come from?

Seedless watermelons were invented over 50 years ago, and they have few or no seeds. When we say seeds, we are talking about mature seeds, the black ones. Oftentimes, the white seed coats where a seed did not mature are assumed to be seeds. But this isn’t the case! They are perfectly safe to swallow while eating, and don’t worry – no seeds will grow in your stomach.

So, how are seedless watermelons grown? Chromosomes are the building blocks that give characteristics, or traits, to living things including plants and watermelons. Watermelon breeders discovered that crossing a diploid plant (bearing the standard two sets of chromosomes) with a tetraploid plant (having four sets of chromosomes) results in a fruit that produces a triploid seed. (Yes, it has three sets of chromosomes). This triploid seed is the seed that produces seedless watermelons!

But then they screw it up. Can you spot the error, or at least the confusion, in the following paragraph?

In other words, a seedless watermelon is a sterile hybrid which is created by crossing male pollen for a watermelon, containing 22 chromosomes per cell, with a female watermelon flower with 44 chromosomes per cell. When this seeded fruit matures, the small, white seed coats inside contain 33 chromosomes, rendering it sterile and incapable of producing seeds. This is similar to the mule, produced by crossing a horse with a donkey. This process does not involve genetic modification.

Note too the somewhat duplicitous statement that seedless watermelon is not a genetically modified food. It sure is! Colchicine treatment, crossing, and so on, are all modifications of the genetic complement of a melon, and what you finally eat has three sets of chromosomes instead of two. How is that  not genetic modification? The fear of GMOs, always greatly exaggerated, has gone too far, scaring even the National Watermelon Promotion Board.

At any rate, eating watermelon is much better for you than eating the other summer treat I love: ice cream (Breyer’s strawberry, please). Watermelon is nutritious and rich in lycopene. Here’s the nutritional information for a “standard serving”: two cups. (I probably eat twice that much.):

Screen Shot 2014-08-16 at 12.58.20 PM

Note: only 80 calories. By comparison, a small pot of Greek yogurt (125 g) has about 144 calories.

Oy vey! I intended to just show the picture of my bargain watermelon, and look what happened. I went too far. But that reminds me of a joke, one I’ve told before:

An elderly rabbi, having just retired from his duties in the congregation, finally decides to fulfill his lifelong fantasy–to taste pork. He goes to a hotel in the Catskills in the off-season (not his usual one, mind you), enters the empty dining hall and sits down at a table far in the corner.  The waiter arrives, and the rabbi orders roast suckling pig.

As the rabbi is waiting, struggling with his conscience, a family from his congregation walks in!  They immediately see the rabbi and, since no one should eat alone, they join him.

Shocked, the rabbi begins to sweat.  At last, the waiter arrives with a huge domed platter. He lifts the lid to reveal-what else?–roast suckling pig, complete with an apple in its mouth.

The family gasp in shock and disgust, and quickly turn to the rabbi for an explanation.

“This place is amazing!” cries the rabbi. “You order a baked apple, and look what you get!”

I’ll be here all year, folks.

68 thoughts on “The wonders of genetics: a seedless melon

    1. Why not? Without seeds there can be no divine watermelon which spells out “There is no god” (see ‘That Mitchell and Webb Look’, Divine Watermelon).

  1. IMHO, there is no watermelon on the planet that tastes better than one of them oblong-shaped monsters from ‘bamy, ripe to the max, cold to the core and bursting with black seeds.

    I always find the meat of the one shown here to be inferior to those. I call ’em “near melons.” They’re the fruit equivalent of near-beer.

    1. I find the degree of ripeness is hugely important when rating a watermelon’s flavor. Many today are probably picked early to survive shipping to market in good condition. You might try giving the ones you buy a few extra days before cutting into them to see if their flavor improves as they reach peak.

      1. That’s why I prefer buying them from farmer’s markets instead of grocery stores. There’s a better chance that the melons are riper that way.

    2. I agree, The watermelons that I had when I was a kid are much bigger and better then today’s seedless. And if you ate one or two black seeds it was no big deal.

