Brilliant Brassica

Photo by Nadine Primeau on Unsplash

The incredible variation of one plant:

Did you know that broccoli and cauliflower are the same species? No? What if I told you that list also includes kale, cabbage, kohlrabi, romanesco, and many more! These vegetables are cultivars of the species Brassica oleracea, which, through selective breeding and a few genetic hurdles, displays the amazing variety of features we see today. Here, I’ll explain what we know about the evolutionary history of B. oleracea, as well as outlining current study to understand the genetic forces behind its extraordinary variation.

If you were to see B. oleracea growing in the wild, perhaps on a limestone cliff, you could be forgiven for not recognising it. Uncultivated, it’s a wild cabbage with high salt and lime tolerance, not easily distinguished from any other leafy green plant. By contrast, modern cultivated B. oleracea presents a variety of unique forms. Selective breeding has given plants with high density of flowering buds (broccoli), oversized flowering buds (brussels sprout, red cabbage, savoy cabbage), swollen stems (kohlrabi), swollen inflorescence (cauliflower), and even combinations of these features (broccoflower). In addition to this range of striking forms there are a number of cultivars like kale and collard greens which still resemble the ancestral variety but have larger leaves.

B. oleracea is just one species of many of the genus Brassica, a member of the cabbage and mustard family Brassicaceae. The members of Brassica are thought to have linked evolutionary relationships. Ancestrally, the genomes of three species of Brassica (B. nigra with a haploid (n) number of chromosomes n=8, B. oleracea: n=9, and B. rapa: n=10), combined in pairs via interspecific breeding to give three new Brassica species, today important oilseed crops (B. carinata: n=17, B. juncea: n=18, and B. napus: n=19). This is formally termed the “Triangle of U” theory by Woo Jang-choon, first published in 1935. Due to the combination of genomes, individuals of the three new species received four sets of chromosomes. These species are described as allotetraploid (4n): “allo” = from several species, and “tetraploid” = having four sets of genetic material, two from each of their diploid (2n) ancestors. Even more striking is that these plants can themselves further interbreed to give plants with six sets of genetic material (allohexaploid, 6n).

Knowing the history of the genus Brassica, you may be wondering why only B. oleracea shows this level of phenotypic (relating to appearance) variation. Brassica genomes are known to have undergone historical genome duplication and triplication events, which increased the sizes of their genomes, allowing for increased variation by natural selection for all species of the genus. Furthermore, according to a 2014 Nature Communications study by Liu et al., when the genomes of B. oleracea and B. rapa were compared, numerous genetic differences were identified. These included rearrangements of chromosomal regions, loss of genes in duplicated and triplicated regions, uneven quantities of transposable elements (which jump around the genome and are known to be strong contributors of variation), and different retention of particular biochemical pathways in different cell types and tissues. The combination of these genetic factors is thought to have permitted large variation in B. oleracea as a result of human-mediated selective breeding.

The selective breeding of B. oleracea before modern history is unknown thus far – the earliest records from ancient Greece and Rome suggest it already exhibited the sort of variation we see today. It’s clear that it has been an important food source for thousands of years, but now it continues to be important in an additional aspect, as agricultural scientists search for insights into drivers of plant phenotypes. Due to the close relationships of B. oleracea cultivars, any differences in phenotype, for example cold tolerance or pest resistance, must lie in the small percentage of variable genetic material. This was the focus of a 2016 Nature Communications paper by Golicz et al (see further reading), which found that 18.7% of the B. oleracea pangenome (the gene set of all the cultivars) is made up of variable genes. It is these genes which have key implications for breeding of new crops and may play a key role in providing robust and resilient future crop varieties to respond to changing climates and ecosystems.

A final and slightly unrelated factoid about Brassica which stands out as interesting is the way that one gene, TAS2R38, relates to taste. This gene encodes a protein that acts as a taste receptor and is bound by several compounds such as PROP and phenylthiocarbamide (PTC). Brassica vegetables contain a compound similar to PROP, and thus, depending on the alleles (versions) of the TAS2R38 gene that one has, we perceive these vegetables differently; some individuals find them highly bitter, some just a little, and others not at all. Due to the simplicity of this, the other compound, PTC, is often used to practically introduce younger students to genetics, allowing them to infer their TAS2R38 alleles by tasting a PTC-soaked piece of paper.

Further Reading:

  • Hank Green from SciShow, one of my favourite YouTube channels, presents an introductory history of Brassica oleracea here in an easily digestible visual format.
  • Kew also has a fact file dedicated to the species including information about the wide number of sub-groups, as well as links to a wide range of additional sources. I found the distribution map interesting; the species has been introduced globally from just a few native regions.
  • Finally, this open access Nature Communications paper by Golicz et al. details further the research efforts into describing the pangenome of Brassica oleracea. It makes for more challenging reading, but I found it to to beautifully visualise the historical relationships between the cultivars, as well as succinctly discuss the importance of variable genes for breeding new crop varieties.

Written by Joshua Williams for the UCL Genetics Society:

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