Water Lily
May Provide A "Missing Link" In The Evolution Of Flowering Plants
National
Science Foundation
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."
Click
here for a complete list of books about endosperms
|
