By Ethan Nielsen
Polyploidy refers to when an organism has more than two complete sets of chromosomes. Polyploidy was first discovered nearly one hundred years ago, however, many new insights are being realized today in how it can be used in plant breeding. Using aspects of polyploidy gives plant breeders more options for developing novel plants and improving existing cultivated varieties.
WHAT IS POLYPLOIDY?
Most higher organisms are diploid having two sets of chromosomes with one set of chromosomes donated by the father and one set from the mother. Variations in the number of sets of chromosomes are frequently encountered in nature. Although very common in plants, polyploidy does occur in animals as well (such as some species of fish, salamanders, lizards, and more recently a polyploid rat was discovered in Argentina). In fact, it is estimated that over one third of flowering plant species are polyploid.
Ploidy or ploidy level refers to the the number of complete sets of chromosomes and is notated by an “x.” A diploid organism is 2x, for example Fragaria vesca, the alpine strawberry, is diploid with 2x=14. Polyploidy can occur in many other combinations such as triploids with three sets of chromosomes (3x), four sets are tetraploids (4x), six sets are hexaploids (6x), and so on. It is also important to understand if someone is referring to the reduced gamete chromosome number following meiosis (when a cell divides into cells called gametes, known as sperm or egg cells), notated as “n.” This is also called haploid, referring to the gametic number of chromosomes, essentially meaning the number of chromosomes in a gamete (an egg or pollen cell) that when combined equal 2n. For example Fragaria x ananassa, the domestic strawberry, is octoploid with 2n=8x=56. Aneuploidy refers to chromosome numbers that are not exact multiples of n.
In natural plant populations polyploidy usually results from a mutation during cell division, which can be spontaneous mutations or or due to damage from bacteria or other external factors. One mutation occurs as an interruption of meiosis when the plant cells are dividing and making egg cells or pollen (gametes). Another mutation can occur due to incomplete meiosis and unreduced gametes are produced with the same number of chromosomes as an adult cell. Assuming the original cell was diploid, then if two diploid (2n) gametes are combined it will result in a tetraploid offspring (2x+2x=4x). If one of the diploid gametes joins a normal haploid gamete (1x), a triploid (3x) offspring will result. A similar interruption during mitosis (when a cell divides into two new adult cells) in the cells of a terminal bud could cause one branch of a tree to be tetraploid while the rest of the tree remains diploid, or the reverse could be true of a tetraploid tree with one diploid branch.
How a polyploid plant originates can often affect its fertility and indicate its usefulness in a breeding program. If a plant shoot spontaneously doubles its chromosomes (called somatic doubling) or, alternatively, if two unreduced gametes from two closely related plants (i.e. same species) combine, the new tetraploid will have four similar sets of chromosomes (referred to as homologous chromosomes). Although they arrive from different situations, both tetraploid forms are similar in reproductive aspects and are called “autotetraploids.” These autopolyploids may or may not be fertile. During meiosis of diploid plants, the chromosomes form homologous pairs and separate into two gametes, each with one set of chromosomes. Infertility can arise because the new tetraploid form has more than two sets of homologous chromosomes, and during meiosis there can be trouble in how the cell pairs the chromosomes, resulting in unpaired chromosomes and unbalanced sets of chromosomes (aneuploids).
If a new plant is produced from sexual reproduction of two unreduced gametes from different diploid parents, or somatic doubling of a diploid hybrid (of two unrelated parents or different species), both versions will have four versions of each chromosome. Because the set of chromosomes from one parent are sufficiently different from the other parent, they generally don’t mix during meiosis. These type of polyploids are called allopolyploids, and are generally fertile. During meiosis, each chromosome can pair with its homologous partner resulting in fertile gametes.
POLYPLOIDY IN PLANT EVOLUTION
In contrast with the relatively slow process of isolated populations developing into new species, polyploidy plants can rapidly develop into new species. If the offspring of diploid parents become tetraploids, they will often become reproductively isolated from the parent population, as crossbreeding often develops sterile triploid hybrids. The tetraploid population could interbreed independent of the parental population and may eventually become a new species. In many plant families, closely related plant species often have differing chromosome counts. In the genus Chrysanthemum, different species have chromosome numbers of 2n=18, 36, 54, 72, 90, and 198, all multiples of a base number of 9.
There are different reasons why polyploid plants can have adaptive advantages. In the case of allotetraploids, they have increased heterozygosity (having varied sets of chromosomes). They are essentially two plants in one, and can have four different copies of a gene at the same place on a chromosome, all expressing different things, affecting things such as growth rate, performance and adaptability. In the case of an autopolyploid, because there can be multiple sets of the same gene, the result is similar to that of inbreeding and can produce a plant with weakened vigor and fertility. Because all polyploids exhibit some genetic redundancy, mutations can occur without damaging essential functions, resulting in new traits. Over time, some polyploid populations can mutate and shed extra genes to the point that they become “diploidized” with the original diploid number restored.
Polyploid plants are often self-fertile, and many are apomictic, meaning that seeds are derived from maternal tissue without fertilization. Molecular studies have shown that polyploid plants can have “enzyme multiplicity” in the case of allopolyploids. This means that in theory they can produce the enzymes of each of the parent plants as well as possibly producing new, hybrid enzymes. This could result in the plants having greater environmental adaptability, possibly extending the range in which they could grow, as well as other unforeseen advantages that are yet to be discovered.
