Triple threat watermelon

Will seedless watermelons make us superhuman or turn our children into giants?  Hardly, but they do give home cooks the power to count chromosomes without a microscope.   Just a knife or a hard thunk on the sidewalk are enough to get a watermelon to spill its genetic guts.

If you were reading a Hearst Corporation newspaper in late 1937, you might have thought humanity would eventually be swallowed up by giant carnivorous plants, unwittingly unleashed by uncontrolled biotechnology.  The San Francisco Examiner reported on November 21st of that year that the discovery of an “elixir of growth,” meant that “…science may at last have a grip on the steering wheel of evolution, and be able to produce at will almost any kind of species…”  including “…a plague of man-eating ones.”  In 1937 Americans had much more important things to worry about, just as we do now.  Still, that discovery may in fact have threatened one cherished aspect of the American way of life by triggering the slow demise of late summer state fair watermelon seed spitting contests.  It doubtlessly paved the way for seedless watermelons, and in 2014 the total harvest of seedless watermelons on American farms – nearly 700 thousand tons – outweighed the seeded watermelon harvest more than 13 to 1 (USDA National Watermelon Report). A similar pattern is emerging this year.  Is there no stopping the attack of the seedless watermelons?

Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

CLICK to read. Image from microfilm of an actual page in the San Francisco Examiner, published Sunday November 21, 1937. Found in the Media and Microtext Center of Stanford University Libraries.

And more important, how is it even possible to get seedless fruit from an annual plant?  From a plant whose only mode of reproduction is through those very seeds?  From a plant that cannot make suckers as bananas do and cannot be perpetuated endlessly through grafts like fruit trees and vines?   Such is the challenge posed every single year by watermelons, but thanks to the “elixir of growth” discovered by Albert Blakeslee and subsequent work by Hitoshi Kihara, one of the most prominent agricultural geneticists of the 20th century, the world has an elegant solution. Breeders continually improve the varieties available, and consumer demand keeps growing, yet seedless watermelon production methods have remained essentially unchanged for three quarters of a century.

This watermelon definitely has seeds

This watermelon definitely has seeds

Pollination without procreation

Seeds normally develop when pollen grains germinate on the stigma of a flower and pollen tubes grow down into the ovary to deliver sperm cells to the eggs inside the potential seeds (ovules).  (see Jeanne’s fuller explanation here) The surrounding ovary is then stimulated to develop into a fruit.  You might think that seeds could be avoided if pollination could be avoided.  No pollen, no sperm, no seeds.  But also no fruit.  Large sweet fleshy fruits are expensive for a plant to produce, but the investment pays off if the fruit entices animals to disperse its seeds.  Natural selection generally favors a high return on investment, and accordingly, watermelon fruits develop only if their flowers are amply pollinated and the chance of producing seeds seems high. (Although wild watermelon fruits are smaller than commercial varieties and not sweet, they still represent a serious investment of resources; see Paris 2015)

Preventing pollination is not a solution.  It turns out, though, that a big load of pollen is enough to send the hormonal signals that promote watermelon fruit development, even if seeds ultimately never develop.  What’s needed, then, is some way to trick the plant by providing a full pollen load while preventing the sperm in the pollen from fertilizing the egg and producing a viable embryo-filled seed. 

Right now some of you are imagining thousands of microscopic condoms for the pollen tubes.  It’s an amusing image, but the much more effective way to block embryo development is to start with defective egg cells.  Plant egg cells are produced inside ovules (which would become seeds if fertilized) after a sequence of cell divisions that includes meiosis, the type of cell division that halves the number of chromosomes in a cell.  Messing up meiosis will produce defective eggs.  The geneticist Hitoshi Kihara knew that meiosis would be disrupted in every ovule of a plant if the plant body had three copies of each chromosome (a condition known as triploidy) instead of the usual two.  On that point he seems to have been inspired by triploid seedless bananas (Crow, 1994).  His great contribution was developing a technique to produce those triploid watermelon plants that would bear seedless fruit.

