Polyploidy in crop evolution and breeding
Euploidy and aneuploidy, auto- and allopolyploids, how wheat, cotton and Brassica crops arose, colchicine, seedless triploids, and chromosome sums.
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Bread wheat carries three complete genomes from three different wild grasses. Potato and alfalfa have four copies of each chromosome, the common banana three, and strawberry eight. Many of the world's most important crops are polyploids: they carry more than two complete sets of chromosomes. Polyploidy has shaped plant evolution, created several major crops, and gives breeders tools for making seedless fruit, bridging species and restoring fertility in wide hybrids.
Polyploidy and the related changes in chromosome number are standard exam topics in cytogenetics and plant breeding.
The vocabulary
- x: the basic chromosome number, the number in one basic set (7 in wheat and its relatives).
- n: the gametic number, the chromosomes in a gamete.
- 2n: the somatic number, in body cells.
In a diploid, 2n = 2x. In hexaploid bread wheat, 2n = 6x = 42, so x = 7 and n = 21.
Euploidy and aneuploidy
- Euploids have whole sets: monoploid (x), diploid (2x), triploid (3x), tetraploid (4x) and so on. Plants with more than two sets are polyploids. Haploids have the gametic number, n, whatever the ploidy of the species.
- Aneuploids have extra or missing single chromosomes:
- monosomic 2n − 1, nullisomic 2n − 2 (both copies of one chromosome missing);
- trisomic 2n + 1, tetrasomic 2n + 2.
Hexaploid wheat tolerates the loss of a chromosome pair because its other two genomes carry similar genes; E. R. Sears used complete sets of nullisomic and other aneuploid lines from the 1950s to locate wheat genes on chromosomes.
Autopolyploids and allopolyploids
- Autopolyploids have multiple sets from the same species, such as AAAA. Examples: potato (4x), alfalfa (4x), cultivated banana (mostly 3x), some grapes and apples (3x), tetraploid ryegrass and red clover, seedless watermelon (3x). Because four or more similar chromosomes pair in meiosis, forming multivalents, autopolyploids often have reduced fertility, and they show tetrasomic inheritance.
- Allopolyploids combine sets from different species, such as AABB. They arise when two species hybridise and the hybrid's chromosome number doubles. Because each chromosome has a partner from the same species, pairing is regular (bivalents), and they behave like diploids, with disomic inheritance. Examples: bread wheat, durum wheat, cotton, oat, tobacco, canola and brown mustard, peanut, arabica coffee and strawberry.
How polyploid crops arose
Wheat
- Durum (pasta) wheat, Triticum turgidum, 2n = 4x = 28, genomes AABB, arose when wild einkorn-related T. urartu (A genome) hybridised with a goatgrass related to Aegilops speltoides (B genome), long before agriculture.
- Bread wheat, T. aestivum, 2n = 6x = 42, AABBDD, arose about 8,000 to 10,000 years ago in farmers' fields, when cultivated tetraploid wheat crossed with the goatgrass Aegilops tauschii (D genome). The D genome brought adaptation to continental climates and the gluten qualities used in bread.
Brassica crops: the triangle of U
Three diploid species and their three allotetraploid hybrids include most of the world's cabbages, mustards and oilseed rape.
Cotton
Upland and Egyptian cottons (Gossypium hirsutum and G. barbadense, 2n = 4x = 52, AADD) arose from an ancient hybrid between an African–Asian A-genome species and an American D-genome species.
Synthetic polyploids
- Raphanobrassica: in 1928 G. D. Karpechenko crossed radish with cabbage and doubled the chromosomes, making the first fertile synthetic allopolyploid; it had a radish's leaves and a cabbage's root, the reverse of what he hoped.
- Triticale: wheat × rye, with doubled chromosomes. Hexaploid triticale (AABBRR, 2n = 42) is grown on several million hectares, mainly in Europe, as feed grain.
Making polyploids
- Unreduced gametes (2n gametes), formed when meiosis fails, are the main natural route.