      1. Our parents would line the kitchen table with newspaper, put the watermelon on them and cut it, and then leave hurriedly, closing the door behind them. By that time, we were already spitting seeds at each other. 🙂

          1. Even though I never really liked the taste, I did learn to spit the seeds in self- defense against three younger brothers:-). Seems like we had to do it out on the porch, though.

    3. I concur. The best watermelon is indeed seeded, huge, ruby red and juicy and crunchy inside, with full black seeds, a single melon enough to last a small family for a week of watermelon debauchery.

      So much better then the bland tasteless, often unripe ubiquitous seedless watermelon, a terrible blight on all those who truly care about the food they eat.

      Luckily there are still enough of those who choose the best, so the great green striped seeded beasts haven’t completely disappeared from supermarkets, although their natural habitat is quickly being overrun by the nasty insipid replacements. How that can be when they are often sterile, producing only a small pittance of pathetic white and brown sickly looking seeds that issue from their pale meat…

      I choose seeded watermelons by gently slapping them, finding something in the middle of the road, not too high pitched, not too low.

      The best time of the year is summertime when watermelons are in season.

    4. In theory seeded watermelons might taste better but avoiding the black seeds is such a pain that I always buy seedless.

      I love all sorts of melons but not quite to Jerry’s scale. I also like strawberries (with cream and castor sugar) and bananas. Very much nicer if refrigerated. I had intended to buy some watermelons today but didn’t like the look of the few on offer, so I bought six punnets of strawberries instead.

    5. While I would rather eat seedless, I have also noticed that the sweetest strongest flavour is always to be found immediately surrounding the seeds. It is a compromise I am prepared to live with though.

  2. Wow! What a bargain! I’d be pretty excited if I got a watermelon for that price too. I hope you eat it quickly though, I once had one go bad in a refrigerator when I was a kid and it stank so bad it actually woke me up from sleep. Yuck!

    1. I keep my refrigerator on its coldest setting, so melons last over a week. Once one did go off and I had to bin it, but it only looked bad, the smell wasn’t noticeable.

  3. Cool post! The Watermelon Promotion board got the ploidy count of the respective gametes wrong. But what’s a few chromosomes among fruit lovers?

  4. I love the genetics aspect, but I must be the only person on this planet who does not like watermelon. Love cantaloupe, honeydew, crenshaw, all the other melons ( and almost all other fruits – peaches, raspberries, papayas- yum) but not watermelon. I’m with ya on the Breyer’s strawberry ice cream, though. Stumbled on some incredibly delicious Blue Bunny brand salted caramel ice cream in ND last month. I had never heard of that brand, but my Southern-born bf knew it well. Had some great blood-orange gelato at a Japanese crêpe joint in Kensington Market, Toronto yesterday. How global is that??

    1. I had a bad experience with honey dew melon when I was a teenager working in an ice cream place. I got food poisoning & it kicked in while I was making honey dew melon sundaes. I didn’t throw up but I felt horrible so now I just can’t deal with them.

  5. I have never eaten a seedless watermelon that was “tasty” they are pretty bland in flavor. I don’t mind the seeds; those I can’t pick out I just swallow.

    1. The seedless watermelon that I bought at the local farmer’s market a few weeks ago was by far the best, tastiest watermelon I have ever eaten. Maybe you simply haven’t found any good ones. Have you tried local or regional? The freshest fruit is usually the best!

      1. I agree! I think these seeded watermelon boasters simply are remembering wrong, or else haven’t had a good seedless one. In principle, there is no reason why a seedless melon should be worse than a seeded one.

        It’s the luck of the draw, not the seeds!

        1. I’ve eaten watermelon occasionally for years but only recently become an addict, which perhaps explains why I don’t have these memories of how wonderful seeded melons were back in the good old days.

  6. I agree that such watermelons are genetically modified in the literal sense, as are all crop plants. They are perhaps a bit more genetically modified than most crop plants developed by traditional selective breeding because of the application of colchicine to create the polyploid condition. However, I think the average layperson understands “genetically modified organism” to mean “chimeric organism”, such as Bt corn, which includes a gene inserted from the bacterium Bacillus thurengiensis.

