Brussels sprouts and cauliflowers look nothing alike. Despite this, botanically, they are the same species: Brassica oleracea. And their ancestor grows on the rocky coasts of England and France: the wild cabbage. However, the types of cabbage we eat today did not originate from England or France at all, but from 4,000 kilometres away. How is this possible?
The answer is not just interesting for historians, but for agriculturalists too, who, due to climate change, are working on creating sustainable and resilient crops that are better able to withstand drought and heat.
A lot of variation
Guusje Bonnema, Plant Breeding researcher at Wageningen University & Research, can be rightly called a ‘Brassica specialist’. She has been working with cabbages for more than 20 years. “Their diversity is fascinating. With tomatoes, there are big ones and small ones, but you always eat the fruit. With cabbages, you can eat the leaves (kale, head cabbage), the stalks (kohlrabi), the axillary buds (Brussels sprouts) or the flowers (cauliflower, broccoli). Earlier, we researched how this huge variation could have come about. But that still did not answer the question of where it happened.”
In an article in the scientific journal Horticulture Research , PhD student Chengcheng Cai and Bonnema, together with colleagues from Wageningen University & Research, describe their search. “It was only possible because we managed to collect an unprecedented number of accessions from all different Brassica oleracea vegetables; modern hybrid varieties and old land races from gene banks from all over the world,” says Bonnema.
It took a lot of effort to gather all of the varieties, but it turned out to be a gold mine. The researchers made DNA fingerprints (a kind of barcode) and were able to use markers to estimate the relationships between the different varieties.
There were two steps to domestication (turning a wild plant into a cultivated one). “Greek and Roman writers mentioned these Brassica vegetables as far back as 400 BC. They described very diverse kale like varieties, probably like curly kale and palm cabbage but also described already very large cabbages,” she says.
After the first step in domestication in Western Europe, the second took place in the Middle East. The genetic research shows that these old, kale-like plants from Western Europe played a role. But how did they get there?
Bonnema: “It is highly likely that the tin trade played a large part. Tin was mined in Cornwall and Galicia and brought to the Middle East by ship around 2500 BC. The boatmen took vegetables and seeds with them for the journey.”
This is how early kale varieties ended up in the Middle East. It is likely that both head cabbages and cauliflowers were selected from those first leafy kale crops. Spontaneous crossbreeding with other wild local Brassica varieties may have played a role too. These old varieties developed into modern cabbages. “So our current cabbages all come from there,” she says.
Why is it important to know all this? “We see that genetic variation in modern hybrid varieties is not that substantial, while the variation in the gene banks is much greater. If a plant breeder is looking for new crop characteristics, it is best to start here. In Turkey, Syria and Lebanon - where our cabbages originated from - many land varieties with a great deal of diversity still exist. These are countries with warmer and drier climates. To respond to climate change, we need varieties that can better withstand hot summers in the field. Now you can look more specifically at where to get these characteristics from,” says Bonnema.
The strange case of cauliflower
There have been numerous interesting insights during the research. For example, the cauliflower turned out to be a strange case: it is an inflorescence that no longer blossoms, but continues to grow.
“A whole series of mutations were necessary for this particular form. It was kind of a genetic bottleneck the species passed through. As a result, all varieties of cauliflowers are very similar; there is very little genetic variation. But the genetic distance to other Brassica oleracea vegetables is still very large. Every other vegetable (cabbage, kohlrabi etc) has more in common with the wild cabbage than with the cauliflower,” she says.