422 3 MAY 2019 • VOL 364 ISSUE 6439 sciencemag.org SCIENCE

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Wheat and other plants that feed

much of the world today were

likely first domesticated in the

Fertile Crescent of the Middle

East. But another early cradle

of agriculture lay thousands of

kilometers away, around West Africa’s Niger

River Basin, a flurry of plant genomic stud-

ies is showing. Several of the continent’s

traditional food crops got their start there:

a cereal called pearl millet and Africa’s own

version of rice. Now, a report out this week

in Science Advances adds yams to the list

of African crops domesticated thousands of

years ago in that same area. A drying cli-

mate may have spurred the move to farm-

ing, says Yves Vigouroux, a

population geneticist at the

French Research Institute

for Development (IRD) in

Montpellier who led some

of the new work.

The recent findings pin-

point the wild ancestors of

some of Africa’s most impor-

tant crops, highlighting res-

ervoirs of genes that could

be exploited to boost the

productivity and disease re-

sistance of the domesticated

varieties, he adds. Such im-

provements could be life

savers on a continent where population is

expanding, and climate change threatens

crop yields. “When we study evolution of

crops across time, it helps us to see varieties

[that] are more resilient,” says Alemseged

Beldados, an archaeobotanist at Addis

Ababa University. “It will help us single out

better breeds.”

Generations of archaeologists have stud-

ied plant domestication in the Middle East

as well as in Asia and the Americas. “But

Africa has very much lagged behind,” says

Dorian Fuller, an archaeobotanist at Univer-

sity College London. Plant fossils and farm-

ing artifacts are less likely to be preserved

in Africa’s warm, moist environments, fund-

ing is scarce, and field research often faces

political and logistical challenges.

Genetic studies, however,

bypass some of these difficul-

ties. In 2002, Nora Scarcelli,

a population geneticist col-

league of Vigouroux’s at IRD,

took an interest in yams, the

most important root crop in

Africa before the introduc-

tion of cassava in the 1500s

and still more important

than maize in parts of Africa.

The vines of the African

yam (Dioscorea rotundata)

produce large tubers that

look a bit like American

sweet potatoes (sometimes

mistakenly called yams), but the plant is a

different species that is also distinct from

Asian yams. But whether the modern Afri-

can crop was derived from D. abyssinica,

a wild yam that grows in the savanna, or

D. praehensilis, which thrives in the wet-

ter rainforests, was not known. Hoping to

resolve the issue, Scarcelli and colleagues

recently sequenced and compared 167 ge-

nomes of wild and domesticated yams

gathered from Ghana, Benin, Nigeria, and

Cameroon. The DNA of savanna wild yams

was fairly similar, but the forest wild yams

split into two groups, one centered in Cam-

eroon and another much farther west.

Scarcelli, Vigouroux, and their colleagues

further identified forest yams in the Niger

River Basin, between eastern Ghana and

western Nigeria, as the source of the mod-

ern domesticate. Their analysis could not

pinpoint the date of domestication, but it

did identify genes that changed along the

way. Variations in genes for water regu-

lation probably helped convert a forest

dweller into a plant that thrives in open

sun. Alterations in root development and

starch production genes also likely made tu-

bers regularly shaped and richer in starch.

A similar study of pearl millet (Cenchrus

americanus), the most important cereal for

arid areas of Africa and Asia with poor soils,

also pointed to a West African origin. When

Vigouroux and his colleagues sequenced

and compared the genomes of 221 wild

(Pennisetum glaucum monodii) and domes-

ticated millets, they concluded that all do-

mesticated pearl millet varieties came from

a single ancestor growing north of the Niger

River in part of the western Sahara Desert

that today includes northern Mali and Mau-

ritania. The genetic work, reported last year

in Nature Ecology & Evolution, dovetails

nicely with a 2011 discovery of 4500-year-

old pearl millet remains in an archaeologi-

cal site in southeastern Mali, Fuller adds.

Previous studies had shown that about

6000 years ago, probably before millet was

domesticated, a moist climate created a net-

work of lakes in the region, yielding abundant

wild food. As the climate dried and those

lakes vanished, Vigouroux hypothesizes, the

local people began to cultivate plants.

And over more time, people and plants

shifted southward, with cultivated plants in-

termittently interbreeding with wild plants.

The mingling slowed full domestication but

added genetic variation to the millet. “Our

findings stressed the importance of wild-to-

crop gene flow during and after crop domes-

tication,” Vigouroux and his colleagues wrote.

African rice (Oryza glaberrima) is less

Plant genomics unearths Africa’s ‘fertile crescent’Several traditional crops traced to West African river basin

AGRICULTURE

As it was domesticated, the African yam got bigger,

starchier, and more regular in shape.

By Elizabeth Pennisi

Wild yams such as these were

cultivated into an African crop.

Published by AAAS

Corrected 3 May 2019. See full text.

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3 MAY 2019 • VOL 364 ISSUE 6439 423SCIENCE sciencemag.org

important today than the other traditional

crops, having been mostly replaced with

Asian rice. But it, too, got a West African start.

Last year in Current Biology, Vigouroux and

his colleagues analyzed 163 varieties of the

domesticated African rice and 83 samples of

wild rice collected from west and east Af-

rica. Their analysis showed the cultivated

species has about half as much genetic di-

versity as the wild species, and that it arose

from wild relatives in northern Mali. They

also found that some of the same genetic

changes central to the taming of Asian rice,

such as a gene deletion that made the plant

grow more erect, also played a part in the

domestication of African rice.