    1. But the hybrid fruits produced by Luther Burbank and his ilk (nectarines, plumcots, and so on) are just chimeric. He just had work harder and longer, with many generations of backcrossing, to isolate the genes he wanted.

  7. I love the seeds, myself. Crunchy and yummy and you can plant them. My experience with seedless is rather narrow but most of them have been rubbery and bland in comparison. Also, they are seedless!

  8. “Oy vey! I intended to just show the picture of my bargain watermelon, and look what happened. I went too far.”

    Au contraire! All the interest is in the science–greatly enhances the picture. 🙂

  9. This was interesting! Now I knew that seedless varieties of fruits were polyploid, but I did not know that it involved annual crosses to make triploids in the cases of melons and bananas. I just thought they were like some other sterile polyploid plants — sterile b/c they were highly polypoid like 32n or something.

  10. I agree that the seedless watermelons of tody are not as good as the watermelons I had as a kid. But then that is true of most things.

    I have occasionally found a good looking seed in a seedless melon.

  11. I haz a big melon

    Well, of course you do — you’re a big-name university professor, after all; not too many dumb people get those jobs.

    But what’s with the cucurbit you’re hiding behind?


  12. I echo the sentiment that seedless watermelons are far inferior in flavor.

    Furthermore, they’re not seedless! I don’t mind the big black seeds, which I can easily remove or spit out. I despise the small white immature seeds, and that’s all the “seedless” melons have.

    Mind you, most store-bought watermelon pales in comparison to the kinds you can grow in a garden. I have Crimson Sweet, Moon and Stars, and Congo growing right now. The first is probably the best variety you can grow, though they don’t last long after ripening. The second has a distinct flavor and flesh on the pinker side of red. As for Congo, this is my first year growing those, so I’ll have to see in a month or so.

    If you do grow your own, there are two markers for ripeness. First, there’s a vine tendril directly opposite the melon’s attachment stem. That tendril must be completely dead – brown and falling off. If any of it is still green, the melon is definitely not ripe. Second, the bottom of the melon (where it rests on the ground) will have a colorless spot. That spot will start out cream-colored, and turn yellow when the melon ripens. If you have a completely dead tendril and a yellow bottom, the melon is probably ripe. But not definitely. The only way to be absolutely certain is to take a core sample. If the melon isn’t ripe, put the plug back in (make sure you don’t get dirt get in the hole, of course). Watermelons do not ripen off the vine.

    As someone else pointed out, the difference in flavor between ripe and not-quite-ripe is tremendous. If you don’t like watermelon, odds are you’ve never had ripe watermelon.

    1. There is a farm near here, which my daughter participate in a CSA, which has yellow flesh watermelons. My two-year-old granddaughter refused to eat the yellow ones until this week.

    2. I read once of someone who figured out that melons get heavier and heavier when they are growing, and when they plateau, that’s when they are ripe.

      Of course, making practical use of this knowledge would be completely crazy.

  13. What’s the fun of a seedless melon? Part of the ritual of enjoying a melon is to see how far and how rapidly you can shoot out the seeds. Well, I guess that doesn’t work out so well in a city.

  14. Seedless watermelon? What sorcery is this? Next you’ll tell me there’s seedless grapes.

    It seems like you can get a lot of seedless varieties using induced tetrapolyploidy + multi-ploidy crosses. I wonder why it hasn’t been tried on every vegetable? Also, are seeds really that bad to eat? They never really bothered me.

  15. Glad to see the opinions are running >90% against seedless in taste tests. I concur.

    No problem whatsoever with seedless grapes, though.

  16. If you like Bryer’s strawberry, you should try Tillamook’s strawberry. The PNW (Pacific North-West) has superior berry’s.

  17. While lack of seeds is convenient there is no better watermelon than the seeded one. It have superior taste and is more sweet. Nowadays it is more and more difficult to find seeded variety in stores which really saddening.

  18. By the way, a pretty easy way to de-seed diploid watermelon is to take a pretty thin slice of it and poke the seeds out the other side with a chopstick. You’ll get just about all of them in a very short time.

    But there will still be a few hiding in there for spitting purposes.

    Though I’ve always been more fond of squeezing them out between thumb and forefinger and seeing how far they could go.

