Polyploids in Plants

Polyploids in Plants

• Polyploids are organisms possessing multiple sets of chromosomes exceeding the diploid number.
• Polyploidy is widespread in nature, crucial for adaptation and speciation.
• Approximately 50-70% of angiosperms, encompassing numerous crops, have experienced polyploidy during their evolutionary journey.

Classification of Polyploids

• Polyploids are categorized based on their chromosomal composition into euploids and aneuploids.
• Euploids, comprising the majority, have complete multiple sets of chromosomes specific to a species.
• Euploids are further classified into autopolyploids and allopolyploids based on genome composition.

Autopolyploidy

• Autopolyploidy involves the presence of multiple copies of the same basic set of chromosomes within a species.
• This arises from the failure of chromosomes to separate during meiosis, resulting in unreduced gametes.
• Natural examples include tetraploid alfalfa, peanuts, potatoes, coffee, and triploid bananas.

Allopolyploidy

• Allopolyploidy results from the combination of genomes from distinct species.
• This typically involves hybridization followed by chromosome doubling.
• Economically significant allopolyploid crops include strawberries, wheat, oats, upland cotton, oilseed rape, blueberries, and mustard.

Aneuploidy

• Aneuploids differ from euploids by possessing additions or subtractions of one or more specific chromosomes.
• Aneuploids originate from meiotic errors, such as non-disjunction, leading to an uneven distribution of chromosomes.
• Different categories of aneuploids include monosomy (2n-1), nullisomy (2n-2), trisomy (2n+1), tetrasomy (2n+2), and pentasomy (2n+3).

Applications of Polyploids

• Mutation Breeding: Polyploids tolerate deleterious allele modifications post-mutation and possess higher mutation frequency due to duplicated genes. This higher frequency can be utilized to induce mutations in diploid cultivars that do not generate enough genetic variation after treatment with mutagens.

• Seedless Fruits: Polyploids, such as induced autotetraploids in watermelons, are used to produce seedless triploid hybrids.

• Bridge Crossing: This method utilizes the reproductive superiority of polyploids to overcome sexual incompatibility issues between two species by creating fertile hybrid bridge species.

• Ornamental and Forage Breeding: Polyploidy results in larger plant organs, termed the "giga effect," owing primarily to increased water content. While this provides enlarged seeds and elevated seed protein contents, it is often offset by lower seed set. Enhanced qualities in blossoms and flower periods make polyploids valuable in ornamental breeding.

• Disease Resistance: Aneuploidy, used in breeding strategies for disease-resistant crops (e.g., leaf rust resistance in wheat). This might entail chromosome deletion, substitution, or the presence of supernumerary chromosomes.

• Industrial Applications: Polyploids, like tetraploid Dioscorea zingiberensis, enhance the production of secondary metabolites (e.g., alkaloids and terpenes), boosting the synthesis of sex hormones and corticosteroids. This also offers resistance to pests and pathogens.

Advantages of Polyploids

• Enlarged Size and Increased Vigor: Polyploids often display enhanced vigor, an advantage exploited in breeding.
• Restoration of Fertility: Polyploids can restore fertility in species that exhibit infertility during hybridization.
• Overcoming Hybridization Barriers: They can bridge the reproductive gap found between incompatible species.
• Pest and Stress Tolerance: Some polyploids exhibit improved tolerance to pest or stress.
• Creation of Sterile Triploids: Polyploids are vital for generating sterile hybrid triploid species.

Disadvantages of Polyploids

• Inbreeding Problems: Inbreeding can result in various issues with polyploid crops.
• Sterility Issues: Polyploidy can negatively impact sterility rates or lead to infertility to varying degrees.
• Inheritance Problems: Polyploidy has implications on the patterns of inheritance, impacting their use in population genetics work.

Chimeras

• Plants or parts of a plant that are composed of genetically distinct cell layers.

• Mericlinal: A mutation occurs in one layer causing changes along the side of the apex.

• Periclinal: A mutation occurs within a layer at the top of the plant's apical region affecting the whole layer and is stable.

• Sectorial: Mutations occur in multiple layers. The area affected is stable.