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Monkeyflower Mutation Provides Evolution Insight

John Roach
for National Geographic News
November 12, 2003
 
For years scientists have grappled to understand the number and type of
genetic mutations required for a new species to evolve. Does it require
the accumulation of many minute mutations? Or can a single mutation
spark a big change?

Now researchers studying pink and red flowers in the monkeyflower (Mimulus) family have found a persuasive answer: A single mutation can recruit a whole new set of pollinators, serving as the fork in the road that leads to a new species.


"Many mutations are required to get all the way, but the original step can be produced by one mutation," said Toby Bradshaw, an evolutionary biologist at the University of Washington in Seattle.

Bradshaw and his colleague Douglas Schemske, a plant biologist at Michigan State University in East Lansing, mimicked a mutation by switching gene variants that control color from one monkeyflower to another.

Mimulus lewisii are pink and usually pollinated by bees. Mimulus cardinalis are red and usually pollinated by hummingbirds. The birds and bees rarely confuse the flowers, and the flowers almost never interbreed in nature.

The scientists crossed the flowers in a laboratory, breeding red monkeyflower gene variants that control for color into pink monkeyflowers and vice versa.

The results were an orange M. lewisii that drew many hummingbirds and some bees and a dark pink M. cardinalis that drew many bees and some hummingbirds.

"It doesn't switch pollinators as much as it recruits additional pollinators," said Bradshaw.

Writing in a paper published tomorrow in the science journal Nature, Bradshaw concludes with Schemske that "an adaptive shift in pollinator preference may be initiated by a single mutation."

Jerry Coyne, an evolutionary biologist at the University of Chicago, Illinios, said the conclusions of Bradshaw and Schemske "are very sound" and the elegance with which they crossed the species and observed the effect in the field amounts to a "solid and brilliant piece of research."

Crossbreeding

From previous research, Bradshaw and Schemske knew that genes in a certain section of the monkeyflower genome called the yellow upper, or YUP, control the presence or absence of yellow pigments in the petals of M. lewisii and M. cardinalis, and thus whether or not they have their normal pink and red colors.

To determine if a single mutation at the YUP locus causing the color of the flower to change would be sufficient to recruit a different set of pollinators to the plants, the researchers crossbred and then backcrossed the flowers until each flower had the YUP of the other.

"What you are trying to do is get rid of all the genes from the donor species except for the one gene controlling whether or not yellow pigment is in the flowers," said Bradshaw.

After successfully achieving this step, the researchers took the flowers to a field site in the Sierra Nevada mountains of California where both M. lewisii and M. cardinalis are normally found. The pair observed pollinators recruited by the mutant flowers.

Speciation

According to calculations presented by Bradshaw and Schemske in Nature, if this mutation were to occur in the wild, the mutants would likely die off because they are visited much less than their non-mutant sisters.

"That is sort of to be expected," said Bradshaw. "We expect the normal, non-mutant flower to be optimally adapted."

However, in their paper, the researchers calculated the circumstances under which the mutant could take hold and evolve to a new species of monkeyflower by fully adapting traits specific to their pollinators.

For example, if a bunch of the bumblebees died off owing to a climate change or parasite infestation and the hummingbirds remained stable, suddenly the yellow-orange mutant M. lewisii could recruit a sufficient number of hummingbirds to survive alongside the wild-type pink-flowered ancestor.

"There are circumstances under which the mutant could be favored by natural selection," said Bradshaw.

Over time, the mutant M. lewisii would adapt other traits specific to hummingbird pollinators, such as flower shape and the quantity of nectar they store, becoming a distinct species of monkeyflower, said Bradshaw.

According to Coyne, it makes sense that mutations of large effect are important for monkeyflower speciation, the evolutionary changes that lead to new species. Changes in pollinators are likely to occur when a population becomes isolated from others in a new environment and are suddenly exposed to a new regime of pollinators.

So, if a monkeyflower normally pollinated by bees was suddenly stranded in an area dominated by hummingbirds, it must either adapt to hummingbird pollination or die. "When you change pollinators, it is hard to do that without having a mutation of large effect," said Coyne.

However, there are certain circumstances when a change of large effect would be of no benefit to a species.

For example, males of various duck species differ in color from females, an important cue in attracting mates, said Coyne. If a male duck suddenly changes color, it won't find a mate and will die without leaving offspring. So large color mutations will not be passed on. In the case of duck speciation, the process is likely to involve a gradual accumulation of small changes.

"Like evolution itself, there are no general rules that apply to the formation of species," he said.
 

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