Jumping Genes Determine What A Cabbage Looks Like

A cauliflower and a pointed cabbage are genetically more different from each other than a human and a chimpanzee. Yet they belong to the same species. Wageningen and Chinese researchers mapped the enormous genetic variation of coal. More knowledge makes it possible to breed in a more targeted manner, for example for resistance to diseases or better nutritional value. Scientific journal Nature Genetics pays extensive attention to this special research.

Cabbage vegetables are an important part of our menu. The variation is very large. Yet cauliflower, broccoli, Brussels sprouts, red cabbage, white cabbage, kohlrabi and pointed cabbage are all variations of the same species, Brassica oleracea . How can such a great diversity of shapes within one species be explained?

And the variation goes further than the outer form. The ingredients (such as vitamins and antioxidants) or the resistance to drought, cold and diseases also differ considerably.

There was already quite a lot of insight into the genome (the totality of genetic information) of different types of cabbage, but it was still unclear how this variation in the genome is related to the diversity in vegetables.

International cooperation
Researchers from Wageningen University & Research and the Chinese Academy of Agricultural Sciences Beijing have therefore joined forces. They determined the DNA sequence of 23 different cabbage crops and analyzed them together with existing data. “We have constructed a so-called pan-genome: that is the overview of all the different genes within the cabbage crops. We then looked at which of those genes occur in each cabbage crop, which occur in the majority and which are unique to a particular crop," says Guusje Bonnema, breeding researcher at Wageningen University & Research. In recent years, she has worked intensively on research from the Netherlands together with fellow researcher ChengCheng Cai. This gave very surprising results: only one third of the genes are present in all cabbage crops. And half of all those genes only occur in some of the crops and are absent in the rest.

“Now Brassica has a lot of genes. Cauliflower, for example, has about 60,000 (humans have 20,000). This is because the genome tripled fifteen million years ago, while the original genome was already sufficient for the plant to function properly. We want to understand where the origin of the variation is and then you can experiment with it to get better varieties,” says Bonnema.

Jumping genes
A remarkable detail is that more than half of the genome consists of transposons. These are small pieces of DNA that 'jump around' in the genome. So they can occur in all kinds of different places. In Dutch they are called 'jumping genes'. They have a bad reputation among people because they are the cause of diseases such as hemophilia. This is different with plants, where they are an important source of natural variation.

“We have now discovered that these transposons often regulate the activity of nearby genes. They increase or decrease their activity. Previously, when asking the question 'what makes cauliflower a cauliflower', we looked for the determining genes for that specific shape: a compact flower structure that does not grow out. But now we know that you not only need to have an overview of the genes, but certainly also how they operate. So the transposons. They are the on/off switches and the dimmers of the genes they are close to,” she says.

Breakthrough in insight
The fact that a pan-genome is now available makes it possible to categorize the transposons and other structural variations. “They turn the genes up or down. And not just the genes that determine the specific shapes of the different coals. Also the genes that determine resistance or nutritional value. And resilience to climate conditions. Cauliflower, for example, is very sensitive to temperature. If you understand how the process works, you can manage it more easily and arrive at varieties that are less sensitive to temperature,” says Bonnema.

“This is truly a breakthrough in insights. We always focused on the variations within the genes. Now we know that it is much more subtle. Regulating the activity of the genes has a huge influence.”