Evolution and Systematic Significance of Polyploidy

Any deviation from the normal chromosome number of an organism at any stage in its life cycle is termed heteroploidy. Changes in chromosome number which either increase or decrease the number by a complete genome are termed euploidy. Polyploidy is a type of euploidy that occurs when an organism possesses three or more complete genomes. These definitions apply to whole organisms, organs, tissues, or single cells. Reports of polyploidy in a variety of organisms came after the development of suitable techniques for staining chromosomes. Much of the early work was concerned with production of polyploidy and study of resultant changes in cell and organ shape and size. Changes in chromosome number were explained usually in terms of geometrical increase of somatic chromosomes, and there was little information on mechanisms of arithmetical increase until the publication of Winge's (159) classic paper. Using Chrysanthemum as a model, Winge set up a series of theoretical species and then suggested the possible outcome of various crosses. By crossing species A and B and then doubling the chromosome number in the F1 hybrid, theoretical tetraploid species D was produced. Species D was then backcrossed to A to produce a triploid, thus giving an arithmetical series. Winge speculated that his theoretical triploid might behave cytologically so that species A chromosomes paired with A's from the tetraploid and the B genome might remain unpaired, a phenomenon that fitted published cytological observations on certain Drosera hybrids (120). Additional examples of arithmetical increases in genomes were soon observed in nature (57, 160, 161), but experimental verification of Winge's hypothesis was obtained by spontaneous doubling in an artificial hybrid of Nicotiana glutinosus X N. tobacum (29). This method of origin and the chromosome behavior were essentially those anticipated by Winge's model, but the doubled hybrid was genomically hexaploid instead of tetraploid as described.