Who killed the bullseye?
Flowers are fascinating part of plants. Where the rest of the plant is dressed stylish green, flowers are more abundant in their style choice. With lots of colours and guises. Raising the question: how do they manage this?
In the recent publication ‘The genetic basis of replicated bullseye pattern reduction across the Hibiscus trionum complex’ this is precisely what the researchers led by Edwige Moyroud try to find out for the bullseye pattern in the flower-of-an-hour flowers.
Those flowers like the name suggests are only flowering for a few hours. But what is making this group of flowers so interesting is that their bullseyes are of different sizes and colours. To use this the group of Moyroud first analysed how the different flower-of-an-hour flowers are related to each other. They did this by reading the DNA of 11 related species. Subsequently they compared each of the 11 species with each other. This resulted in a phylogenetic tree visualizing the relatedness.
Subsequently the researchers studied the flowers of the 11 species. The analysed the pigments in the bullseye, but also the form and texture of the cells. Together with the information of the phylogenetic tree the researchers concluded that pigmentation was regulated separately from cell shape and texture.
Smaller bullseye
In addition, the researchers noticed that two extremes in bullseye pattern were closely related. In Hibiscus trionum CUBG the bullseye took up 15% of the petal space, while in H. richardsonii the bullseye covered only 3% of the petal. Resulting in the question: which gene or genes are responsible for the small bullseye in H. richardsonii?
The first step that the researchers took to solve this was analysing the effect of crossing those two species. The prodigy appeared to look similar to H. trionum, but not completely. The next generation segeregated in three groups, in the first group the flowers looked like those of H. trionum, in the second group they looked like those of H. richardsonii, and in the third group looked just like the first generation to be somewhat in between. This, so say the researchers, suggests that a single gene is responsible for the bullseye size difference between H. trionum and H. richardsonii.
Subsequently the researchers looked at the first suspects: the pigment biosynthesis genes. Early in the petal development the gene for pigmentation was active in H. trionum, but only at the petal base. But now the researchers found something strange, when the researchers turned on the gene for pigmentation in the whole plant the plant did not get dark flowers. There was still a bullseye visible, but the rest of the petals were now pink-purple instead of white. But what really surprised the researchers that in half of the plants in which the pigmentation gene should be turned on, it wasn’t. Those flowers did not have any pigmentation at all. This, so explain the researchers, is showing that there is a regulating factor that turns of the gene.
New suspects
To find this regulating factor the researchers analysed which gen regulators were active in the different petal regio’s. Three got their attention. Two of those, BERRY1 and 2, were exclusively active in the petal base, while in contrast the third, CREAM1, was only active in the petal tip.
To figure out the effect of BERRY1 and CREAM1, the researchers developed plants in which BERRY1 or CREAM1 were always active. The flowers with the always active BERRY1 had pink petals instead of white, but with a just as noticeable bullseye. In plants with extra CREAM1 in contrasts, the bullseye was a few shades lighter but not absent.
Subsequently the researchers studied the effect of BERRY1 and CREAM1 on the pigment biosynthesis genes. Looking at how strongly the genes were active with extra BERRY1 or CREAM1. In petal tips of plants with extra BERRY1 more pigmentation genes were active. While in the petal base of plants with extra CREAM1 those genes were less active than normally. This, the researchers say, suggests that BERRY1 regulates the pigment biosynthesis genes, while CREAM1 (indirectly) inhibits those.
Lastly the researchers analysed if BERRY1 or CREAM1 were responsible for the smaller bullseye in H. richardsonii. For this they studied the difference in gene sequence between the genes from H. trionum and H. richardsonii. The gene for CREAM1 resulted in both species in the same protein. For BERRY1 however, the researchers found differences. One of those was responsible for a shorter inactive protein in H. richardsonii.
The culprit
To prove that BERRY1 was indeed responsible for the smaller bullseye the researchers determined the BERRY1 variant the prodigy of the crosses between H. trionum and H. richardsonii. Plants with flowers that look like those of H. trionum had two versions of the H. trionum BERRY1 variant. While plants with flowers that looked like those of H. richardsonii had two versions of the H. richardsonii BERRY1 variant. And the hybrid? I hear you think, those had one of each.
Interestingly this study once again proves that the most variation between flowers are due to loss of fuction. Remember those 11 flower-of-an-hour variants? In between those there were also five closely related variants with either red or yellow bullseyes. The group of Moyroud analysed BERRY1 in those variants as well. It turned out that variants with a red bullseye had a working BERRY1 variant, while the BERRY1 variant of those with a yellow bullseye was turned off. BERRY1 therefore regulates the size of the bullseye.
Literature
Yeo, M.T.S., Fairnie, A.L.M., Travaglia, V., Walker, J.F., Riglet, L., Zeyrek, S. and Moyroud, E. (2025), The genetic basis of replicated bullseye pattern reduction across the Hibiscus trionum complex. New Phytol, 247: 863-883. https://doi.org/10.1111/nph.70168
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