A seedless watermelon

A seedless watermelon

Why triploids don’t make seeds

Humans normally have 23 pairs of homologous chromosomes, for a total of 46 chromosomes in most cells.  One set of 23 is inherited from one parent, and the homologous set comes from the other.  Similarly, watermelons normally have 11 pairs of chromosomes, for a total of 22.  Having two copies of each chromosome makes us and watermelons diploid (Greek for 2-ply, written as 2N).  Unless our parents are completely genetically identical (impossible in humans), the chromosomes in a pair are similar – “homologous” – but not identical.  They carry the same sequence of genes, but will have different versions (alleles) of many of those genes.  For example, both homologs of chromosome 6 in watermelon carry a gene influencing striping pattern, but a watermelon plant may have inherited the allele for defined stripes from one parent and the allele for diffuse stripes from the other.


A full set of human chromosomes, with homologous pairs brought together and digitally arranged in order of chromosome number.

When it comes time to make haploid gametes (eggs and sperm), the homologs have to be separated again so that each gamete carries a complete set of chromosomes, one of each pair.  It’s almost funny how straightforward and logical the meiotic process is.  The homologs find each other in the cell based on their similar gene sequences and become physically entwined.  Each pair is then guided to the equator of the cell by tiny tubes (microtubules).  When all the pairs are lined up properly, the homologs let go of each other and are pulled to opposite sides of the dividing cell.  (A second round of division follows, but only the first round is critical for our watermelons.)

Like humans, watermelons are normally diploid; however (unlike humans) a plant with three copies of each chromosome (3N, or triploid) does perfectly well doing its everyday thing, growing and photosynthesizing.  Plants in general are remarkably tolerant of extra chromosomes, and polyploidy is rampant in the plant world.  For example, bread wheat is hexaploid (6N) and common commercial strawberries are octoploid (8N).  The trouble starts with meiosis.

Small undeveloped seeds can be found in a seedless watermelon, but most people just eat these

Small undeveloped seeds can be found in a seedless watermelon, but most people just eat these

Polyploids with even numbers of chromosomes, such as wheat and strawberries, can produce normal gametes (more on this below), but triploids (or pentaploids, etc.) mostly do not.  When all three homologs are properly matched and lined up on the equator ready to separate, they cannot be divided equally.  Sometimes only two homologs find each other, but then the third is left floating around on its own, which also prevents normal meiosis.  With 11 sets of three homologs, in a given cell there may be a combination of indivisible triplets and pairs-plus-singles.  In any case, the egg and its supporting cells fail to form, and the ovule remains small and soft and inoffensive enough to be ignored.  The desired watermelon fruit develops in response to the pollen load, but the unwanted seeds never do.

Click to enlarge. Plants that grow from triploid seeds grow normally, but they are sterile. Pollen from a diploid plant is used to stimulate fruit growth, but seeds never develop.

CLICK to enlarge. Plants that grow from triploid seeds grow normally, but they are sterile. Pollen from a diploid plant is used to stimulate fruit growth, but seeds never develop.

How we make triploids (but not giant rats)

Seedless triploids are beautiful in theory, but they don’t occur naturally in watermelons.  How do breeders make a triploid plant from a diploid species? By bringing together a typical haploid (N) gamete from one parent and a diploid (2N) gamete from another.  The most reliable way to produce diploid gametes is to start with a tetraploid parent (4N).  Recall that even-numbered polyploids, unlike triploids, can make functional gametes through meiosis.

Tetraploid plants come about through various mechanisms, both natural and artificial.  Sometimes ploidy is increased through hybridization between species (called “allopolyploidy,” as in bread wheat) and sometimes spontaneously within a species, even within a single plant (“autopolyploidy”). 

The earliest written report of successful chemical induction of autopolyploidy was made in 1937 by Blakeslee and Avery, who were working at the Carnegie Institution at Cold Spring Harbor in New York.  Following up on a tip from a colleague, they found that they could induce chromosome doubling by applying a plant-derived alkaloid called colchicine to either seeds or seedlings.  Some of their experiments charmingly involved an atomizer “purchased in Woolworths for twenty cents.” 

