Taxonomy & Evolution

Pink monkeyflowers are a sign of local adaptation

Botanists find petal colour can signal a monkeyflower’s drought tolerance.

Colourful petals are fantastic for attracting pollinators, but sometimes a particular colour signifies some environmental conditions. One of the best-known examples is the hydrangea. The flowers are pink on alkaline soils but are blue on more acidic soils. 

Some of the environmental conditions known to lead to pigmentation change (not only around the flowers) are drought, extremely hot temperatures and UV radiation. The anthocyanin pigments can protect the plant tissues against tissue damage by UV light, or darker floral pigments could increase the temperature of the flower that acts as a cue for certain pollinators. These adaptations are known on a local scale, but not much is known about the evolution of colour flower change on a macro-scale. 

Dr Dena Grossenbacher and colleagues from the California Polytechnic State University and California Botanic Garden investigated if petal colour change due to environmental conditions has evolved on a macro-evolutionary scale in the highly diverse monkeyflower family (Phrymaceae). Dr Grossenbacher has been working with monkeyflowers for over a decadeIn their latest study, the researchers found that two monkeyflower species had more pink flowers at locations with less precipitation. A larger comparison of 56 species has not found an overall evolutionary pattern in flower pigmentation and environmental conditions.

Two pairs of yellow and pink flowers. The colours are very distinct.
The yellow and pink flowers of two monkeyflower species, Erythranthe discolor and Diplacus mephiticus. Source: Grossenbacher et al., 2021

First, the researchers focused on two monkeyflower species, Erythranthe discolor and Diplacus mephiticus, which are known to have different coloured flowers around California. 

The scientists recorded the petal colours of 14 populations of E. discolor in the field. As D. mephiticus populations span hundreds of kilometres from Sierra Nevada to the Great Basin in Nevada, the researchers examined over 160 herbarium specimens for this species rather than going on field trips. The scientists mapped six environmental conditions relating to temperature, rainfall, UV-B radiation and elevation and looked for correlations between flower colour and extreme abiotic conditions. The team scored the colour states as “anthocyanin present” (red, pink and purple) and “no anthocyanin” (yellow, orange and white).

Secondly, Grossenbacher and colleagues collocated DNA sequences from 56 species belonging to Erythranthe and Diplacus genera and once again noted down whether anthocyanin was present or not. A phylogenetic analysis tested whether anthocyanin evolution correlated with environmental conditions amongst related species. 

Two distribution maps with an overlay of the rainfall on the western half of the United States.
Occurrence of Erythranthe and Diplacus species, presence of anthocyanin (dark circles) and amount of rainfall (red is lowest). Source: Grossenbacher et al., 2021

The flower pigments correlated with all environmental conditions for Diplacus speciesPink colours occurred in cooler areas with less precipitation and less UV radiation than yellow-flowered populations. 

However, for E. discolour, only precipitation seemed to explain the presence of anthocyanins. Pink flowered Erythranthe species occupied areas with less precipitation than yellow-flowered ones.

Petal pigments up close from pink and yellow morphs of Erythranthe discolor (lower panels) and Diplacus mephiticus. The yellow morphs contain chromoplast organelles (arrows) consistent with presence of carotenoid pigments. Source: Grossenbacher et al., 2021

Whilst overall, there was no phylogenetic signal in flower colour and climate traits amongst the 56 species, there was a phylogenetic signal within Erythranthe. Grossenbacher and colleagues have three theories why there is no clear evidence of macroevolutionary patterns in petal pigmentation of monkeyflowers.

“First, despite the role of floral anthocyanins in stress tolerance, it could be that pollinator-mediated selection is the stronger agent at the macroevolutionary scale where it can lead to reproductive isolation, speciation and lineage divergence,” Grossenbacher and colleagues wrote. 

“Second, perhaps the role of anthocyanins in stress tolerance most commonly operates under balancing selection in highly heterogeneous environments, rather than as directional selection.”

“Finally, because flower color is a conserved trait in monkeyflowers, it could be that there is simply low power to detect correlated evolution between flower color and abiotic traits in this system.” 

This study hoped to find a simple answer to whether changes in petal colours are driven by environmental conditions and is a trait that evolved on a macroevolutionary scale. Instead of a clear evolutionary pattern, the researchers found some evidence that pink flowers are a sign of drought tolerance. Future experiments could reveal the effect of drought on flower pigmentation in either the same or other monkeyflowers. 

Continue to follow Grossenbacher’s lab research online

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