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SCIENCE sciencemag.org 30 AUGUST 2019 • VOL 365 ISSUE 6456 855

individual evolution and ecology projects

with Mimulus and, more recently, supported

a $1 million effort to develop efficient tech-

niques for altering traits in those plants.

“A large, organized, and growing research

community is using this system,” says evolu-

tionary geneticist Theodore Morgan, a pro-

gram officer at NSF in Alexandria, Virginia. A

monkeyflower meeting in Providence in June

drew about 70 biologists, more than triple the

number who attended the first one 13 years

ago. The number of publications on Mimulus

still isn’t huge—about 425—but that tally has

grown rapidly in the past decade.

Some researchers are exploring monkey-

flowers’ own unusual adaptations. But other

scientists are turning the flowers into a win-

dow on widespread biological processes.

Yuan, for example, recently teamed up with

another lab to use Mimulus mutants with

odd petal color patterns to provide the most

detailed example yet of mathematician Alan

Turing’s scenario for how zebra stripes, leop-

ard spots, and some floral patterns arise

in nature. Another team examining how

monkeyflowers mutate as they grow re-

vealed a mechanism that may enable many

plants to evolve faster than animals.

The field may even have its first seri-

ous controversy: Some researchers are

rejecting a recent revision of the monkey-

flower family tree that split the more than

100 Mimulus species into multiple genera,

creating confusion in the scientific literature

by renaming the most studied monkeyflower

species. The researchers’ passion is a mea-

sure of the enthusiasm the new model plant

arouses. One opponent of the new tree, John

Willis, an evolutionary geneticist at Duke

University in Durham, North Carolina, says

flatly, “We’re not going to take it anymore.”

LAB SCIENTISTS AREN’T THE FIRST to be fas-

cinated by Mimulus, which is found world-

wide, often in the harshest spots, such as the

bare islands of “serpentine”

soil that dot the forests of

California’s Sierra Nevada

mountains. Plant ecologists

have conducted field studies

of wild Mimulus for 80 years.

Last year, for example, re-

searchers documented popu-

lations of Mimulus guttatus

that contain a mix of in-

dividuals with different

flowering times, flower

sizes, and number of

seeds produced. The late-

flowering plants do better

in wet years, and early-

flowering ones do better in years when

drought hits early in the season. Because

the amount of rain varies from year to year,

the two variants coexist in a population, al-

though the proportions change over time.

The work, reported last year in Science

(3 August 2018, p. 475), provided long-sought

proof of an evolutionary phenomenon

called fluctuating selection, in which chang-

ing conditions cause a species to evolve in

multiple directions. Theorists have proposed

that fluctuating selection helps explain the

extensive variation seen in many other spe-

cies besides monkeyflowers.

At the June Mimulus meeting, Willis re-

vealed a major clue to another monkey-

flower mystery: the plants’ affinity for

serpentine soils. Because they derive from

Earth mantle rock, those soils are rich in

iron and magnesium but low in potassium

and calcium, which plants depend on to

maintain their cell walls. The soils also tend

to have little nitrogen, vital for plants, but

plenty of toxic heavy metals, such as nickel

and chromium.

Willis and his Duke postdoc Jessica Selby

recently crossed serpentine-tolerant monkey-

flowers with versions of the plant that were

not growing on serpentine soil. The duo

tested several generations to identify DNA

important to the trait. To narrow the hunt

for relevant genes, Selby collected M. gutta-

tus specimens from serpentine soils in seven

places across California and Oregon and

compared their DNA with that of populations

of M. guttatus living nearby, on richer soil.

Both approaches pointed to a gene for

an enzyme that makes arabinose, a sugar

found primarily in the plant cell wall, Willis

reported. That gene varies among M. gut-

tatus plants, but every plant that can grow

on serpentine soils has the same mutation.

It may alter how arabinose interacts with

other components of the cell wall, some-

how compensating for the low calcium and

high magnesium and keeping cell walls

intact—an idea Willis’s team is testing with

researchers from the University of Califor-

nia (UC), Berkeley, and

Stanford University.

By harnessing popula-

tion genetics and other

gene-finding techniques,

Willis “was way ahead of

the curve in seeing how ge-

nomics could make the tre-

mendous natural variation

in plants knowable at the

level of the gene,” says Lila

Fishman, an evolutionary

biologist at the University

of Montana in Missoula.

The work could have prac-

tical benefits as well, adds

Benjamin Blackman, an evolutionary biolo-

gist at UC Berkeley: “Learning how plants

have already adapted to cope with marginal

soil environments can inform breeding ef-

forts aimed at developing crops that can

cope with poor soil.”

Besides probing monkeyflowers’ own

special biology, researchers are using them

to glean more general lessons about plants

and animals. Take Yuan and Blackman’s

work on color patterning. Blackman’s lab

originally studied sunflowers. But M. gut-

tatus appealed to him for studies of the ge-

netic basis of patterning because its simple

genome had been sequenced—making it

easier to test the role of particular genes

and proteins by genetically modifying the

plant. Independently, he and Yuan homed

in on the same protein, which they thought

might be a key to flower color patterns.

Yuan and UConn postdoc Baoqing Ding

had recently tracked down the gene that

causes red pigment to appear on the yellow

Red “tongue” Mimulus mutants

revealed how coloration arises.

The great diversity of flowers within

Mimulus is one reason the plants

have become a popular subject of study.

PHOTO: BENJAMIN BLACKMAN

Published by AAAS