Science - USA (2019-08-30)

SCIENCE sciencemag.org 30 AUGUST 2019 • VOL 365 ISSUE 6456 857

more mutations than animal germ cells.

But they don’t. And deleterious mutations

are surprisingly scarce in plants, Schwoch

found when she compared two sets of

monkeyflowers. She produced one by self-

fertilizing Mimulus flowers with each

flower’s own pollen, the other by fertilizing

flowers with pollen from flowers on another

stem of the same plant. The latter is the

equivalent of a cross between different par-

ents, because each stem acquires a unique

set of mutations as it grows.

Self-fertilization, like inbreeding in ani-

mals, should pair up harmful recessive mu-

tations, so Schwoch expected the crosses

that used pollen from one stem on flowers

from another to do better. But some of the

more prolific, healthier plants came from

progeny derived from a single stem, she

reported at the Evolution 2019 meeting,

also in Providence in June. That find-

ing suggested the plants were somehow

eliminating harmful mutations in their

somatic cells and accumulating benefi-

cial ones for their reproductive cells.

To verify that sorting process,

Schwoch grew Mimulus plants for

6 months in saltier-than-normal con-

ditions and then sequenced DNA from

their tips, taking note of new muta-

tions and how often the mutations

appeared in the sequenced material.

Such mutations should occur in low

frequencies, so when she found one

that occurred in many cells of the

plant tip, she inferred that the origi-

nal cell with that mutation had grown

much faster than cells without it and

replaced them.

The rate of mutations doubled under the

salt stress, she reported. Moreover, cells

carrying mutations that improve salt toler-

ance proved more likely to persist in stems,

whereas less well-adapted cells died out.

The survivors made it into the germ line,

so the within-lifetime innovations were

passed on to subsequent flowers and pollen.

The process means “plants can adapt very

quickly” to tough situations, Schwoch said.

Duke evolutionary biologist Jennifer

Coughlan is impressed. “This work has

broad significance for all plants, but in

particular for long-lived perennial plants,

[which] accumulate many mutations

across their lifetimes,” she says. Sweigart

predicts that Schwoch will quickly unearth

the specific mutations that produced the salt

tolerance: Mimulus “has great genetic and

genomic resources, so it should be possible

to identify the precise molecular changes

that have occurred” in her salt-tolerant

plants, Sweigart says. And the lessons from

monkeyflowers may point to ways to make

other plants, including crops, more toler-

ant of salty soils, researchers suggest.

JUST WHEN INVESTIGATORS are flocking to
monkeyflowers, the monkeyflowers may
be scattering—at least taxonomically. A
2012 evaluation of the Mimulus family tree
placed some of the better-studied monkey-
flower species in other genera. For exam-
ple, the popular M. guttatus is now named
Erythranthe guttata. The Mimulus genus
itself kept only seven species of the origi-
nal 165-plus. Adopting new designa-
tions for many Mimulus species will
lead to chaos in the scientific litera-
ture, some researchers in the field say.
Among monkeyflower researchers,
the reclassification provoked a mi-
nor rebellion. Most did not use the
new names in their presentations at
the Mimulus conference or the sub-
sequent Evolution meeting. A paper
in press in the journal Taxon argues
for a different, less disruptive reclas-
sification, and Willis says that makes
sense. “You either rename 150 species,
or you rename 20 somewhat obscure
species and call them all Mimulus,”
he notes.
A Mimulus by any other name
might smell as sweet, but most bio-
logists don’t want to monkey around
PHOTOS: (TOP TO BOTTOM) JESSICA PACKARD SELBY; JAIME SCHWOCH with their new favorite plant. j

Growing monkeyflowers in salty conditions has helped reveal how

plants weed out deleterious mutations.

Many monkeyf owers thrive in inhospitable, mineral-laden soils, like this spot in Lake County in California, and plant biologists are starting to understand how they do it.

Published by AAAS