Researchers have found an answer to a centuries-old floral mystery, using a mathematical model to explain how striped tulips get their distinctive pattern.
Often referred to as "broken tulips," the striped variations of the popular flower were coveted in the 17th century for their beautiful markings. It's been known since 1928 that the pattern is caused by a viral infection known as the tulip breaking virus, but exactly how the signature stripes are formed remained an unsolved mystery until now.
In a study published in Nature Communications Biology and led by University of Alberta mathematics professor Thomas Hillen, researchers found out the tulip breaking virus inhibits the production of anthocyanins, the pigments that give tulips their vibrant colours.
"The plant wants to produce a pigment and the virus wants to produce a virus. And if the virus is very strong, it takes over the machinery completely and there's no more resources to produce any colouration," says Hillen.
The striped pattern arises because the areas of the tulip petals that are most infected become almost colourless, while the areas that have less extensive infection keep their colour.
To simulate the interaction between the virus, pigment production and the cellular resources within the plant, Hillen and his collaborators designed a mathematical model that incorporates two key mechanisms, the substrate-activator mechanism and Wolpert's positional information mechanism.
The substrate-activator mechanism is similar to a well-known mathematical concept called the Turing instability, which is responsible for other patterns in nature such as stripes on zebras or spots on leopards. This mechanism helps explain how the virus moves at different rates within the tulip, creating areas with more or less infection, and consequently more or less pigmentation.
Hillen likens it to a mall where some stores are having big Black Friday sales and others aren't. The products for sale are the substrate, the shoppers are the activators, and the mechanism describes their behaviour as they rush to the stores with the best sales, congregating in clumps there while the other stores remain empty. In tulip terms, the substrate is the cellular resources the plant uses to make the pigments, and the activator is the virus.
Wolpert's positional information mechanism was initially conceived to explain the chemical signalling taking place within a developing embryo. As organs are developing, Hillen explains, the cells need information on where to go. "Let's say there is a heart cell — where does it need to go to meet with all the other heart cells to settle and form a healthy heart? The mechanism relates to the chemical signalling, where the cells stop if they receive the right signal and settle to, say, form the heart."
Working together in the model, the activator-substrate mechanism kicks things off by creating instability in the tulip that causes the virus to spread unevenly, while Wolpert's mechanism signals how much pigment is needed in each part of the petal. The result? Eye-catching flowers that commanded an eye-watering price during "Tulipmania."
