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lower petal of some Mimulus flowers. The
red usually appears as a band of speckles,
which serve as a “nectar guide” for incom-
ing pollinators. Many types of coloration
in plants and animals are the result of a
network of proteins that activate pigment
genes at specific places and times in the
body. But Yuan and Blackman wondered
whether the monkeyflower spots might
be generated instead through a pattern-
ing mechanism proposed in the 1950s by
Turing, who is best known for breaking the
Germans’ Enigma code in World War II but
was also a theoretical biologist.
Turing predicted that some patterns
emerge from the natural diffusion and in-

teractions of proteins whose concentrations

regulate each other’s production. For spots,

when the gene encoding a cell’s “activator” of

pigment production turns on, the activator

protein stimulates its own production and

that of a “repressor” protein, which diffuses

beyond the pigmented spot. That second

molecule shuts down any activator in the

surrounding cells—causing a white halo. But

the repressor gets more dilute the farther it

travels, eventually losing its effect. Then the

activator can turn on, and a new spot of color

can form. Color patterns emerge depending

on the differences in the two proteins’ diffu-

sion rates (see diagram, below).

Biologists have long assumed that Turing’s

mechanism is responsible for zebra stripes

and leopard spots and perhaps even for

monkeyflower nectar guides. Indeed, Yuan

had identified a monkeyflower protein that

might serve as the activator. But no one had

identified a full activator-repressor system

involved in periodic pigmentation patterns.

Blackman, however, had noticed a clue in

some wild M. guttatus: They either lacked

spots or else had just one large red patch,

which he called a tongue, suggesting part

of the system was missing. Independently,

Yuan uncovered a similar red tongue variety

among mutants he made in another Mimu-

lus species. When they learned of each other’s

work, the two joined forces. And at the June

Mimulus meeting, they reported using the

red tongue varieties to track down a protein

dubbed R3-MYB, the repressor counterpart

to the already known activator protein.

To confirm that R3-MYB really acted as a

repressor, Yuan and Blackman both wielded

molecular tools to block its production.

Yuan relied on RNA interference, whereas

Blackman’s team enlisted CRISPR—the

first use of the genome-editing technology

in Mimulus. Both techniques led to full red

tongues on the plants’ petals, Blackman re-

ported—vivid testimony that the mechanism

Turing hypothesized can account for some

of nature’s tapestry. “This work shows how

Mimulus can provide broad insight into pro-

cesses that shape biodiversity,” Sweigart says.

ANOTHER LINE OF WORK with monkeyflow-

ers sheds light on biodiversity by revealing

a mechanism, unique to plants, for rapidly

adapting to new conditions. Their advan-

tage, graduate student Jaime Schwoch of

Portland State University in Oregon found,

is rooted in the way they produce reproduc-

tive cells. In animals, the cells that mature

into eggs and sperm are sequestered early

in development, and they don’t divide until

the organism sexually matures. That pro-

tects them from division-related mutations

that occur in the organism’s nongerm, or

somatic, tissues. But flowers, which con-

tain both kinds of germ cells—pollen and

ova—form from active somatic tissue at the

tips of growing stems. Any mutations oc-

curring in the dividing cells of a stem will

be locked into the germ cells and can pass

on to the next generation.

Given that somatic tissue mutations

should accumulate in a plant’s germ cells

over successive generations, Schwoch won-

dered why plants don’t wind up with many

more such mutations than animals—and

a greater burden of harmful ones. In fact,

as the somatic parts of both plants and

animals grow, their cells accumulate about

one mutation per million bases per cell di-

vision, so plant germ cells should have far CREDITS: (GRAPHIC) V. ALTOUNIAN/

SCIENCE

; (IMAGES) QIAOSHAN LIN AND BAOQING DING

Initial condition: high

concentration of activator

because of self-activation.

Activator difuses.

Pigment spot develops, but

inhibitor concentration grows

because of activator activity.

Activator levels decline at the

border of the spot because of

competition from faster-difusing

inhibitors. Spot is isolated.

Activator

Inhibitor

Concentration

Pigment

change

occurs

No change

in pigment

from default

Turing’s idea

takes root
Spots, stripes, and
monkeyflower petal
speckles arise through
“reaction-diffusion”
(top right), which
involves an activator
protein that turns on
pigment pathways and
the production of a
second protein, which
inhibits the activator.

Spot on
Unimpeded by the inhibitor, an activator colors a cell and spreads to and colors nearby cells. The inhibitor quickly
diffuses away from the colored cells, causing a nonpigmented halo to form. As this process repeats throughout the
tissue, multiple spots form, resulting in a periodic pattern.

Both the activator and the

inhibitor diffuse away from the

cells that produce them, with the

inhibitor moving faster (above).

The balance of concentrations

between the two proteins

determines whether pigment

is produced.

s o d m a a a o v

Patterns galore
Monkeyflower hybrids mix different species’ activators
and inhibitors, altering petal speckle patterns.

856 30 AUGUST 2019 • VOL 365 ISSUE 6456

Published by AAAS