Polyploidy

The phenotypes of many organisms are modifications from changes in the number of chromosomes in their cells, and sometimes even changes in only a part of them can be important. These numerical changes are usually described as variations in the organism's ploidy (-ploid is a Greek suffix that just means "number of chromosome sets"; hence, diploid would mean two sets of chromosomes).

So, let's learn about polyploidy!

• First, we will look at the definition of polyploidy.
• Then, we will explore the types of polyploidy.
• After, we will look at the difference between aneuploidy and polyploidy.
• Lastly, we will learn about polyploidy in plants, and also look at some examples involving polyploidy.

Polyploidy Definition

The definition of polyploidy is:

For example, if an organism goes from being diploid (2n) to being tetraploid (4n), then it has undergone polyploidy.

To get a better understanding of what polyploidy means, let's revise the concept of "genes". Genes are the blueprint for our biology.

Genes are found in a fixed position (locus) on a chromosome. Each chromosome contains a linear molecule of DNA folded several times on itself, as well as ribonucleic acid (RNA) and proteins.

Now, organisms can have different forms of the same gene, called alleles. For example, G and g are different alleles of the G gene.

• An organism is considered homozygous if it has two identical alleles for a given gene or trait.

• An organism is considered heterozygous if it has two different alleles for a given gene or trait.

Diploid organisms contain a pair of chromosomes, where one chromosome was inherited from the mother, and the other chromosome from the father. Since these chromosomes are the same size, structure and contains the same genes, they are known as homologous chromosomes.

• When a homologous chromosome undergoes DNA replication in the S phase of the cell cycle, it results in the formation of sister chromatids.

Karyotype

The conventional representation of the constitution of homologues within the nucleus is defined as a cell karyotype.

Except egg and sperm cells, which have only 23 chromosomes, all normal human cells contain 23 pairs of chromosomes, or 46 chromosomes in all.

Of the the 23 pairs of human chromosomes, 22 of them are called autosomes, which look the same in males and females. The 23rd pair, called the sex chromosomes, differs between males and females.

There are two sex chromosomes found in humans: X and Y.

• Females have two X chromosomes.

• Males have one X chromosome and one Y chromosome.

The karyotype below human chromosomes lined up in pairs.

During most of the cell cycle, chromosomes are dispersed in the nucleus and cannot be morphologically identified. Only when the cell divides do chromosomes become morphologically evident.

Now that we have this breakdown out of the way, let's hop back to the original track we were on. So, what happens if somehow an error occurs, changing the number of chromosomes or the overall process? We have what is called polyploidy!

Types of Polyploidy

There are two basic types of polyploidy (that are encountered in plants):

• Allopolyploidy
• Autopolyploidy

But before tackling polyploidy, you need to understand a little bit about how bodies make new cells. Plants are a different thing, but all human cells are diploid, so when gametes are created, they must be haploid or have only one set of chromosomes, so that the new organism can be diploid again.

However, during this process, sometimes something goes wrong. The most common occurrence is that a new gamete gets two copies of the chromosomes, which can happen when females produce eggs. When an egg with two sets of chromosomes fuses with a normal haploid sperm, the resulting cell has three sets of chromosomes, that is, it is triploid (3n).

Individuals with an entire supernumerary chromosomal set can be produced by artificial couplings or as a result of accidental events. Under certain circumstances triploid (3n), tetraploid (4n) or higher grade nuclei, can be formed. Each of these levels of polyploidy represents an increase in the number of haploid kits present.

Allopolyploidy

First up is allopolyploidy.

In allopolyploidy, the different chromosomal assets are defined as homeologous, meaning that they are only partially homologous. Problems arise when an individual with a chromosomal mutation mates with an individual that has no chromosomal mutations, resulting in sterility.

• Since the chromosomal sets are not homologous, the hybrid is sterile: pairing is not possible and the gametes are not viable.

It may happen, albeit rarely, that in some F₁ hybrid plants, due to the absence of telophase, the doubling of chromosomes occurs; it follows that the cells of some tissues contain a chromosomal set which is the sum of the two parental sets.

The diploid arrangement, however, ensures that meiosis works regularly. The self-fertilization of these gametes gives rise to fertile plants, as they are produced by regular meiosis, the so-called allotetraploid plants, with two diploid sets.

Consequently, the allotetraploid plant behaves like a normal diploid plant. The doubling of genomes in such crosses leads to the loss of sterility, frequent in hybrids, and to the acquisition of fertility.

