Flowers in the genus Petunia rely on a variety of pollinators, ranging from bees to hummingbirds to moths. Researchers have now identified a single gene that seems to control which pollinators specific petunia species attract. Mutations on this gene impact the flower’s ability to absorb ultraviolet light, an important trait for attracting nocturnal pollinators, according to findings published in Nature Genetics this week.
Flowering plants often depend on animal pollinators, and these partnerships are typically associated with specific sets of floral traits spanning a diversity of colors, scents, shapes, and nectar types. Over evolutionary time, plants can acquire changes that allow them to attract new pollinators, but adaptations to new pollinators – and the speciation events that might follow – are rather involved processes requiring multiple floral traits as well as coordinated changes in the underlying genes.
Despite this complexity, it’s happened frequently over the history of flowering plants. The transition from bee to hummingbird pollination has occurred at least 10 times in the nightshade family alone.
To study how plants evolve new pollinator partnerships, the University of Bern’s Cris Kuhlemeier and colleagues looked at the molecular basis of UV light absorbance in three species of South American petunias, a member of the nightshade family. The most ancestral of the three, Petunia inflata, has small, purple flowers that are pollinated by solitary bees. The bright red, non-scented flowers of P. exserta attract hummingbirds in the daytime, and the white flowers of P. axillaris attract hawkmoths at night.
Out of these three petunia species, white P. axillaris flowers absorb the highest levels of UV light. And their natural hawkmoth pollinators, Manduca sexta, prefer flowers that absorb higher levels of UV light over those with lower levels of absorbance. UV-reflective flowers get fewer visits from these moths.
The differences in UV absorbance that evolved in the divergent petunia species are due to changes in a single gene called MYB-FL. The shift from bee pollination in the ancestral form to hawkmoth pollination in P. axillaris was accompanied by a mutation in MYB-FL that upped the levels of UV-absorbing compounds called flavonols within the flower petals. Later on, a different mutation arose that disabled MYB-FL, and this led to the shift from hawkmoth pollination to the daytime services of hummingbirds for P. exserta.
A flower shown in visible light (left), and a flower shown in UV light (right). Hester Sheehan