Applying colchicine with an atomizer from Woolworth. Excerpt from figure 5 of Blakeslee and Avery 1937.

Applying colchicine with an atomizer from Woolworth. Excerpt from figure 5 of Blakeslee and Avery 1937.

What started innocently enough at Woolworths ended up splashed across the nation’s newspapers soon after Blakeslee and Avery’s discoveries became public.  The reports were so overblown that the editor of the Journal of Heredity was moved to include a comment at the end of the published article to quell “over-enthusiastic popularizations” resulting from “the tidal wave of wierdly [sic] misleading publicity distributed by the Hearst newspapers” involving giant babies and wheat as tall as a pine tree.  See below for an excerpt of his reassuring note.  Yellow journalism aside, colchicine is toxic.  Like many poisons, though, its properties are medically useful in low doses, and humans still take it for gout and some inflammatory diseases. 

Colchicine works by interfering with mitosis, the kind of cell division that goes on constantly in our bodies, creating (ideally) identical cells for growth, maintenance, and healing.  Before cells divide, they replicate their DNA so that each chromosome contains two copies of all the genetic material.  As in meiosis, the chromosomes are moved to the middle of the cell by microtubules.  In mitosis, however, the chromosomes line up single file and the two copies of the DNA are separated, not the homologs. A diploid cell becomes two diploid cells.

Colchicine-treated cells replicate their DNA as usual, but they are blocked from forming microtubules, so the cells do not divide; they just go on living with the extra copies of their chromosomes.  What Blakeslee and Avery identified was the right amount of the chemical for the right length of time to prevent one round of cell division and induce tetraploidy but allow cells to start dividing again soon after that.  With the correct protocol, plant parts that develop after colchicine treatment – including flowers – should all be tetraploid.  Tetraploid flowers will produce diploid gametes (through meiosis) to be joined with haploid gametes from diploid plants to create triploid offspring.

Images of tetraploid female plant and diploid male plant crossed to make triploid embryos

CLICK to enlarge. Triploid watermelon seeds are made by crossing a tetraploid maternal plant with a diploid paternal plant. The resulting embryos are triploid. The fruits containing the seeds are tetraploid and not harvested for food. The triploid seeds are harvested for sale to growers.

It didn’t take long for Hitoshi Kihara to apply the new and exciting colchicine technique to watermelons, but it took a while to perfect it (Crow, 1994).  One problem, as it turns out, is that tetraploid plants do still have some trouble making gametes, especially when all four copies of their chromosomes find each other and get tangled up.  Breeders therefore select the most fertile tetraploid lines to develop, and over time meiosis becomes more normal.  Once established, tetraploid lines can be maintained and propagated through seeds.  There is no need to use colchicine again except to initiate a new tetraploid variety.  Seed companies then use diploid plants to pollinate tetraploid flowers, which develop into fruit containing seeds with triploid embryos.  We in turn buy those seeds to plant in our gardens as seedless varieties.

Growing triploids

With many very tasty seedless varieties available, why would anyone grow seeded watermelons these days?  One reason is that triploid seeds cost about twice as much as diploid seeds because seed companies need to recoup their investment in developing tetraploid lines and generating seeds annually.  Another consideration is the opportunity cost of growing seedless watermelon varieties.  Alongside the triploid plants, farmers and gardeners have to plant diploid “pollinizer” plants, at a ratio of 2-to-1 triploids.  Doing so ensures that there is enough pollen deposited on the stigmas of the triploid flowers to trigger fruit formation.  Not only do farmers pay more for the triploid seeds, but they have to make room for the diploid vines to grow.  Their extra cost is often passed along to consumers at the market or grocery store, who may be reluctant to pay the higher price.

