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Sci Tech
Lily provides missing link
The water lily may represent an intermediate form between haploid gymnosperms and triploid angiosperms.
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 in Nature.
A distinguishing feature of flowering plants is that each seed consists of two parts. An embryo that is similar to that of all other plants, and a unique tissue called `endosperm', which functions to nourish the embryo and which 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 contrasts with the seeds of gymnosperms, in which the nourishing tissue is `haploid' containing a single copy of each chromosome.
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 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 plant embryo to one that essentially feeds the world, notes Friedman.
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