Phenotypic Effects of Aneuploidy
Euploidy
Euploidy
Euploidy in Animals
A genome that contains three or more full copies of the haploid chromosome number are polyploid. As a general rule polyploids can be tolerated in plants, but are rarely found in animals. One reason is that the sex balance is important in animals and variation from the diploid number results in sterility. Those few animals, such as brine shrimp, that avoid the hazards of polyploidy, utilize parthenogenesis, the development of the an individual from an egg without fertilization, to initiate embryo development.
Euploidy in plants
Before we discuss polyploidy in plants in detail, first a distinction must be made between the two major classes of polyploids, autopolyploids and allopolyploids. The following definitions will rely on these chromosomal descriptions. Two species will be considered, A and B. The chromosomal compositions of one species is:A = a1 + a2 + a3 . . . an
where a1, a2, etc. represent individual chromosomes and n is the haploid chromosome number. The chromosomal composition of the second species will be:
B = b1 + b2 + b3 . . . bn
Autopolyploid - an individual that has an additional set of chromosomes that are identical to parental species; an autotriploid would have the chromosomal composition of AAA and an autotetraploid would be AAAA; both of these are in comparison to the diploid with the chromosomal composition of AA
Allopolyploid - an individual that has an additional set of chromosomes derived from another species; these typically occur after chromosomal doubling and their chromosomal composition would be AABB; if both species have the same number of chromosomes then the derived species would be an allotetraploid
An autotriploid could occur if a normal gamete (n) unites with a gamete that has not undergone a reduction and is thus 2n. The zygote would be 3n. Triploids could also be produced by mating a diploid (gametes = n) with a tetraploid (gametes = 2n) to produce an individual that is 3n. The difficulty arises when autotriploids try to mate because unbalanced gametes are produced because of pairing problems with the additional chromosome set. Thus, these are invariably sterile.
Autotetraploids occur from a doubling of the chromosomal composition. This can occur naturally by doubling sometime during the life cycle or artificially through the application of heat, cold or the chemical colchicine. Because an additional set of chromosomes exists, autotetraploids can (but not necessarily in all cases) undergo normal meiosis.
One generalization that has been made is that autopolyploids are larger than their diploid counterpart. For example, their flowers and fruits are larger in size which appears to be the result of larger cell size than cell number. This increased size does offer some commercial advantages. Important triploid plants include, some potatoes, bananas, watermelons and Winesap apples. All of these crops must be propagated asexually. Examples of tetraploids are alfalfa, coffee, peanuts and McIntosh apples. These also are larger and grow more vigorously.
The chromosomal composition of allopolyploids is derived from two different species. The classic experiment that initiated research in allopolyploids was performed by G. Karpechenko in 1928. He knew that cabbage and radish both had a diploid number of 18 chromosomes, and he surmised that if he crossed these two species he should be able to derive offspring with 18 chromosomes. His applied goal was to develop a new plant that contained radish roots and cabbage heads. To his disappointment all of the progeny from the cross appeared to be sterile. It is suggested that this occurred because correct pairing was not possible between the two sets of chromosomes and synapsis and normal disjunction were not possible. Thus, all of the gametes were non-functional.
Surprisingly, though, one day he noticed that some seeds did appear. These were grown, and chromosomal analysis revealed that their diploid number was 36. Apparently, chromosomal doubling had occurred. Therefore balanced gametes were generated because each chromosome had a partner with which to pair. This type of situation where a polyploid is formed from the union of complete sets of chromosomes from two species and their subsequent doubling is called amphidiplpoidy and the species is called an amphidiploid. (As a side note, Karpechenko's experiment produced plants with cabbage roots and radish tops.)
Copyright © 1997. Phillip McClean