Apomixis, hybridization and polyploidy are characteristic processes which are more frequently found in plants when compared to animals.
Polyploidy refers to the presence of three or more sets of chromosomes in a single organism. The phenomenon is present mostly in plants and rare in animals. A few of the animal species which exhibit polyploidy are earthworms, certain species of fishes, lizards, amphibians and some insects.
Causes of polyploidy
The major cause of polyploidy is found to the non disjunction of sister chromatids during the meiotic recombination events. Prior to meiosis chromosome number doubles followed by chromosome separation during gamete formation. Autopolyploids results from failure of segregation of chromosomes during game formation. Somatic doubling in apical meristems is another cause and results from mitotic divisions.
Allopolyploids are formed when the hybridization occur between different genome sets. The initial hybrid diploid products with different sets of chromosomes are usually sterile and genetically unstable. However, this can undergo meiosis prior to which it doubles the chromosomes and there are chances to create a genetically stable polyploidy.
Artificial generation of polyplidy have been found to be useful both in research as well as economic purposes. Colchicine, the chemical which interferes with meiotic spidle formation is widely used. This has been widely used in development of newer crop varieties.
Crosses between ploid genotypes have given rise to sterile offspring which are of economic value due to their vegetative parts. Cultivars of banana, Bermuda grass etc are derived from crosses between diploid and tetraploid species. The resultant progeny depends on clonal propagation for reproduction.
Types of polyploidy
There are mainly two types of polyploidy- autopolyploidy and allo(amphi)polyploidy. There are various types under each of these major divisions.
1. Autopolyploidy is the presence of multiple sets of chromosomes derived from a single genotype. This type results mostly from somatic doubling either in zygotes or cells apical meristem region. Since this involving doubling of the existing number of chromosomes, the resultant ploidy is always even. The chromosomes are similar and hence can lead to multivalent pairing at meiosis. The alleleic relationship is difficult to be deciphered making genetic analysis a little complex.
a. Strict autopolyploids occur as a result of genome doubling within the same individual
b. Interracial autopolyploids are the result of crossing between different individuals of similar genotype.
Potato, alfalfa (4x), Sugar beet (3x), Cassava (4x), etc.
2. Allopolyploidy is the result of different genotypes. This is mainly due to meiotic recombination events. The genotypes can be different and the resultant set can be either even or odd. The type of ploidy can be distinguished by analyzing the pattern of meiotic recombination and segregation of chromosomes. Multivalents are found in autoploidy whereas bivalents are found in allopolyploidy. This condition of bivalent pairing increases the rate of fertile offspring in allopolyploids.
a. Segmental allopolyploidy results from hybridization between different ecospecies in the same cenospecies. Cenospecies is organisms of different species which can still interbreed among themselves. The genomes have partial or segmental homology which is sufficient enough to cause sterility in the resultant progeny.
b. Complete allopolyploids result from hybridization between different genotypes and have a complete set of chromosomes from each parent.
c. True or genomic polyploids are derived from hybridization of parents with dissimilar genome sets.
d. Autoallopolyploids are polyloids which has the characteristics of both autopolyploids and allopolyplids. It has duplicated species of one species along with atleast one genome set from a different species. Usually atleast 2 sets of genomes are involved resulting in formation of tetraploids.
e. Aneuploids are formed by gain or loss of single chromosome.
3. Paleodiploid is the ancient polyploidy species which have become diploid in the evolutionary history. They are characterized by large number of chromosomes and are useful in deciphering evolutionary history.
4. Neopolyploids are the newly formed allo and autopolyploid species.
A ploidy series is one with a number of levels within the same species. The series can be either odd or even with the number of chromosome sets it possesses. Aneuploid series is one such succession of allopolyploids with differing chromosome numbers. Sometimes, polyploids loss one or more of its chromosomes resulting in modified polyploidy series which is referred to as polyploidy drop.
Cotton (6x), Oat (6x), wheat (4x or 6x), soybean (4x), tobacco (4x), and coffee (4x).
Importance of Polyploidy
• Polyploidy has been found to be one of major causes of evolution/speciation.
• Increases vigor and size in crop plants. The phenomenon is similar to heterosis and is phenomenal in developing economically important crops and flowering plants.
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