Euploidy

Variation from the normal diploid number (2n) may either be haploids or polyploidy.

Monoploidy (Haploidy). A haploid organism contains only one set of chromosomes, and is said to be hemizygous. Each chromosome is represented only once. Haploidy may be normal in a particular species, or may arise as an abnormality. Bacteria and fungi pass most of their life cycle in the haploid condition. The gametophytes of lower plants are haploid. Among animals, haploid forms occur in insects and rotifers.

In Datura, monoploids can be formed, but they do not reproduce because of difficulties in meiosis. Because of the presence of only one set of chromosomes, there is random distribution of chromosomes to the two poles during meiosis. The chromosome behavior in haploids differs from the normal condition.

Polypoids. Polypoids contain three or more sets of genomes. Triploids contain three sets, tetraploids four, pentaploids five, hexaploids six, and so on. Polyploids may be classified as primary or secondary. A primary polypoid is always even numbered.



Autoploids. In autopolyploids there is duplication of the same genome. Natural autopolyploids are rare, because of the resulting anomalies in cell division. Autopolyploids can be identified by the behavior of chromosomes during metaphase I of meiosis.

Alloploids. Alloploids arise form a stock that is a heterozygote. The chromosomes sets are non-homologous, and are initially derived from two different species. Allopoids, therefore, contain different genomes that have come together during hybridization. Alloploidy may result in immediate species formation. However, if doubling of chromosomes takes place, then an allotetraploid will be formed.






Polyploidy in plants. With the exception of fungi, polyploidy occurs naturally in all groups of plants. About half the known plant genera contain polypoids. Tradesacntia has several tetraploid species.

Polyploidy in animals. Polyploidy is comparatively rare in animals because animals have a more delicate sex balance mechanism than plants. Any departure from the diploid number usually results in sterility. Polyproidy is found in the brine shrimp Artemia salina. This crustacean has avoided the difficulties resulting from anomalous gametes by adopting parthenogenesis as the mode of reproduction.
 
Practical application of polyploidy. Tetraploids are usually larger more vigorous than corresponding diploids, and bear larger fruits and seeds.

(1) High temperature. Polypoidy results if certain plants are subjected to higher than normal temperature. Polyploidy has been achieved in certain cells of the maize plant by this method.

(2) Decapitation has been used in tomatoes. After removing the bud, some tissues developing from scar tissue were found to be tetraploid.

(3) Chemicals. Chromosome division without cytoplasmic division results in the doubling of the chromosome number. Other chemical like acenapthene and indole acetic acid have been used to induce polyploidy.