Polyploidy Part 2. And Crop Plants

Polyploidy is whole genome duplication, where all the chromosomes double. Animals rarely survive major chromosome changes, but plants usually do, leading to aspects of plant genetics that are quite different from animals. 

strawberry, Fragaria ananassa

Even though you cannot easily spot a polyploid by looking at it, polyploidy is not obscure. Many common, important plants are polyploid, for example bread wheat (Triticum aestivum), white potatoes (Solanum tuberosum), coffee (Coffea arabica), sweet potatoes (Ipomoea batatas), strawberries (Fragaria x ananassa) and cotton (Gossypium tomentosum). Researchers estimate that 75% of all plants, and likewise, 75% of crops, are polyploids.  

These plants underwent a whole genome duplication in the relatively recent past, so that their cells have three or more copies of the whole genome, not the two copies of humans and familiar animals. 

Two copies are called diploid. More than two copies of the genome are grouped as polyploids and can be specifically named. For example four copies are tetraploid. Or you can say 2x and 4x. Potatoes, coffee, sweet potatoes, and cotton are tetraploid (4 copies of the basic genome), bread wheat is hexaploid (6x) and strawberries are octoploid (8x). 

potato plant, Solanum tuberosum
growing from the eyes on a potato that was thrown into the compost

I was specific above, giving genus and species for the polyploids, because polyploidy varies among plants. Often some species in a group (genus) are polyploid while others not. In each of the genera mentioned above, there are also diploid (2x) plant species. So polyploidy is a form of variation for botanists to compare between plants and wonder about. 

Polyploids have extra genes but they grow and flower like other plants. Where polyploidy changes things is when people try to cross or breed polyploids. Polyploidy changes inheritance, as you might expect. Plants that are diploids, like humans, get one gene from each parent and have two copies. Tetraploids, such as potatoes, get two copies of each chromosome from its parents and have four copies of all its genes all its life. If one of the genes gives a dominant color, then polyploids seem very uniform, because that color gene, whether in one copy or four, makes all the flowers or all the tubers or all whatever, the same color. If you can look inside at the actual DNA sequences, polyploids have more variation than diploids, since each of the four copies of each gene can be different. 

I will put a comparison of diploid and tetraploid inheritance at the end of this post, for those of you who, like me, like transmission genetics.

Beyond genetics, however, polyploids are physically different from diploids, it is just extremely difficult to observe the differences without lots of measurements. The logic is easy: with more chromosomes in the nucleus polyploids have bigger nuclei, bigger cells, and bigger plants. For crop plants, this is usually a good thing. We like bigger potatoes, huge strawberries, and larger flowers.

banana, Musa paradisiaca

Polyploidy sometimes results in sterile plants. That is especially true of uneven multiples such as 3x (triploids) or 5x (pentaploids). Bananas (Musa paradisiaca) have 33 chromosomes, a multiple of three over their wild ancestors. At sexual cell division, the chromosomes go some to one new cell and some to the other. Producing a balanced genome of 11 or 22 is very rare. Cells with only part of the genome will not make a healthy embryos, creating sterility. But banana seeds were annoying to humans; we wanted to eat the fruit around them. So failure to make seeds was a good thing and bananas are propagated by taking cuttings. 

Working around crop sterility caused by polyploidy is common, usually by cloning the crop. Cloning also allows greater uniformity, which makes large scale production much easier. 

Polyploidy is so common in plants that it is very diverse. The plants can have different numbers of chromosomes. The chromosomes can all be derived from one parent species, or could come from a hybrid between two different species which doubled its chromosomes. For example, potatoes, as described above, have four nearly identical copies of each chromosome. Bread wheat has six copies of the basic seven chromosomes, but got them from three different wild wheats. First two diploid wild wheats (Triticum monococcum with chromosome set A and an unknown Triticum with chromosome set B) formed an AB hybrid which later doubled all its chromosomes. The original AB hybrid was sterile, the tetrapolid, AABB, fertile. This species exists, it is Triticum turgidum. A second round of hybridization combined T. turgidum’s AB chromosomes with another set of chromosomes, called D, from Triticum taushii. Again, the hybrid ABD was sterile, but when all the chromosomes doubled, it created our modern bread wheat Triticum aestivum, AABBDD, which is fertile, Both processes, internal doubling (autopolyploidy) and hybridization followed by doubling (allopolyploidy), occur frequently. 

wheat, Triticum aestivum, ripe 

One result of the amount of polyploidy among crop plants is that breeding particular plants is a specialty. Shifting from say, potatoes to wheat requires learning a lot of wheat-specific details of inheritance. In contrast the mammals we know well, people, dogs, cats, horses, and cows, for example, are all diploids, so we tend to think of inheritance as simple and obvious. Polyploidy and the diversity of polyploids among economically important plants means plant breeding is much more specialized and a breakthrough in one species often cannot be immediately applied to others. 

I have loved genetics since grade school, and I think the complexities of polyploid inheritance fascinating. But I do see that plant breeders probably find it aggravating. 

In crops polyploidy feels like a problem to work around. So why is it there? Is it adaptive? Surely 75% of the plants aren't growing at an evolutionary disadvantage. In future posts I'll consider ideas about the functions of polyploidy.

Comments and corrections welcome.

Botanical Details. Comparison of Diploid and Tetraploid Inheritance

Diploids: Cross two pure lines of the same plant species, one with red (WW) and the other with white (ww) flowers. The plants raised from their seeds are red flowered, genotype Ww,  because W red is dominant to white.  Now cross those: Ww diploid heterozygote makes gametes ½ W and ½ w

 

Those combine like this, W with W and w, w with W and w, making ¼ WW, ½ Ww and ¼ ww offspring: 

 

	½ W	½ w
½ W	¼ WW	¼ Ww
½ w	¼ Ww	¼ ww

 

3 red-flowered plants to 1 white-flowered plant. 

Tetraploids: Imagine this is a close relative of the diploid in the example above, so it is the same gene, W for red, dominant over w, white flowers. Cross two pure lines, one red WWWW and one white wwww. The progeny have red flowers, and are WWww heterozygotes. When they are crossed, each plant makes gametes that are ¼ WW ½ Ww and ¼ ww.

 

Those combine 

 

	¼ WW	½ Ww	¼ ww
¼ WW	1/16 WWWW	1/8 WWWw	1/16 WWww
½ Ww	1/8 WWWw	¼ WWww	1/8 Wwww
¼ ww	1/16 WWww	1/8 Wwww	1/16 wwww

 

15 red-flowered plants to 1 white-flowered plant. 

Ploidy changes inheritance.

Note that inheritance in triploids (3x) and hexaploids (6x) and ... are slightly to very different from diploids and tetraploids.

References

Muenchrath, D., A. Campbell, L. Merrick, T. Lübberstedt, and S. Fei. (2023). Ploidy: Polyploidy, aneuploidy, haploidy. In W. P. Suza, & K. R. Lamkey (Eds.), Crop Genetics. Iowa State University Digital Press. DOI: 10.31274/isudp.2023.130 link (Accessed 2/27/24).

years of studying polyploid plants

Kathy Keeler

A Wandering Botanist

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