Some Flowers Evolved to be ‘Pretty in Pink’

Scientists uncovered a new molecular mechanism by which pink flowers evolved

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Take a relaxing walk through nature and you cannot miss the beautiful diversity of flower colors. Have you ever stopped and wondered what has led to this diversity in color? How and why did different colored flowers evolve? Scientists have been pondering these questions and are diligently working to uncover the molecular basis for how evolution occurs.

Some scientists study the evolution of flower color by using related plant species from the genus Mimulus (or monkeyflowers) as a model system. This genus works well because it contains many species that come in a variety of colors including Mimulus lewisii and Mimulus parishii, which are pink, and Mimulus cardinalis, which is red. Early research with M. lewisii and M. cardinalis helped develop one of the current theories for why flowers evolved different colors – to attract pollinators.

A flower’s color (and shape and odor) can have a major impact on which type of pollinator it attracts. For example, a study in 1999 showed that the pink M. lewisii attracts bees as its pollinator because bees prefer larger flowers with lighter coloration; whereas, the red M. cardinalis attracts hummingbirds as its pollinator because hummingbirds prefer nectar-rich flowers with rich coloration. Further support of pollinators being attracted by color came from a field study in 2003, where scientists swapped a color related gene between M. lewisii and M. cardinalis. This gene swap caused M. cardinalis to be lighter in color and attract more bees, whereas M. lewisii became more yellow-orange and attracted more hummingbirds.

The reason different pollinators prefer different colored flowers is most likely due to their different perceptions of color. For example, different species of insects have varying sensitivity to different wavelengths of light, which means not all insects see the same colors (or color at all). So it seems attracting diverse pollinators with different sensory systems is driving the development of variation in flower color evolution.

More recent research using the Mimulus genus has been tackling the molecular mechanisms of how flower color evolves. Flower coloration is encoded in a plant’s genes. Genes responsible for coloration work by contributing to the production and depositing of pigments in flower petals; more production of pigments can cause darker coloring (e.g. red), whereas less production of pigment can cause lighter coloring (e.g. pink). One mechanism by which a new flower color could evolve is if a mutation occurs in a gene that disrupts or enhances pigment production. A group of researchers at the University of Connecticut have uncovered just that in pink monkeyflowers.

In a study from 2013, the scientists revealed how M. lewisii evolved pink coloration relative to red colored monkeyflowers. This occurred by a mutation that affected the gene expression (or how turned ‘on’ a gene is) of the gene ROI1, which decreased production of pigments. An intriguing finding, and like any good scientific discovery, led to a more interesting question. Is the mutation in ROI1 widespread across pink monkeyflower species – or – have many different mutations across species driven the evolution of pink coloration? If a widespread mutation is behind pink coloration, it would suggest pink evolved once (or very few times); whereas, if many different mutations occurred in different species, then it would suggest multiple species are evolving the pink coloration more quickly and independently.

…Is the mutation in ROI1 widespread across pink monkeyflower species – or – have many different mutations across species driven the evolution of pink coloration…

The researchers addressed this question in their more recent study (2022) published in the journal Science Advances. A closely related species of M. lewisii – M. parishii – is also pink so the researchers tested if both species evolved their pink coloration by the same mechanism [a mutation that increased the gene expression of ROI1]. The scientists used a molecular technique to compare the gene expression of ROI1 between the two pink species relative to red species. They found ROI1 gene expression in M. parishii was comparable to red flowers and not elevated like in the related pink flower. This informed the scientists that a different mutation and gene must account for the pink coloring in M. parishii.

So what gene is responsible? To narrow down the many possibilities, the scientists used an extensive approach. This approach was based on selective breeding, where the scientists bred M. parishii with a red flower to produce offspring with darker coloring and then looked at what genes changed to cause the darker coloring. Eventually, the scientists were able to focus on a candidate gene that seemed to correlate with the pink petal coloring called PELAN (a gene involved in pigment formation). Correlations are great and can give powerful insights in science, but they do not guarantee a cause. To more definitively know the gene identified by correlation is actually causing pink coloring, the scientists needed to perform molecular experiments to show the PELAN gene functions differently in M. parshii relative to red flowers.

…To more definitively know the gene identified by correlation is actually causing pink coloring, the scientists needed to perform molecular experiments…

To start, the scientists guessed that there might be a mutation in PELAN that affects its gene expression, but they tested this and found it was not the case. Their next guess was that the mutation may be altering the structure or function of the protein produced by the PELAN gene. The researchers compared the protein structure by looking at protein sequences of PELAN between M. parshii and other flowers and noticed some differences, however, these differences did not seem to have a functional consequence – especially when they were experimentally tested. Another swing and a miss, which is a common occurrence in the scientific process.

So, the scientists were forced to take another guess, and based on their data, they believed there must be a mutation in the PELAN gene that was affecting the construction process of the PELAN protein. When genes are expressed (turned on), they produce a ‘blueprint’ called messenger RNA (or mRNA). The mRNA ‘blueprint’ is then used by cellular machinery to construct proteins (a process known as translation). If a mutation disrupted the translation of PELAN, then it could lead to reduced pigment formation. So, the scientists went back to the sequence of the PELAN gene and found a mutation that would most likely trigger a disruption in translation [it occurred in the 5’ untranslated region]. They tested this mutation by several biochemical experiments and showed that it was indeed disrupting translation and causing pink coloring in M. parshii. 

…the scientists uncovered at the molecular level that two closely related flowers evolved to be pink by independent methods…

This means the scientists uncovered at the molecular level that two closely related flowers evolved to be pink by independent methods – one by a mutation that altered gene expression of ROI1 (in M. lewisii), while the other was a mutation in the gene PELAN that perturbs protein translation (in M. parshii). Interestingly, the reason these two species evolved to be pink are also different. M. parshii is not pink to attract bee pollinators like in M. lewisii because it is self-pollinated. It may be that the ancestor of M. parshii evolved to self-pollinate before changing color, and then, due to not needing to attract hummingbirds anymore, lost the ability to produce large amounts of ‘red’ pigment (which is wasted energy if not serving a purpose).

In future work, it would be interesting to continue exploring other pink monkeyflower species to see how many different ways pink evolved and if any mutations are shared. If explored, would scientists continue to verify that flower color changes occur frequently through independent methods? What about other colors? Have any evolved only once or were they all evolved through many frequent and unique mutations? I do wonder what color the original ancestor of monkeyflowers was as well. What color did the story start from? For now, there are still many interesting questions to address and perhaps one day their answers will lead scientists to identify every mutation responsible for the beautiful diversity of colors we see in nature.