For all of October, we at Nature Genetics have been admiring the lovely cabbages on our cover. The image, created by photographer Keyong Chang, was contributed by the authors of the study on page 1218 of the issue.
But what is the story behind these pretty green cabbages?
Xiaowu Wang, corresponding author of the study, gave us a behind-the-scenes look at the process that led to the picture on our cover.
The image conveys the main idea of the study, namely that Brassica oleracea (cabbage, left) and Brassica rapa (Chinese cabbage, right) have taken similar evolutionary paths to arrive at their similar, but distinct, appearances. During domestication, farmers selected for cabbages of both species to have the large, leafy heads for which they are known. As shown in the study, the farmers were unknowingly selecting for orthologous genes in these two species.
Brassica genomes are the product of an ancient whole genome triplication event, meaning that they contain 3 copies of their genome and each copy has diverged from the others over evolutionary time. When the authors looked closer, they saw that genes selected for during domestication often shared the same ancestral gene across the different genome copies as well, demonstrating parallel evolution both within and between species.
Although the cover shows images of 2 cabbages, B. rapa and B. oleracea also come in many different forms, including turnips, broccoli, kale and pak choi.
To illustrate the idea of convergent evolution between species, the authors set to work choosing the ideal cabbages.
Step 1: head to the market!
Step 2: find the perfect cabbage.
“To take the photo, we cut over 100 kilos of Chinese cabbages and cabbages into hundreds of pieces,” said Xiaowu Wang. “We tried many different thicknesses and different ways to take the photo.”
“Actually, we didn’t expect that the way we took the photo as shown below would produce the effect of ‘jade’!”
We asked whether the photographer, Keyong Chang, was also a scientist working on Brassica species. “Keyong is not a scientist,” said Dr. Wang, “but a pure photographer having an interest in science. He is very good at taking photos of industry products. Before taking the photo, we discussed the ideas together how we should show the concepts of Heading and Convergent.”
The original concept for the cover was as shown below (blue background). The editorial staff together with our art director decided to go with the image you see on the cover, for aesthetic reasons, but Dr. Wang explains the idea behind the initial concept: “We cut the Chinese cabbage/cabbage to show better the trait of heading. We then placed the Chinese cabbage/cabbage in a symmetrical way to imply the convergent evolution. The Chinese cabbage and cabbage rooting together means they share the same ancestor.”
In addition to getting the inside scoop on our cover image, we asked Dr. Wang a few questions about the Nature Genetics paper as well.
How did you become interested in studying Brassica species?
I started working on Brassica species because my supervisor was working on them, but I decided to continue my work on them because of their fascinating morphological polymorphism.
What was the most challenging aspect of the study?
The most challenging aspect of the study was to collect a large set of germplasm and purify them. We started collecting and purifying Brassica germplasm by either generating double-haploids through microspore culture or selfing starting in 2000. We have now over 1,000 accessions which we have sequenced. We hope to use the data to decipher not only heading, but also the shape of heading and many other traits.
What was the most surprising result for you?
The surprising result came from the sub-genome analysis. When we compared the selection signals for the trait of heading of B. rapa and B. oleracea at the genome level, we noticed that a few selected genes were paralogous genes within a species and were orthologous genes between the two species. This brought us to believe that behind the convergent traits of the two crops, there might be parallel selection of orthologous genes at the sub-genomes level. But, indeed, we didn’t expect that the number of signals were doubled when we looked at the sub-genome level compared to those at the genome level.
How do you see others using your results in the future?
We provided for the first time evidence that sub-genome parallel selection is associated with morphotype diversification and convergent crop domestication in plants. We also developed tools to analyze sub-genome selection signals. Many crops are either polyploid or ancient polyploids. We believe that the conclusions and tools can be extended to many other crops. Furthermore, we generated a large set of SNPs which should be very useful for Brassica breeders.
For even more Brassica science, see the study by Jinghua Zhang et al. on page 1225 “The genome sequence of allopolyploid Brassica juncea and analysis of differential homoeolog gene expression influencing selection.”