In this article we will discuss about:- 1. Distribution and Occurrence of Polyploidy 2. Classification of Polyploidy.
Distribution and Occurrence of Polyploidy:
Polyploidy is wide spread in plant kingdom but is less common in animal kingdom. Darlington and Janaki-Ammal in 1945 and Stebbins in 950 estimated that polyploids make up 30 to 35% of Angiosperms. In certain families, very high frequency of polyploid species are found, such as Rosaceae, Polygonaceae, Malvaceae, Crassuiaceae, Nymphaeaceae; nearly 75% of the Gramine are polyploid.
In general, polyploidy is the most prevalent in herbaceous perennials.
Classification of Polyploidy:
Polyploids can be classified into four types. For ease in understanding and explanation, similar genomes are represented by the same capital letter bearing different numbers as subscripts, e.g., A1,A2, A3, etc. while distinct genomes with only limited or no similarities are denoted by different letters, e.g., A, B, C etc.
1. Auto-polyploids (AAAA)
2. Allopolyploids:
(i) Segmental allopolyploids (AXAXA2A2)
(ii) Genome allopolyploids (AABB)
(iii) Auto-allopolyploids (AAAABB, AABBBB AAAABBBB, etc.)
The origin of different kinds of polyploids and their interrelationships are diagrammatically represented in Fig. 17.4.
A specialized situation, called mixoploidy denotes the presence of cells having different chromosome numbers in the same cell population or individual. It has been reported in many plants, such as, maize, barley, Datura, Lycopersicon, and others. Mixoploidy is genetically controlled and it occurs through cellular or nuclear fusion or through endomitosis.
A special kind of genetically controlled mixoploid mutant was detected by Takahashi et al. in 1955 in barley cultivar Kairyobozu. This mutant named “minute” was extremely dwarf and never produced ear. Mixoploid condition resulted from incomplete formation of cell-walls after nuclear divisions leading to the production of polyploid cells ranging from 4x to 30x, besides 2x cells.
The same authors isolated a number of “semi-minute” mixoploid strains from the progeny of a cross between Kairyobozu heterozygous for “minute” and variety Colses, in 1959. The semi- minute plant was much taller than the minute and could produce heads and seeds.
Ploidy level in this plant varied from 2x to 8x in root tip cells and it was found that the mixoploid condition was governed by a recessive gene min located on chromosome 4. Prasad et. al., in 1983 reported that this plant (semi-minute) produces about 3% 2x pollen; this is due to the fusion of haploid nuclei facilitated by the failure of cell wall formation during meiosis.