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#polyploidy

12 public questions tagged with this topic.

Plant Breeding 4

Autotetraploids are generally:

Autotetraploids contain four homologous chromosome sets of same species genome, AAAA, derived from doubling of diploid AA genome via colchicine treatment that inhibits spindle formation. Presence of four allele copies increases cell size, nuclear volume, and organ dimensions termed gigas effect due to increased gene dosage and protein production per cell. Leaves become thicker, stomata larger, flowers bigger, fruits heavier, and biomass enhanced, contributing to increased vigor in forage crops like alfalfa and rye grass and ornamental flowers. Photosynthetic rate may rise due to larger chlorop

Ref: Randolph LF. 1932 Autopolyploid vigor; Stebbins GL. Gigas effect. Acquaah Autotetraploid breeding

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Plant Breeding 4

Major limitation of newly synthesized allopolyploids is:

Newly synthesized allopolyploids face meiotic irregularities despite possession of homologous sets because homoeologous chromosomes from related parental genomes may still pair multivalently when Ph-like controls absent, leading to interchanges, aneuploid gametes, and reduced pollen viability. Genomic shock disrupts gene regulation: duplicate homoeologs exhibit expression bias, silencing, and epigenetic repatterning through methylation changes, small RNA perturbations, and transposon mobilization, causing phenotypic instability and transcriptome shock. Initial polyploids often show low seed fe

Ref: McClintock B. Genomic shock; Otto & Whitton 2000 Annual Rev Genetics – limitations of synthetic polyploids

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Plant Breeding 4

Allopolyploids originate due to:

Allopolyploids arise when two distinct species hybridize and their combined chromosome sets undergo doubling, producing new species containing both parental genomes in duplicated form. Initial interspecific F1 hybrid is typically sterile because homoeologous chromosomes from different species lack sufficient homology for regular bivalent pairing at meiosis I, causing univalents and aborted gametes due to irregular segregation. Chance somatic doubling via nondisjunction or unreduced gamete fusion doubles each parental complement, providing each chromosome a homologous partner to pair as bivalen

Ref: Stebbins GL. 1947 Types of polyploids; Chen ZJ. Genetics of allopolyploid formation. Nature Reviews Genetics

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Plant Breeding 4

Optimum ploidy level for sugar beet yield is:

In sugar beet Beta vulgaris, yield comprises root biomass and sucrose percentage, traits influenced by cell size, leaf area, and breeding system efficiency. Diploid 2n=18, triploid 3n=27, and tetraploid 4n=36 have been evaluated extensively in European trials. Tetraploids display reduced fertility, irregular meiosis with quadrivalents, slower growth, and lower seed set, while diploids have normal fertility but smaller cells and less vigor. Triploid hybrid produced by crossing tetraploid female × diploid male combines benefits: increased cell volume and leaf vigor from polyploid gigas effect, i

Ref: Munerati et al. Sugar beet polyploid breeding; Bosemark NO. 1993 – Triploid optimum J Sugar Beet Res

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Plant Breeding 4

Seedless watermelon is an example of:

Seedless watermelon is classic application of triploid sterility exploiting 3x block. Normal diploid watermelon has 2n=22. Using colchicine, breeders create tetraploid lines 4n=44 by doubling diploid seedlings and selecting fertile tetraploids. Tetraploid is then used as female parent crossed to diploid male, yielding triploid hybrid 3n=33. Triploid hybrid grows vigorously with larger leaves due to gigas effect of polyploidy but meiosis is chaotic because three homologs cannot segregate evenly, causing ovule and pollen abortion and highly sterile flowers. Fruit develops through stimulation by

Ref: Kihara H. 1951 Triploid watermelon history; Maynard DN. Watermelon breeding. USDA

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Plant Breeding 4

Triploid plants are generally:

Triploids carry three complete sets of chromosomes, 3x, resulting from cross between tetraploid 4x and diploid 2x parents or from failure of reduction in meiosis producing unreduced 2n gamete fertilized by normal n gamete. During prophase I homologous pairing must partition three copies of each chromosome; trivalent formation, or bivalent plus univalent, is common, leading to unbalanced segregation and aneuploid gametes with variable chromosome numbers ranging from n to 2n. Gametes receiving incomplete complement are typically non-viable, pollen abortion high, embryo sac degeneration frequent.

