Monkeyflowers prove that evolution is unpredictable

The process of speciation, where a new species evolves, is usually considered to be slow and to involve many different genes that affect the behavior, physiology, ecology or even the anatomy of the evolving species. Speciation due to a mutation in a single gene that isolates an individual reproductively is considered highly unlikely. However, botanists from the University of Connecticut have recently identified just such an alteration in the genome of some species of monkeyflowers; they suggest that this led to a change in the pollinating agent and gave rise to new species. 

Monkeyflowers (genus Mimulus) grow in harsh, mineral-rich soils where other plants do not thrive. The flowers are famously diverse in shape and color, and have been the subject of much research into the genetic origins of patterns and shades on the petals. Researchers already know that, around 5 million years ago, a monkeyflower species underwent a mutation that caused the loss of yellow pigments in the petals; instead, the mutation produced shades of pink that attracted bees as pollination agents. 

Subsequent to this, a descendent species accumulated new mutations in a gene called YUP that had the effect of recovering the ability to form yellow pigments, and this led to the production of red flowers. The species no longer attracted bees but, instead, hummingbirds became the pollinators, thus isolating the plants with red flowers genetically and leading to the development of a new species.

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 it is that changed to prevent monkeyflowers from making the yellow pigment all those years ago. Their research, published in the journal Science, adds weight to the theory that speciation can indeed occur through changes to single genes. 

The YUP gene in question is located in a region (locus) of the monkeyflower genome that has three new genes. 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 duplicate of a pre-existing gene that has nothing to do with producing colored pigments.

Standard genetics thought is that partial duplicate genes regulate the genes they are derived from; the researchers thought it was very unlikely that these genes would have an impact on the unrelated genes for colored pigments. Liang decided to investigate what these genes were doing anyway, against the advice of Yuan, who thought it was a waste of time.  

Liang’s intuition paid off: she discovered that the YUP gene was actually targeting the plants’ master regulator of carotenoids, the pigments that make monkeyflowers, and the petals of other plants, yellow. YUP produced many small RNAs that suppressed the carotenoid gene. There are very few examples of genes that produce small RNAs affecting traits important to the creation of a new species.

“This experience really taught me how important it is not to constrain oneself with ‘conventional wisdom,’” said Yuan. Not only does YUP regulate a gene it is entirely unrelated to it, but 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 monkeyflower species, is an important clue as to how new species evolve.

“Almost every single species has unique genes,” said Yuan. These are known as ‘taxon specific’ genes because they are only found in a small group of species. “For the most part, we have no idea what these genes do.”

This research shows that these taxon specific genes can be the keys to the formation of a new species. Previously, many geneticists and evolutionary biologists thought that it was changes in the expression of common genes shared by many different species that differentiated them, and that the small number of idiosyncratic genes were unlikely to be important.

“We think we understand evolution well enough to make predictions. But now we are realizing we really don’t. Evolution is just so unpredictable,” said Yuan.

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 want to know how the plants suppress pigment only in certain parts of the flower.

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By Alison Bosman, Earth.com Staff Writer

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