Fan chose to focus initially on a weed
that seems to have evolved its crop-
mimicking characteristics in China:
barnyard grass (Echinochloa crus-galli),
a variety of which takes on a rice-like
appearance early in its lifecycle. This
mimicry had been documented in the
early 1980s by University of Toronto
plant evolutionary biologist Spencer
Barrett, who proposed that hand weed-
ing of rice fields had exerted selective
pressure on the variety, driving it to grow
upright like rice rather than spread out
along the ground like its close relatives
(Econ Bot, 37:255–82, 1983).
Fan’s team collected 328 specimens
of barnyard grass from paddy fields
throughout the Yangtze River basin—
the epicenter of rice cultivation in China
for centuries—and grew their seeds in a
field and in a greenhouse to determine
whether their seedlings had the rice-
mimicking phenotype. The research-
ers also sequenced each plant’s genomeand used the data to construct a phylo-
genetic tree of barnyard grass varieties.
The mimic and non-mimic varieties
diverged from each other about 1,000
years ago, the research team concluded,
and the mimic’s genome showed evi-dence of selection in 87 genes thought
to control plant architecture. The time-
line, the researchers note in their paper,
coincides with intensified rice cultiva-
tion in the Yangtze basin during China’s
Song dynasty (Nat Ecol Evol, 3:1474–
82, 2019).The results are consistent with the
idea that humans, through hand weed-
ing, provided the selective pressure that
drove the evolution of the rice-imitating
variety. It’s remarkable that Vavilov’s
decades-old predictions align so well
with the data, says Manyuan Long, a
University of Chicago evolutionary biolo-
gist who was not involved in the research
but wrote a review of the paper for the
site F1000.com. Fan’s study, he says, is
“really wonderful work.” Like Fan, Long
remembers learning about Vavilov as an
undergraduate in China in the 1980s.
“Vavilov is my hero in science,” he says,
although he notes that he hadn’t been
aware of the idea of Vavilovian mimicry
until he read Fan’s paper.
Fan’s collaborator on the study,
Washington University in St. Louis
plant biologist Kenneth Olsen, says it
was exciting to revisit Barrett’s work
with the benefit of modern sequencing
technology. “It’s really satisfying in aIt’s just such a fasci nating
phenomenon, this evolution
of these weedy species
that are exploiting crop
environments.
—Kenneth Olsen, Washington University in
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