CLICK to enlarge. From top left, moving clockwise, the stages of creating a seedless watermelon. To make a tetraploid line, colchicine is applied to diploid seedlings. Resulting tetraploid growth is allowed to flower and produce seeds. Those seeds are planted and the best versions are selected for several generations to establish a strong tetraploid line. Every year, seed producers fertilize tetraploid plants with pollen from diploid plants to create triploid seeds. Triploid seeds are sold to growers as seedless varieties. These plants must be pollinated by a diploid plant in order to make the fruit we eat.

CLICK to enlarge. From top left, moving clockwise, the stages of creating a seedless watermelon. To make a tetraploid line, colchicine is applied to diploid seedlings. Resulting tetraploid growth is allowed to flower and produce seeds. Those seeds are planted and the best versions are selected for several generations to establish a strong tetraploid line. Every year, seed producers fertilize tetraploid plants with pollen from diploid plants to create triploid seeds. Triploid seeds are sold to growers as seedless varieties. These plants must be pollinated by a diploid plant in order to make the fruit we eat.

Counting chromosomes and thinking of bees

Through the magic of colchicine, you have the power to tell a diploid from a triploid watermelon in your kitchen.  If it has full-grown shiny black seeds, it’s a diploid.  If it does not, it’s a triploid.  That said, it is possible that in production, a tetraploid flower could be contaminated with pollen from another tetraploid flower and thus produce tetraploid seeds that accidentally get sold as triploids.  However, according to the Cucurbit Breeding Project at North Carolina State University, tetraploids are usually bred to have a grey rind exactly so that any tetraploid seeds sneaking through would be evident to growers as soon as fruits started to develop.

I’ll admit that old fashioned seed-filled watermelons have their charms.  Watermelon seeds are beautiful, especially against the deep red and sweet background of watermelon flesh.  And the zone of seeds conveniently demarcates the heart. On the other hand, I love to contemplate the genetic gymnastics required to produce a seedless watermelon.  If the biology of seedlessness is not enough to capture your own imagination, consider the bees.  One study found that bees have to visit a triploid flower 16 to 24 times before it will produce a good fruit (Walters, 2005).  That’s twice as many visits as a seeded variety requires.  Humans may have escaped a nightmare plague of six-foot-long carnivorous caterpillars, but the bees really are in serious trouble, and we’d better do all we can to keep those bees happy and healthy and abundant.

Excerpt of a note by the editor of the Journal of Heredity, 1937, at the end of the article by Blakeslee and Avery:

“MANY readers of the JOURNAL will be interested, we feel sure, in some expansion and explanation of certain points not touched on in Dr. Blakeslee’s article, which have come up in connection with this most interesting subject. The tidal wave of wierdly misleading publicity distributed by the Hearst newspapers on November 21, has doubtless aroused hopes and fears that have no foundation whatever. Colchicine is not a “growth elixer,” and evidence is lacking as to its effects on animal chromosomes, so that doctors may still make their calls safe from attack by giant rats, and ladies are in no danger of being gobbled up by caterpillars suddenly gone carnivorous.

Colchicine is a narcotic alkaloid, related chemically to morphine and codeine. It is a very potent and very poisonous substance whose immediate effect on growing tissues even in very small concentrations is to produce stunting and distortion. It offers no Magical Royal Road to the production of new varieties of plants and animals. In spite of the cold water which must be thrown on over-enthusiastic popularizations, the effect which colchicine has on doubling the number of chromosomes in the cells makes it unquestionably one of the most important genetic discoveries of the year.”


Agricultural Research Service, USDA. (1971) Production of seedless watermelons.  Technical Bulletin no. 1425.

Blakeslee, A. F., & Avery, A. G. (1937). Methods of inducing doubling of chromosomes in plants by treatment with colchicine. Journal of Heredity, 28(12), 393-411.

Crow, J.F. (1994) Hitoshi Kihara, Japan’s pioneer geneticist. Genetics 137: 891-894.