- Colchicine, an alkaloid from the autumn crocus, blocks spindle formation, so chromosomes double without the cell dividing. A. F. Blakeslee and A. G. Avery introduced it to plant breeding in 1937. Herbicides such as oryzalin and trifluralin act similarly.
- Doubled haploids: haploid plants from anther or microspore culture or wide crosses are doubled to give completely homozygous lines in one step.
Effects of polyploidy
- Gigas effect: larger cells, often larger leaves, flowers, fruits and seeds, although not always higher yield.
- Slower growth and later flowering in many induced polyploids.
- Reduced fertility in autopolyploids and in odd-numbered polyploids.
- Buffering: duplicate genes mask harmful mutations and allow new functions to evolve.
- Hybrid vigour fixed: allopolyploids keep the combination of their parental genomes in every generation.
Why triploid watermelons are seedless
A diploid watermelon has 2n = 22. Breeders double it with colchicine to make a tetraploid (2n = 44) and cross the tetraploid, as the female, with a diploid.
- Tetraploid gametes: 22 chromosomes; diploid gametes: 11.
- Triploid seed: 22 + 11 = 33 chromosomes (3x).
In the triploid's meiosis, each of the 11 chromosome types has three copies, which go one to one pole and two to the other at random. A gamete is balanced only if it gets exactly one copy of every chromosome or exactly two of every one:
- Probability = 2 × (1⁄2)11 = 2 ÷ 2,048 = 1 in 1,024.
Almost all gametes are unbalanced and fail, so the fruit develops without viable seeds. Triploid plants still need pollen from diploid plants planted nearby to stimulate fruit set. This method was developed by H. Kihara in Japan around 1950.
Key points
- x = basic number, n = gametic number, 2n = somatic number; bread wheat is 2n = 6x = 42.
- Euploids have whole sets; aneuploids are monosomic (2n − 1), nullisomic (2n − 2), trisomic (2n + 1) or tetrasomic (2n + 2).
- Autopolyploids (potato, alfalfa, banana) have repeated sets of one genome and often reduced fertility; allopolyploids (wheat, cotton, canola) combine genomes of different species and behave like diploids.
- Bread wheat is AABBDD; the triangle of U explains Brassica species.
- Colchicine doubles chromosomes; triploids are seedless because almost all their gametes are unbalanced.
What the exams ask
- School-leaving and entrance papers: definitions of polyploidy, examples of polyploid crops, and the use of colchicine.
- Degree and research-fellowship entrances: euploidy and aneuploidy, auto- and allopolyploids, the origin of wheat and Brassica (triangle of U), Raphanobrassica, triticale, seedless watermelon, and chromosome calculations.
- Breeding and research examinations: doubled haploids, aneuploid gene mapping, and bridging crosses with wild relatives.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Are polyploid crops genetically modified?
No. Polyploidy occurs naturally and is very common in plants; most of the polyploid crops people eat arose without human intervention, and colchicine doubling does not insert foreign genes.
Is bigger always better in polyploids?
No. Induced polyploids often have larger organs but grow more slowly and set fewer seeds. Successful polyploid crops have been selected over many generations.
Why does banana have no seeds?
Cultivated bananas are mostly triploid and set fruit without fertilisation (parthenocarpy). Their unbalanced gametes rarely produce seeds, which is why bananas are propagated from suckers and tissue culture.
Sources
- U, N. "Genome analysis in Brassica with special reference to the experimental formation of B. napus and peculiar mode of fertilization." Japanese Journal of Botany 7, 389–452 (1935).
- Blakeslee, A. F. and Avery, A. G. "Methods of inducing doubling of chromosomes in plants by treatment with colchicine." Journal of Heredity 28, 393–411 (1937).
- Marcussen, T. et al. "Ancient hybridizations among the ancestral genomes of bread wheat." Science 345, 1250092 (2014).
- Soltis, P. S. and Soltis, D. E. "The role of hybridization and polyploidy in plant evolution." Annual Review of Plant Biology 60, 561–588 (2009).
- Singh, R. J. Plant Cytogenetics, 3rd edition. CRC Press, 2016.
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