These wildflowers changed color to attract bees

Hummingbirds normally patrol the scarlet blooms of the western United States, sipping nectar and carrying pollen from one monkeyflower to the next.

Yet along the outer edges of that crimson range botanists once noticed something odd: scattered stands of Mimulus cardinalis and Mimulus verbenaceus (two monkeyflower species) had traded their fiery petals for butter-yellow ones.

Earlier fieldwork hinted that bumblebees, not hummingbirds, seemed drawn to these rare morphs, but decades ago researchers lacked the genomic and biochemical tools to learn why.

A team led by Kelsey Byers at the John Innes Center has now reopened the case. In a recent study, the group combined controlled pollinator trials, floral chemistry, and comparative genetics.

The findings reveal how color, scent, and shape interact when a plant begins shifting from one pollination strategy to another. Researchers documented an adaptive walk – evolution’s slow path from ancestral traits to new ecological niches.

Bees prefer a yellow color

Inside climate-regulated glasshouses, the researchers offered captive bumblebees paired arrays of yellow and red flowers from both species.

The insects showed a clear preference, visiting yellow twice as often as red.

Chemical analysis then uncovered a clue: the yellow morphs emitted higher volumes of volatile compounds, a likely adaptation because bees, unlike hummingbirds, rely heavily on scent when foraging.

Bees struggle despite color

Color and fragrance alone do not guarantee reproductive success. High-speed video revealed that bees wrestled awkwardly with the tubular corollas, structures sculpted through millennia of hummingbird visitation.

Many bees tore the delicate petals while probing for nectar and often failed to brush against the floral reproductive organs, a prerequisite for pollen transfer. As a result, the bees’ frequent visits did not translate into efficient pollination.

The researchers interpret this mismatch as evidence that the yellow monkeyflowers represent an early stage in the switch from bird to bee service: color mutation and heightened scent have arisen first, but corolla architecture has yet to catch up.

Different genes, same traits

A long-standing question was whether the yellow trait in the two Mimulus species emerged through the same molecular pathway. The team’s genomic scans show the answer is partly yes and partly no.

Both species boosted yellow and orange carotenoid pigments by changes to the same suite of genes, a textbook example of convergent evolution. Yet other alterations – loss of red anthocyanin pigments and shifts in scent chemistry – arose through different genetic tweaks in each lineage.

“Even though these species are closely related, they have followed different paths to the same endpoint for many of their traits,” Byers said.

“We have identified ways that flowers can change and affect pollinator visitation, as well as some genetic pathways that can underlie these changes and how they have evolved in different but closely related species.”

A timeline for trait evolution

Because floral color strongly influences pollinator choice, mutations in pigment genes may surface first during a pollinator shift. Secondary adjustments, such as boosted scent emission or re-shaped petals, could then fine-tune the interaction.

The Mimulus data support that sequence. Large-effect color mutations preceded subtler changes in odor bouquet, while corolla morphology still lags behind, leaving the bee–flower fit imperfect.

The researchers argue that this hierarchy of changes – big steps followed by smaller refinements – may be typical whenever plants embark on new mutualisms.

Bee-friendly color affects farming

Pollinator partnerships underpin terrestrial biodiversity and sustain many food crops. By dissecting how wild species recalibrate those relationships, scientists gain conceptual tools for conservation and agriculture.

“By understanding how traits evolve in the wild, we understand biodiversity better, and by understanding how these traits can be affected by plant genetics, we have the foundation of the ability to engineer traits to impact pollinator preference and thus crop yield,” Byers said.

Tuning color and scent may let breeders align crops with local pollinators, improving output without extra chemicals.

Next on the research agenda

The team plans gene-editing and transgenic experiments to verify the precise functions of the carotenoid biosynthesis genes that acted convergently and the anthocyanin regulators that diverged.

They are also surveying another yellow population of Mimulus cardinalis to see whether it repeated the same genetic tricks or invented new ones. Such comparative work will refine our picture of how predictable – or idiosyncratic – evolutionary innovation can be.

Floral traits still changing

For now the yellow monkeyflowers stand as living waypoints on an evolutionary journey not yet complete. Bumblebees like their color and perfume, but the floral entrance still suits a hovering hummingbird more than a burrowing bee.

With time and mutations, petal tubes may evolve to guide bees directly to stamens and stigmas. Should that happen, future botanists might find a fully bee-adapted Mimulus lineage where once only hummingbirds ruled – and they will have these early studies to trace how the transformation began.

The study is published in the journal Nature Communications.

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