Gama, R. et al. (2015) Microsatellite markers linked to the locus of the watermelon fruit stripe patternGenetics and Molecular Research 14 (1): 269-276 (2015)

Paris, H. S. (2015). Origin and emergence of the sweet dessert watermelon, Citrullus lanatus. Annals of botany, mcv077.

Science opens the way to make our children giants. San Francisco Examiner (1937, Nov. 21). Media and Microtext Center, Stanford University Libraries.

USDA National Watermelon Report.  Week ending Sept 4, 2015.

Walters, S. A. (2005). Honey bee pollination requirements for triploid watermelon. HortScience, 40(5), 1268-1270.

The new apples: an explosion of crisp pink honey sweet snow white candy crunch

What’s in a name?  An apple with an old fashioned name could taste as sweet, but it might not sell.  The most sought after branded varieties reveal what people look for in an apple: sweet and crunchy and bright white inside.  Do the fruits live up to their names?  Are Honeycrisp apples crunchier than others?  Do Arctics actually stay white?  We zoom in on the cells to find out.

Some of you will remember the era when the Superbowl halftime show repeatedly featured Up With People.  That was around the time when Granny Smiths arrived in our supermarkets and finally gave Americans a third apple, a tart and crunchy alternative to red and golden delicious.  Those were simple days.  Continue reading

Taking advantage of convergent terpene evolution in the kitchen

The Cooks Illustrated recipe masters recently added nutmeg and orange zest to a pepper-crusted steak to replace two flavorful terpenes, pinene and limonene, lost from black pepper when simmered in oil. In doing so they take advantage of convergent evolution of terpenoids, the most diverse group of chemical products produced by plants. Nutmeg and orange zest, though, were hardly their only options.

The terpene swap

Black pepper (Piper nigrum) growing in Cambodia (photo by L. Osnas)

Black pepper growing (photo by L. Osnas)

To develop satisfying crunch, the Cooks Illustrated recipe for pepper-crusted beef tenderloin requires a prodigious quantity of coarsely ground black pepper (Piper nigrum; family Piperaceae). If applied to the meat raw, however, in the recipe authors’ view, this heap of pepper generates an unwelcome amount of spicy heat. To mellow it, the recipe authors recommend simmering the pepper in oil and straining it out of the oil before adding it to the dry rub. The hot oil draws out the alkaloid piperine, which makes black pepper taste hot, from the cracked black pepper fruits (peppercorns).

Nutmeg seed showing brown seed coat folded within the ruminate endosperm

Nutmeg seed

To their dismay, however, the recipe authors discovered that the hot oil also removes flavorful compounds from the cracked pepper, in particular the terpenes pinene and limonene. To rectify this flavor problem, the recipe authors added pinene-rich nutmeg (Myristica fragrans; Myristicaceae) and limonene-rich orange (Citrus x sinensis; Rutaceae) zest to the dry rub, along with the simmered black pepper. In doing so they take advantage of widespread and diverse array of terpenoids in the plant kingdom. Continue reading


The rapunzel plant (Campanula rapunculus; Campanulaceae). Photo from Wikipedia.

The rapunzel plant (Campanula rapunculus; Campanulaceae). Photo from Wikipedia.

I never suspected that I’d learn something about edible botany by indulging my 3-year-old’s princess obsession, but I have. According to the Brothers Grimm, Princess Rapunzel is named after the cultivated  vegetable of the same name, growing in a witch’s garden. The wording of the story suggested to me that the Grimms’ contemporaries would be familiar with the plant as a vegetable, that it wasn’t a fantastical invented thing. Apparently rapunzel was a popular vegetable in the Grimm’s Europe. Continue reading

Apples: the ultimate everyday accessory

Infinity scarves? No. They won’t keep doctors away. Apples are the ultimate everyday accessory (fruit). Katherine explains where the star in the apple comes from. Could it be due to a random doubling of chromosomes? We also give readers the chance to test their apple knowledge with a video quiz.

