Monkeyflower is a common name for plants in the Mimulus genus, which belongs to the family Phrymaceae. The name “monkeyflower” comes from the shape of the flowers, which are often two-lipped and resemble a monkey’s face. There are over 150 species of Mimulus, and they are native to North and South America, Europe, Asia, and Australia.
Monkeyflowers glow in a rich assortment of colors, from yellow to pink to deep red-orange. But about 5 million years ago, some of them lost their yellow. In the Feb. 10 issue of Science, UConn botanists explain what happened genetically to jettison the yellow pigment, and the implications for the evolution of species.
Monkeyflowers are famous for growing in harsh, mineral-rich soils where other plants can’t. They are also famously diverse in shape and color. The flowers can be yellow, orange, pink, red, purple, or white, and are usually about 1-2 inches wide. The leaves are typically green and toothed, and can be either opposite or alternate on the stem.
A classic illustration of how a single gene alteration can create a new species is provided by monkeyflowers. A monkeyflower species in this instance lost the yellow pigments in the petals about 5 million years ago but gained pink, luring bees for fertilization.
Later, a descendant species developed mutations in the YUP gene, restoring the yellow pigments and resulting in the development of red flowers. Bees ceased being drawn to the species. Hummingbirds pollinated it instead, genetically isolating the red blossoms to produce a new species.
Almost every single species has unique genes, called ‘taxon specific’ because they are only found in a small group of species. For the most part, we have no idea what these genes do.
Yaowu Yuan
UConn botanist Yaowu Yuan and postdoctoral researcher Mei Liang (currently a professor at South China Agricultural University), with collaborators from four other institutes, have now shown exactly which gene changed to prevent monkeyflowers from making yellow. Their study, which was just published in Science, supports the idea that additional genes can lead to phenotypic variety and possibly the emergence of new species.
The in issue YUP gene is located at a locus (region) of the monkeyflower genome where three additional genes are also present. These new genes are not found in species outside of this group. They are duplicates of other genes from other parts of the monkeyflower genome. In particular, YUP is a partial copy of an unrelated gene that already exists.
It was extremely unlikely that these genes would affect an unrelated gene because it is a common misconception in genetics that partial duplicate genes regulate the genes they are descended from. In spite of Yuan’s suggestion that it would be a waste of time, Liang made the decision to look into what these genes were doing.
But Liang’s persistence paid off: she discovered that the YUP gene was actually targeting the plants’ master regulator of carotenoids, the pigments that make monkeyflowers and other plants yellow.
YUP produced many small RNAs that suppressed the carotenoid gene. There are very few examples of tiny RNA-producing genes influencing features critical to the emergence of new species.
“This experience really taught me how important it is not to constrain oneself with ‘conventional wisdom,’” Yuan says. Not only does YUP regulate a gene to which it is entirely unrelated; the other two genes at this same locus also affect monkeyflower color, Yuan says.
The uniqueness of these three genes, only found in a few closely related monkeyflowers, is an important clue as to how new species evolve.
“Almost every single species has unique genes, called ‘taxon specific’ because they are only found in a small group of species. For the most part, we have no idea what these genes do,” says Yuan.
This research shows that these taxon specific genes can be the keys to the new species. Prior until this, many geneticists and evolutionary biologists believed that the few idiosyncratic genes were unlikely to play a significant role in evolution and that species differentiation was instead caused by variations in the expression of common genes shared by many distinct species.
“We think we understand evolution well enough to make predictions. But now we are realizing we really don’t. Evolution is just so unpredictable,” Yuan says.
His lab is now looking at how the monkeyflower genome controls the production of pigment spatially. For example, some monkeyflowers have upper petals that are entirely white, but lower petals with color. Yuan and his colleagues are interested in learning how plants only inhibit pigment in specific areas of the bloom.
