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.
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
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.
#160 of 200 images that have made the IYL Showcase shortlist. We’ll be posting 2 images/day in the run up to the London Showcase (October 23rd) when the final 20 images will be announced and presented as chosen by the showcase jury..