Ref: Marks GE. The sterility of triploids – meiotic basis. Ramsey & Schemske 1998 Ecology of polyploids

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Plant Breeding 4

Colchicine induces polyploidy by inhibiting:

Colchicine, an alkaloid from Colchicum autumnale, binds tubulin dimers and prevents microtubule polymerization essential for spindle fibre formation during mitosis and meiosis. Without functional spindle apparatus formed from microtubules organizing at kinetochores, sister chromatids or homologous chromosomes fail to separate to opposite poles at anaphase; nuclear membrane reforms around doubled chromosome complement. Cell undergoes endoreduplication producing diplochromosome and polyploid nucleus with doubled DNA content. Treatment of meristematic tissue or seedlings with 0.1-0.5% colchicine

Ref: Eigsti OJ & Dustin P. Colchicine in Agriculture – mechanism spindle inhibition. Blakeslee 1937 Science

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SPECIATION

Polyploidy commonly causes sympatric speciation in:

Sympatric speciation arises within same geographic area without physical barrier, often via polyploidy in plants where chromosome doubling creates instant reproductive isolation because tetraploids produce sterile triploid hybrids with diploid parents. Host shifts in insects and disruptive selection also drive sympatric divergence. Molecular studies show cichlid fishes diverging in same lake. This mode demonstrates speciation without geographic separation, defined as Plants. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-NET, highligh

Ref: Coyne & Orr, Speciation, Chapter 1: Species Concepts and Isolation.

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Chromosome variations

Colchicine induces polyploidy by

Colchicine, an alkaloid from Colchicum autumnale, binds to tubulin dimers and blocks microtubule polymerization. Without functional spindle fibers, duplicated chromatids cannot migrate to poles at anaphase, and kinetochore tension fails. Cells exit mitosis with doubled chromosome number in a single restitution nucleus, creating polyploid or C-mitotic cells. In plant breeding, seedling meristems soaked in low colchicine produce tetraploids with larger organs, thicker leaves and increased vigor. Oryzalin and trifluralin act similarly. Effect is specific to spindle disruption, not direct mutagene

Ref: NCBI Bookshelf, Plant Breeding: Polyploid Induction; Griffiths, Chapter 8: Experimental Polyploidy

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Chromosome variations

Polyploidy refers to

Polyploidy is a numerical chromosome variation involving whole genome addition beyond the diploid complement. Euploid changes produce 3n, 4n, 6n sets arising from failure of mitotic or meiotic segregation, polyspermy, or somatic doubling. Unlike aneuploidy where single chromosomes are added or lost, polyploidy duplicates entire sets, often better tolerated in plants than animals due to more flexible development and gene redundancy. Triploids, tetraploids and higher ploids show increased cell size, altered gene dosage, and frequent sterility when odd-numbered ploidies disrupt pairing.

Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 12: Chromosome Number Variations – Polyploidy

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Basics of Inheritance

Polyploidy refers to condition with

Polyploidy refers to condition carrying more than two chromosome sets, for example triploid 3n, tetraploid 4n, hexaploid 6n, arising via failure of mitotic or meiotic division or interspecific hybridisation. Autopolyploids duplicate same genome, allopolyploids combine distinct parental genomes. Extra dosage influences gene expression, cell size, fertility and adaptation, exploited in agriculture for wheat, potato and cotton. One set monoploid, two diploid, only sex chromosomes irrelevant, whole-genome multiplication represents major evolutionary mechanism. Understanding this mechanism aids ped

Ref: Snustad & Simmons, Principles of Genetics, 7th ed., Chapter 9: Polyploidy

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