Although apples are not particularly American – nor is apple pie – they color our landscape from New York City to Washington State, all thanks to Johnny Appleseed. Or so goes the legend. Everyone already knows a lot about apples, and for those wanting more, there are many engaging and beautifully written stories of their cultural history, diversity, and uses. See the reference list below for some good ones. There is no way I could cover the same ground, so instead I’ll keep this post short and sweet (or crisp and tart) by focusing on apple fruit structure and some interesting new studies that shed light on it.
Of course if you do want to learn more about apple history but have only 5 minutes, or if you want to test your current knowledge, take our video quiz! It’s at the bottom of this page. Continue reading

Alliums, Brimstone Tart, and the raison d’etre of spices

If it smells like onion or garlic, it’s in the genus Allium, and it smells that way because of an ancient arms raceThose alliaceous aromas have a lot of sulfur in them, like their counterparts in the crucifers. You can combine them into a Brimstone Tart, if you can get past the tears.

The alliums


garlic curing

The genus Allium is one of the largest genera on the planet, boasting (probably) over 800 species (Friesen et al. 2006, Hirschegger et al. 2009, Mashayehki and Columbus 2014), with most species clustered around central Asia or western North America. Like all of the very speciose genera, Allium includes tremendous variation and internal evolutionary diversification within the genus, and 15 monophyletic (derived from a single common ancestor) subgenera within Allium are currently recognized (Friesen et al. 2006). Only a few have commonly cultivated (or wildharvested by me) species, however, shown on the phylogeny below. Continue reading

The Extreme Monocots

Coconut palms grow some of the biggest seeds on the planet (coconuts), and the tiny black specks in very good real vanilla ice cream are clumps of some of the smallest, seeds from the fruit of the vanilla orchid (the vanilla “bean”). Both palms and orchids are in the large clade of plants called monocots. About a sixth of flowering plant species are monocots, and among them are several noteworthy botanical record-holders and important food plants, all subject to biological factors pushing the size of their seeds to the extremes. Continue reading

Walnut nostalgia

Walnuts may not seem like summer fruits, but they are – as long as you have the right recipe.  Katherine takes you to the heart of French walnut country for green walnut season.

France 1154 Eng newAnnotation fullRes 2

Public domain, via wikimedia commons

English walnuts do not come from England. The English walnut came to American shores from England, but the English got them from the French. The (now) French adopted walnut cultivation from the Romans two millennia ago, back when they were still citizens of Gallia Aquitania. Some people call this common walnut species “Persian walnut,” a slightly better name, as it does seem to have evolved originally somewhere east of the Mediterranean. But the most accurate name for the common walnut is Juglans regia, which means something like “Jove’s kingly nuts.” I think of them as queenly nuts, in honor of Eleanor of Aquitaine, because if any queen had nuts, she certainly did. During her lifetime the Aquitaine region of France became a major exporter of walnuts and walnut oil to northern Europe, and it remains so more than 800 years later. Continue reading

An apple for the teacher

With her fellow educators in mind, Katherine tells a story of virtual botany in the dining hall and letting students be teachers.

When we botanists in the kitchen are quiet for a little while, it usually means we are focusing all of our attention on our day jobs.  Like a garden, the academic calendar has a rhythm that cannot be ignored, and from April through June, I pour most of my time and creative energy into my small seminar class, where we dig into the evolutionary and ecological connections between humans and plants across many time scales and topics. It’s a fun class and the debate is usually lively, but because the journal articles we discuss are often dense and technical, I sometimes worry that we are squelching some opportunities for joy. Continue reading

A biologist eating for two

This is a bit tangential to our usual fare, but I think it’s fun, and you may as well. A friend of mine, Cara Bertron, edits the creative and delightful quarterly compendium Pocket Guide. I submitted this image, entitled “A biologist eating for two,” for the current issue, which is themed “secret recipes.” It’s a cladogram of the phylogenetic relationships among all the (multicellular) organisms I (knowingly) ate when I was pregnant with my now two-year-old daughter. Continue reading