The “mule” of summer

Ada Park Playground June 05, 2010

Photo by Steven Depolo via Flickr (CC BY 2.0)

It’s the time of year when mules start showing up at grocery stores, farmers’ markets, and produce stands. Not actual mules, nor the refreshing Moscow Mule popularized by Oprah Winfrey*. No, I’m talking about seedless watermelons. Ever wonder how a plant with no seeds came to be? Or where those hard black seeds went? Well, the seeds got stopped before they could start, thanks to a little chemical intervention and some careful breeding.

As this “Ask a Scientist!” post from Cornell University explains,

Producing a seedless watermelon involves three steps. First, a plant is treated with colchicine, a substance that allows chromosomes to duplicate, but prevents the copies from being distributed properly to dividing cells. As a result, a plant with four sets of chromosomes is created, a “tetraploid.” In the second step, a tetraploid plant is crossed with a [regular] diploid to produce offspring that are … triploid, with three sets. They get half the number of chromosomes from each parent. Finally, the triploid seeds are grown into plants.

The triploid abnormality means that the watermelons can’t reproduce, so their seeds never mature and develop the hard black exterior like a diploid watermelon. As NPR’s Andrea Seabrook says in this piece, “it’s the watermelon version of a mule…. It can’t reproduce but it exists.”

For all the details, including why you still need diploid watermelon plants around for seedless triploids to bear fruit, check out the NPR story (audio or transcript) or the Cornell post.

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* If you haven’t yet learned the best way to squeeze a lime when there’s no juicer on hand, check out this video of Oprah in Yosemite making Moscow Mules with Gayle for their campsite neighbors. Honestly, J swears by her technique! (You can skip to minute mark 1:20 if you don’t want to first watch her comically try to figure out how to open a bottle of ginger beer.)

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