Biologists have discovered that the water
lily may be a critical "missing link"
in the evolution of flowering plants.
One of the great mysteries of evolutionary
biology is how, 150 or more million years
ago, modern-day angiosperms (flowering
plants) diverged from their closest relatives,
the gymnosperms (seed-bearing plants without
flowers, such as pine trees with pine
cones). A developmental study of the water
lily, Nuphar polysepalum, may provide
an important clue.
Joseph Williams and William Friedman of
the University of Colorado report their
National Science Foundation (NSF) supported
findings in the January 31 issue of the
journal Nature. "This is a significant
first-time discovery because now we are
a big step closer to understanding the
evolution of flowering plants," says Erik
Nilsen, the NSF project officer.
An established and distinguishing feature
of flowering plants is that each seed
consists of two parts: 1) an embryo that
is similar to that of all other plants,
and 2) a unique tissue called the "endosperm",
which functions to nourish the embryo
and which most people know as "grain".
Virtually all angiosperms have endosperm
that is "triploid", that is, it contains
three copies of each chromosome: two from
the mother and one from the father's sperm.
This triploid endosperm contrasts dramatically
with the seeds of gymnosperms, in which
the nourishing tissue is "haploid" containing
a single copy of each chromosome. "The
question of how endosperm originated,
in an evolutionary sense, has perplexed
biologists for over a century," states
Friedman.
Williams and Friedman focused on the water
lily family because fossil records and
recent molecular analyses place it among
the most ancient of flowering plants.
They measured DNA contents of embryo and
endosperm cells using fluorescence microscopy
to discover that the water lily has a
diploid endosperm, with one set of chromosomes
each from the mother and the father. Thus,
the diploid water lily endosperm may represent
an intermediate form between haploid gymnosperms
and triploid angiosperms.
Understanding the origin and genetic constitution
of endosperm is critical to improving
the world's food supply. Two-thirds of
the calories that people consume come
from endosperm filled seeds of wheat,
corn, rice and barley - all of which are
flowering plants.
"Humans co-opted endosperm from its original
purpose of nourishing the plant embryo
to one that essentially feeds the world,"
notes Friedman. Williams adds, "Every
major civilization (except for that of
the Maoris, a people indigenous to New
Zealand) originated on the back of triploid
endosperm."
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All photos: Joseph Williams
The flower of Nuphar polysepalum,
or pond lily.
Photo Credit: Joseph Williams
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(Size: 151KB)
Nuphar polysepalum, or pond lily,
occurs throughout the northern hemisphere
in cold climates, and is pictured here
in a high altitude lake in Colorado, USA.
Photo Credit: Joseph Williams
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Double fertilization in Nuphar involves
the fusion of sperm and egg (not shown)
as well as
a second fertilization event: the fusion
of a sperm nucleus and a central cell
nucleus as
shown here.
Photo Credit: Joseph Williams
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(Size: 52KB)
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