    Which brings to mind this: what are we going to do to replace ‘like squeezing out a watermelon seed’ as the infuriating thing to say to a woman who’s had a quick childbirth?

    [p.s. my advice is not to say this, or maybe even any fruit-based analogy, to a woman who has had a quick childbirth, lol.]

  19. I see a lot of people saying that the seeded ones were better. But they should taste the same! The difference must come from either the producer picks up the seeded watermelon much later because he is not a big producer, so the fruit has time to get more taste. The second reason can come from the false memory, we have a tendency of enhancing good memories from the past, if you remember eating the watermelon and having a lot of fun when you were young, that had a lot of seeds, you put in your memory that it tasted so good, but now you remember it was even better than it actually was. This phenomenon is known and happens to everybody.

    1. Indeed, I’ve eaten a lot of watermelons in my life, and of course they vary, but the seedless ones I’ve had lately are just as good as the best seeded ones I’ve had. I suspect a trick of memory for those who say the seedless ones are worse.

      1. I disagree. The first seedless one was bought without realizing it was seedless. Shrug. But it tasted like cardboard. Shrug again – so does the occasional seeded one. But then the next time it happened, same thing. And the third. So I just don’t buy them anymore if the sticker says seedless. At the same time I’m of the opinion that whoever developed seedless red table grapes (which I think is a phenomenon of our lifetime) should be considered for a Nobel Prize, so it’s not like I have an agenda on this.

        But here’s a possible explanation why it seems that seeds impart a certain je ne sais quoi to the mesocarp. Watermelon seeds contain a novel amino acid, pyrazolylalanine (think phenylalanine, but with a pyrazole ring instead of the benzene ring) that constitutes up to 0.1% of the dry weight of the seeds. (I tripped across its presence in watermelons once long ago while looking up something about pyrazole, which is an inhibitor of alcohol dehydrogenase), but wasn’t sure what part of the melon it was in. I suspected seeds, & just found that ref. It’s made by cysteine synthase, but other curcurbits don’t have it.

        So, here’s my hypothesis, which is mine. There’s something in the seeds, perhaps pyrazolylalanine, that diffuses into the mesocarp in low levels & contributes to the flavor.

        And while poking around on this, I ran across quote from Mark Twain, thanks to the South Carolina Watermelon Board. Watermelon are “chief of this world’s luxuries, king by the grace of God over all the fruits of the earth. When one has tasted it, he knows what the angels eat. It was not a Southern watermelon that Eve took; we know it because she repented.”

        1. Choosing seeded over seedless in the grocery store is hardly a controlled experiment, since you have no idea of the pedigree of either melon.

          To conclude that seeds account for the perceived difference in flavor, you’d need to take your favorite seeded variety, form a seedless triploid variety from that, grow them to full ripeness under identical conditions, remove the seeds, and do a blind tasting on the pulp. If you can still reliably tell them apart, then your pyrazolylalanine hypothesis might have some traction.

          My hypothesis is that early seedless varieties were optimized for storage, transport, and machine processing rather than flavor. But a recent resurgence of interest in heirloom varieties has prompted growers to turn to more flavorful seeded stock to produce seedless melons for the gourmet market.

          Bottom line: any difference in flavor probably has more to do with the genetics of different cultivars bred for different purposes, rather than with the presence or absence of seeds.

    2. I agree. The seedless variety is the most widely available in my area markets, which suggests there could be a harvesting/production bias as growers rush the melons to market. Finding a smaller-scale, local grower should improve your chances of finding sweeter fruit, seedless or not.

  20. My parents were friends with Orie (OJ) and Agnes Eigsti, and I met Orie a couple of times. Orie is considered the father of the seedless watermelon and the Tribune has a nice article about OJ here:

    There were many lean years as Orie tried to get his work accepted commercially, and after many decades, he finally made some decent income from the seedless watermelon. Orie ran track at Goshen College (IN) many moons ago (in the 1920’s I believe), and after he made his much-deserved monetary rewards, he gave money to build a fully modern track and field facility at the college – rightfully named the Orie Eigsti Memorial Track.

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