Other researchers have tracked down

additional genes that aided African rice

domestication. In the March issue of PLOS

Genetics, they described the evolution of

genes that make rice seeds less likely to

fall off the stalks. Such a detailed history is

easier to piece together for African rice than

Asian rice because so many wild populations

are still intact, says evolutionary geneticist

Michael Purugganan of New York University

in New York City, who led the work. Studies

in Africa “may tell us more about crop do-

mestication than what people have learned

about other crops elsewhere,” he adds.

While Vigouroux focuses on climate to ex-

plain why agriculture arose in this part of Af-

rica, archaeologist Sylvain Ozainne from the

University of Geneva in Switzerland suggests

movements of Saharan pastoralists helped

initiate and spread a culture of crop grow-

ing, particularly of pearl millet. “Rather than

a direct response to abrupt climatic change,

the expansion of African agriculture may be

better explained through a more complex

process, which involved socio-economic

transformations,” he says.

In Africa, as elsewhere, crop domestica-

tion was a long, drawn-out process (Science,

29 June 2007, p. 1830). It happened out-

side the Niger River Basin as well. Sor-

ghum was likely tamed in East Africa. Last

month, Nature Plants published a genetic

analysis of wild and domesticated sorghum

samples excavated from an archaeological

site in Egypt that spans several thousands

years. It showed that early farmers either

deliberately crossed this crop—now the

sixth most widely grown in the world—

with wild relatives and with domesticates

from other places, or cultivated varieties

that naturally interbred.

All this genetic exchange ultimately helped

improve the final crop, says Emuobosa Akpo

Orijemie, an archaeobotanist at the Univer-

sity of Ibadan in Nigeria. The new findings,

he adds, “speak about how intelligent our an-

cestors were in terms of selecting and manag-

ing crops for a variety of uses.” j

NEWS | IN DEPTH

NIH limits reign of chiefsGreater gender and ethnic diversity is goal of 12-year cap

SCIENTIFIC WORKFORCE

Able to pursue open-ended research

without relentless grant deadlines,

some scientists who work directly

for the National Institutes of Health

joke that NIH stands for “nerds in

heaven.” But the main NIH campus

in Bethesda, Maryland, and its other in-

tramural research sites are also known as

stodgy places where the scientific manage-

ment, mostly white men, tends to stay in

place for decades. Now, NIH is aiming to

shake up its intramural program, the larg-

est collection of biomedical researchers in

the world, by imposing term limits on mid-

level leadership positions.

Starting next year, the 272 lab and

branch chiefs who oversee NIH’s intra-

mural research will be limited to 12-year

terms. The policy, now being refined by

the directors of NIH’s 23 institutes with

in-house science programs,

means up to half of the chiefs

will turn over in the next

5 years, says Michael Gottes-

man, NIH’s deputy director

for intramural research. “We

see this as an opportunity

for diversity in the leader-

ship at NIH, especially gen-

der and ethnic diversity,” says

Hannah Valantine, NIH’s chief

officer for scientific workforce diversity.

The changes are roiling the campus, with

some grumbling they will have little im-

pact and others questioning whether good

leaders should automatically be replaced.

“The appointment of more women … could

be a plus, but the ‘coin of the realm’ still re-

mains scientific excellence and productiv-

ity,” says Malcolm Martin, who has headed

a lab at the National Institute of Allergy

and Infectious Diseases for 37 years.

At most institutes, NIH’s intramural lab

and branch chiefs oversee several labs or

groups. Although they don’t control re-

searchers’ budgets directly, they handle

administrative matters, mentoring, and re-

cruitment. Chiefs overseeing clinical stud-

ies and shared facilities hold even more

sway. “These are fiefdoms where [chiefs

have] power and resources,” Valantine says.

Many chiefs (54 of 272) have served at

least 20 years, and 17 for more than 30

years, Gottesman says. Although 26% are

women—comparable to the 24% women

among all NIH tenured researchers—men

tend to lead larger programs. Because

of the lack of turnover, “People feel

like there’s no way they’ll ever have a

leadership position,” says Gisela Storz,

chair of NIH’s equity committee, which

pushed for the changes. “And trainees need

to see people in those positions who look

like them.”

Under the draft policy released in Janu-

ary, the chiefs will have to step down after at

most three 4-year terms. The positions that

become vacant will be filled through “open

and transparent processes,” the draft policy

states. While some institutes already do that,

at others, the scientific director overseeing

the intramural program “plucks an heir ap-

parent” from internal staff, Storz says.

To help build the pipeline, NIH will rely

on a recently launched program aimed at re-

cruiting more tenure-track female and mi-

nority faculty. In the long term, NIH hopes

its intramural leadership

will more closely reflect that

women now earn more than

50% of new Ph.D. degrees

in the biological sciences,

Valantine says.

Individual institutes are

now figuring out how to enact

the term limits “in a way that’s

not disruptive,” Gottesman

says. Some chiefs may be ex-

empt, he says, if a change would have “se-

rious consequences” for science programs,

for example because there is no pool of

candidates for the job.

One former NIH veteran is skepti-

cal. “How much have they thought this

through?” asks Story Landis, who was sci-

entific director and later director of the

National Institute of Neurological Disor-

ders and Stroke. She questions why NIH

would want to replace a midcareer chief

doing a stellar job. And, she wonders,

will the job searches truly be open? Will

women get the training they need to move

into leadership positions?

Others point out that NIH’s scientific

directors—seven of whom are now

women—are the true feudal lords, and

the new policy does not affect them.

Gottesman has held his job for 25 years.

But he and the scientific directors he

oversees may be next: NIH term limits

could “move up to other kinds of leader-

ship,” Valantine says. j

By Jocelyn Kaiser

“We see this as an opportunity for diversity in the leadership at NIH …”Hannah Valantine, NIH

Published by AAAS

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Plant genomics unearths Africa's 'fertile crescent'Elizabeth Pennisi

DOI: 10.1126/science.364.6439.422 (6439), 422-423.364Science 

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