Turnips, black mustard, and cabbage together form a triangle of multi-chromosome hybrids: it’s where we get Indian mustard, Ethiopian mustard, and canola.
I’ve written before about the weird fact that kale, broccoli, cabbage, Brussels sprouts, and cauliflower are all the same species. Well, it turns out that the brassica genus is even more flexible than that.
Jumping back to 1935, a Japanese-Korean botanist named Woo Jang-choon noticed an interesting pattern in the number of chromosome pairs in some common brassica species, and he developed a theory from that pattern called – quite poetically, I might add – the Triangle of U. Here’s how it works.
Turnips (brassica rapa) have ten chromosome pairs. Black mustard (brassica nigra) has eight. Cabbages – and all the other cabbage cultivars – have nine chromosome pairs. None of this is unusual; different species commonly have different numbers of chromosomes all the time. But Woo also noted the counts of three other species: Indian mustard, Ethiopian mustard, and canola plants.
Indian mustard has eighteen pairs of chromosomes, of two different types. What do I mean by that Indian mustard, like many other plant species, is polyploid. In contrast to species with just one set of chromosomes (like humans!) it contains two different sets of chromosomes. Ethiopian mustard is the same – it has seventeen pairs. Canola has nineteen pairs. The pattern that Woo noticed was this: these polyploid species contained chromosomes from two other species. They were ancient hybrids of turnip, cabbage, and black mustard.
Consider this: turnip (ten pairs) and cabbage (nine pairs) together make canola (nineteen pairs). Cabbage (nine pairs) plus black mustard (eight pairs) makes Ethiopian mustard (seventeen pairs). Black mustard (eight) plus turnip (ten) makes Indian mustard (eighteen). The three base species combined together to make the three hybrid species.
The relationship between these six species is summarised well in this picture from Wikipedia:
Polyploid plants are not rare, by the way: both cotton and peanuts have multiple chromosome pairs in their genetic makeup. Woo came to his conclusion based on his own crossbreeding experiments, but genetic analysis has since confirmed the truth of the Triangle of U.
[Thanks to Eub for suggesting this topic.]