This year my husband planted a little patch of glass gem corn (apparently a type of “flint” corn) in our vege garden. We harvested the cobs today, and you can see where the name comes from.
I noticed that you could get a good idea of what the main colour of each cob would be, even before freeing it from the enveloping husks. The few cobs with pale, pastel kernels all had pale silks¹ (the long fine hairs protruding from the tip of a cob, each of which connects the sticky pollen-collecting stigma to the ovary of the female flower) while dark brown/pink silks indicated the kernels would mostly be dark. We saw a similar thing in our “Ruby” popcorn, which had beautiful pink silk.
As the images show, individual kernels show a wide range of colours, & I wondered about the history of this variety of corn, and the genetics that generates these random patterns.
As you might expect, Wikipedia has an overview of the history of glass gem corn. It was first developed in the 1980s by Oklahoma farmer Carl Barnes, by selectively breeding from several heritage varieties of corn that were originally used by First Nations tribes. However, the corn really took off as a home garden plant after an image went viral on the internet in 2012!
But what underlies those gorgeous mixes of coloured kernels? There are quite a few publications relating to kernel colour: apparently there’s some interest in using the natural colorants from the kernel coats to replace synthetic dyes in foodstuffs, so a knowledge of the genetic underpinnings of corn pigmentation would be useful. But obviously the most useful corn varieties for that purpose would be those of uniform kernel colour, which would rule out the rainbows of glass gem corn.
I wondered if we were looking at the result of transposons – mobile genetic elements – the discovery of which resulted in a Nobel Prize for biologist Barbara McClintock. She found that the action of transposons carrying genes that coded for kernel colour resulted in cobs with a mix of kernels that were solid purple (or brown), solid white (or yellow), and pale & streaked or spotted with dark pigment. You can find images of some of McClintock’s original cobs here; once dried, in the right conditions they’ve lasted for ages. But, does that by itself explain the wider range of colours seen in many of the cobs we harvested this morning?
Each kernel on a cob is the result of a separate pollination event & subsequent fertilisation. As Jerry Coyne points out, all the ears (cobs) of any given plant will have the same female genotype (since the plant grew from a single seed). Corn is wind-pollinated and pollen from one of any number of male flowers² could pollinate a given egg. Most of the cobs from our garden tend towards a main ‘background’ colour: usually dark blue or green, or red, although some have a pale cream theme. Coyne suggests that that may reflect a female genotype carrying “red” or “purple” genes, or perhaps limited pollination …, the dominance relationships of some color alleles, or all of the above. On top of that they have kernels of other, contrasting colours – this is particularly obvious in the pale cobs, which have kernels that are yellow, red or pink, lilac, green, and dark blue.
Unfortunately I couldn’t find any other sources on-line that told me much more about the genetics of kernel colour in glass gem corn. We might try a bit of selective breeding in our own garden next year, to see if things become clearer! If any readers know more about this, please share what you know in the comments section.
¹ Before I wrote this post I had absolutely no idea of just how many webpages out there claim health benefits from consuming corn silk!
² If you’ve had a close look at sweetcorn plants, you may have noticed that they have male flowers at the tip of the plant while the female ‘flowers’ are found along the developing cob, enclosed by leaves.