Breeding of Fruit and Plantation Crops

Polyploidy

• An organism having more than two sets of homologous chromosomes is known as a polyploid. Polyploidy is of general occurrence in plants while it is rare amongst animals.
• If the somatic chromosome sets in a diploid be represented by AA BB CC then the genome, i.e., the number in the genomes will be A B C. If this is represented by ‘n’ then the simple polyploid series would be:
• 2n – diploid
• 3n – triploid
• 4n – tetraploid
• 5n – pentaploid
• 6n – hexaploid
• 7n – heptaploid
• 8n – octaploid
• 9n – Nonaploid
• 10n – decaploid and so on
• Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploids, and many lineage show evidence of ancient polyploidy (paleopolyploidy) in their genomes.
• Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (e.g. Potato) and allopolyploids (e.g. canola, wheat and cotton) can be found among both wild and domesticated plant species.
• Most polyploids display heterosis relative to their parental species. The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and or expression levels. Many of these rapid changes contribute to reproductive isolation and speciation.