Autopolyploidy

Now, let's look at autopolyploidy.

For example, let's say that you have two diploid (2n) plants of the same species that produce haploid (n) gametes; however, due to the lack of meiosis in one of the two parent plants, diploid gametes (2n) can be generated.

The crossing between diploid (2n) and haploid (1n) gametes produces a zygote triploid (3n), from which, in turn, haploid and diploid gametes will derive!

Following self-fertilization (2n × 2n), the autotetraploid zygote (4n) is obtained; the latter, like the other even-numbered autopolyploids, are more likely to be fertile because they can have regular meiosis!

Aneuploidy vs. Polyploidy

As we learned above, organisms with complete or normal arrangements of chromosomes are called euploids (derived from the Greek words meaning "well" and "once").

Organisms carrying extra sets of chromosomes are called polyploids (from the Greek words meaning "many" and "time") and the level of polyploidy is described with reference to a base number of chromosomes, usually indicated by "n" is defined as the number of chromosomes in an arrangement. Thus, diploids with two sets of chromosomes have 2n chromosomes, triploids with three have 3n arrangements, the tetraploids with 4 have 4n, and so on.

On the other hand, organisms with chromosomes, or segments of chromosomes, that are under- or overrepresented are called aneuploids.

Trisomy and Monosomy are examples of aneuploidy. Trisomy means having 2n+1 chromosomes, while monosomy means having 2n-1 chromosomes.

Therefore, the main distinction between aneuploidy and polyploidy is that aneuploidy refers to the partial numerical change of the genome, usually a single chromosome, whereas polyploidy refers to the change of an entire set of chromosomes. Aneuploidy implies a genetic imbalance, while polyploidy does not.

Cytogenetics have also cataloged various types of structural changes in chromosomes.

For example, a piece of a chromosome can be joined to another chromosome or a segment within a chromosome can be reversed. These structural changes are called rearrangements.

Because these rearrangements segregate irregularly during meiosis, they are often associated with aneuploidy.

• Somatic cells have pairs of homologous chromosomes diploid (2n).

• The germ cells that is the oocytes and the spermatozoa (gametes) are called haploid (n).

Polyploidy in Plants

What organisms have polyploidy? Most often it is observed in the plant kingdom. Thousands of years of selective cultivation and plant breeding have resulted in the creation of fertile food plants, which are usually tetraploid and hexaploid.

Scientists have suggested that two-thirds of flowering plants are polyploid. Most ferns and herbs are polyploid, as are potatoes, apples, and strawberries.

Many succulents, as well as ornamental ones, those from seeds, and many fruit trees, are polyploid. Polyploidy is associated, typically, with infertility due to the difficulty of pairing of the chromosomes during meiosis.

The most visible effect of this condition is the lack of seeds in edible fruits, such as bananas (3n). The evolutionary success of polyploid plant species may result from the fact that polyploidy, like gene duplication, provides additional copies of genes; while one copy continues to perform its original function, the others can evolve by developing new functions!

Polyploid organisms can appear spontaneously or be generated by experimentally altering the spindle apparatus during cell division. This result is achieved with the use of colchicine, an alkaloid isolated from the seeds of autumnal Colchicum, which, added during plant cell culture, inhibits the formation of the spindle with consequent blockage of mitosis.

In this way, the absence of migration of chromosomes to the poles determines the formation of a tetraploid nucleus (4n). By repeating the same operation several times, polyploid nuclei are generated.

Among the most well-known polyploid plants are some varieties of ornamental plants with large flowers, such as hyacinths, tulips, and narcissus. In particular, triploids and tetraploids have characteristics of greater value than haploid or diploid plants, relating above all to the increase in cell volume, fruit, and stamens.

Polyploidy Examples

There are far fewer types of polyploid animals than plants. The exact reason for this is not fully known. Some scientists believe that this may be due to an increase in the complexity of the structure of animal organisms compared to plants.

Others suggest that polyploidy can interfere with gamete formation, cell division, or genome regulation. However, there are some exceptions. Examples of polyploidy in the animal kingdom are fish, reptiles, and insects.

Bananas are also an interesting example of polyploidy. Bananas are triploid and usually triploid organisms cannot reproduce, since they are sterile. This means that it is not possible to get banana seeds to sow more bananas. Therefore, farmers cut the shoots off the plant, before they produce fruit and finish their cycle, and plant a new generation.