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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