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Aneuploidy

All organisms, tissues or cells that contain n chromosomes or a multiple of it are termed euploid in contrast to sets that have either an additional chromosome or lack one. These are called aneuploid. The most common case are trisomies (sing. trisomy) where a single additional chromosome is present.

Chromosomal sets that contain either n or a multiple of it (2n, 3n,....Xn) are called euploid while those that have an additional chromosome or lack one are called aneuploid (e. g. 2n+1, 2n-1, 3n+1, 3n+2, 3n-1 etc.).

In aneuploid sets, the balance of the chromosomes is disturbed so that aneuploidy can nearly always be detected by growth anomalies if it is not lethal from the outset. Single additional chromosomes, so-called trisomies (sing. trisomy), cause the smallest damages. Such mutants have been found and characterized in nearly all cultivated species. Especially impressive are the disorders found in Datura stramonium (Thorn apple; n = 12). Every single chromosome of the set leads to abnormal changes in the shape of the fruit that are typical for the respective chromosome (A. F. BLAKESLEE, 1921, 1934).

Aneuploidy: Fruits of Datura-Plants

On top: Control plant (2n) Below: Mutants that are characterized by one additional chromosome each. Just like in Oenothera, Datura-Chromosomes are characterized by complex heterozygosis. They are accordingly written with a point between two numbers. (according to A. F. BLAKESLEE and A. G. AVERY et al., 1959)

The loss of a chromosome has more severe effects. Numerous such mutants have been observed in maize but all of them proved to be sterile. Fertile mutants were only found in highly polyploid species like the hexaploid wheat (Triticum aestivum). Self-pollination of mutants that lack a chromosome can generate plants that are characterized by the loss of a pair of chromosomes so-called nullisomics.

Nullisomics-Mutants of the Ear in Wheat (Triticum aestivum)

Ear shape of the wild type (WT) and of several mutants lacking single pairs of chromosomes (nullisomics). Due to the hexaploidy, the lack of a pair of chromosomes is tolerated. The effects are usually different though stunted growth is usually one of them. (E. R. SEARS, 1953)