Polyploid (in Greek: πολλαπλόν - multiple) cells or organisms contain more than one copy (ploidy) of their chromosomes. Polyploidy occurs in animals but is especially common among flowering plants, including both wild and cultivated species. Wheat, for example, after millennia of hybridization and modification by humans, has strains that are diploid (two sets of chromosomes), tetraploid (four sets of chromosomes) with the common name of durum or macaroni wheat, and hexaploid (six sets of chromosomes) with the common name of bread wheat.
Polyploidy can be induced in cell culture by some chemicals: the best known is colchicine, which causes chromosome doubling.
Usually, multiple methods are used, identifying tetraploid seedlings using their phenotype in flats before transplanting, the chloroplast number in the stomatal guard cells of the true leaves in seedling flats and greenhouse pots, and by the appearance of the fruit and seeds at harvest after self-pollination in the greenhouse.
Tetraploid plants are selected (using methods such as leaf guard cell chloroplast number) in the T1 generation (plants from colchicine treated diploids) from the greenhouse flats where they were treated with colchicine.
A useful combination is for the tetraploid parent to have fruit with gray rind, the diploid parent to have fruit with wide stripes, so the resulting triploid hybrid will striped fruit, easily distinguished from the gray-fruited tetraploids that result from self- or sib-pollination of the female parent.
The usefulness of diploid maps in the breeding of tetraploid alfalfa depends on a high degree of similarity between the genomes of diploid and tetraploid alfalfa.
This map, which represents the genome of cultivated, tetraploid alfalfa, reduced to the diploid level, was constructed in a backcross population and consists of a map for each parent, joined by common markers.
Evidence that supports conservation of the diploid and tetraploid alfalfa genomes is the fact that they interbreed freely, and are considered to be subspecies of one another.
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