POLYPLOIDY IN PLANT BREEDING
Polyploidy can have many effects on plants, but specific effects vary greatly from species to species, as well as from functions of gene expression, heterozygosity, ploidy level, and different traits of individual plants. Because polyploidy can have such a significant effect on plant evolution, it is understandable that when mitosis inhibitors were discovered in the 1930’s there was considerable interest in producing man-made polyploids. Most of the methods involve somatic doubling, and the resulting plants aren’t much improved by having extra copies of the same chromosomes. In fact, many undesirable traits emerge. The increased genetic material results in larger cells, which can result in physiological differences. Some effects are brittle wood, watery fruit, and erratic bearing. High level polyploids, like octoploids or decaploids, can suffer from stunted or abnormal growth. Although there are negative aspects of these autopolyploids, they can be useful in breeding programs to increase the genetic diversity and further select for desirable traits. It appears that in most cases autopolyploids offer few advantages unless their heterozygosity can be increased.
ADVANTAGES OF POLYPLOIDY
There are many advantages of polyploidy that offer great insights and benefits if these secrets can be unlocked from the plants’ DNA. Many uses have been discovered and many more wait to be discovered. These are some of the main reasons why plant breeders use polyploidy in their breeding programs:
Enlargement and Increased Vigor –– Enlarged cell sizes can have negative effects on some plants, but can be advantageous in certain situations. The large cell size can result in thicker leaves, stems, and wider branch angles. Flowers can be larger with thicker petals resulting in longer lasting flowers, and fruit that can be bigger than normal. Fruit from tetraploid apple trees can be twice as big as the diploid apples, unfortunately the fruit quality is diminished, being watery and misshapen. Triploid apples combine to make a happy medium, with larger fruit but still have the eating quality of diploid apples.
Creation of Sterile Triploids –– There are many beautiful landscape plants that can be improved by inducing sterility. For example, plants that produce desirable flowers but are followed by messy fruit would be improved by eliminating the fruit. A landscape plant that is weedy and possibly invasive could be more freely planted if it didn’t produce seed. Perhaps a plant that wasn’t allocating resources for seed creation could make more flowers.
Restoring Fertility in Hybrids –– Sometimes new hybrids are created by crossing plants that aren’t closely related. The genetic line is then stopped because the chromosome pairs are different enough that they don’t properly pair up during meiosis. If the chromosomes are doubled, creating an allopolyploid, the chromosomes now have homologous pairs and fertility is restored. This has been used successfully in Rhododendron ‘Fragrant Affinity’ and xChitalpa tashkentensis. However, this application doesn’t always work.
Overcoming Barriers in Hybridization –– Sometimes a desirable cross cannot be made because of the difference in ploidy level between the two parents. If one parent can be altered to match the other a successful cross could take place. For example a diploid plant could be doubled in order to attempt a cross with a tetraploid plant.
Enhancing Pest Resistance and Stress Tolerance –– Some polyploid plants exhibit greater pest resistance, drought tolerance, increased nutrient uptake, and increased cold tolerance. However, the opposite may also be true. It is impossible to tell what the result will be as they could be improved or diminished, or a mixture of the two. In some cases the pest resistance of one parent can be combined with the pest resistance of the other, with offspring having both traits expressed simultaneously. This could also be true of other environmental stresses, such as heat resistance or drought tolerance.
In the 1930’s it was discovered that a certain chemical applied during cell division can disrupt mitosis creating copies of the chromosomes without dividing the cell resulting in polyploid cells. The chemical first used was colchicine, derived from the autumn crocus, Colchicum autumnale. Since then, other mitotic inhibitors (doubling agents) have been discovered including oryzalin, trifluralin, amiprophos-methyl, and N2O gas.
Methods vary in applying these treatments. Typically a large number of seedlings with small, actively growing meristems are either entirely submerged or just the apical meristems can be treated with a doubling agent. Results and methods vary by species, but often the nature of the chemicals can stunt or even kill the plants. A portion of the remaining survivors might be polyploid. Shoots of mature plants can also be treated but often will result in chimaeras with tissue being a mix of polyploid cells and some incompletely polyploid cells. Surfactants or wetting agents are sometimes applied to help the doubling agent penetrate the cell for a more consistent result.
Plants that have had their ploidy levels increased will often display morphological characteristics such as thicker leaves and stems, shorter internodes, and sometimes decreased growth. Other forms of identification include increased size of pollen, counting the number of chloroplasts in a guard cell, and larger guard cells and stomates. Flow cytometry for measuring DNA content, or using traditional cytology by accurately measuring the cells in young leaves, root tips, and anthers are the best ways to measure ploidy content.
Polyploidy is not in itself a solution for creating better plants, but is a useful tool to incorporate into breeding programs. Understanding the origins and variations in polyploid plants gives the plant breeder an opportunity to use this knowledge to create new and improved plants. Restoring fertility, inducing sterility, creating bigger or stronger plants, developing plants that are more pest resistant or better adapted to their environment are only the beginning of what we know we can do with polyploidy and more applications are yet to be discovered.
Ranney, Thomas G. Polyploidy: From Evolution to New Plant Development. Combined Proceedings International Plant Propagators’ Society, Volume 56, 2006, pp.137-142. http://www.ces.ncsu.edu/fletcher/mcilab/publications/ranney-2006.pdf