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Mirroring flowers, from bees to genes

If asked to draw a flower, most people will draw a simple flower with a round centre and a few petals, but bilateral symmetric flower would be rare. The so-called zygomorphic flowers can be found in a lot of different plant groups, but are still understudied. Botanists and evolutionary biologists wanted to figure out what might be the reason plants have evolved to grow mirroring flowers and what genes might play a role in this process. This is a study of botanical evolution and development at its finest.

In extant plants we find zygomorphic flowers predominantly in the families of the legumes (Fabaceae), mints and allies (Lamiaceae) and the most stunning examples in the orchids (Orchidaceae). Opinions on why these plants have evolved to have such interesting flowers might differ from scientist to scientist, so let’s focus on the most accepted idea. First of all it’s worth mentioning that the first flowering plants in prehistoric times were actinomorphic (so radial symmetric), so all zygomorphy is evolved from this form. It is also pretty much taken for granted that flowers evolved together with insect pollinators.

A possible explanation of the first case of zygomorphy can be traced back to an ancestral plant that had, like many plants we know now, horizontal branches. The flowers of these plants would have to be subjected to gravity, bending the stamens (the male reproductive organs) and style (the female productive organ) slightly downwards. While the organs were not easily accessible to bee pollinators, the flowers had to come up with a different kind of way to attract pollinators. A landing platform, or botanically called a “lip”, was formed in the plants. In addition to this mechanical evolution between pollinators and plants, bees are apparently more attracted to bilateral symmetric objects than to radial symmetric objects.

In the Fabaceae and Lamiaceae, the constant co-evolution with bees has lead to a specific type of zygomorphic flower. The legumes have developed a flower with five petals, of which two make up the lower lip, designed for the bees to land on. The stamen and style have evolved as well: they make up some sort of spring-loaded pollination-system that only heavy bees can trigger. The Lamiaceae have developed a similar system, but with the reproductive organs on the upper side. Orchids are more famous for their pollinator-specific flowers, think for example of the Darwin orchid, only pollinated by a specific hawk-moth with an extremely long proboscis.

The genes for zygomorphic flowers were studied in the common snapdragon - Antirrhinum majus. They found out that there are a few gene families operating the flower symmetry, especially in the top and bottom half of the flower. By mutations, Evo-Devo scientists could get plants in which the genes that code for the upper half didn’t work. What they saw was that the flowers grew radial symmetric flowers instead of their original mirrored flowers. In other plants of the same family this system is similar, but in other families the gene family differs quite much. In completely different groups like the orchids we still don’t know what happens genetically. As for now, we are still looking for the mystery genes of flower symmetry.

-Werner

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