Which statement about polyploidy in plants is true?

A. Polyploid plants are usually less vigorous than diploid plants of the same species. B. Tetraploid and diploid plants of a given species can mate to produce viable triploid offspring. C. Allopolyploids are hybrid plants that result from the mating of two closely related species. D. Tetraploid plants can mate with other tetraploid plants of the same species, but not with themselves. E. Polyploidy is rare in plants.

The correct answer is C. Allopolyploids are hybrid plants that result from the mating of two closely related species.

Diagram showing how a polyploid (4x) can be produced from diploid (2n) cells
Diagram showing how a polyploid (4x) can be produced from diploid (2n) cells (wikimedia [public domain])

Chromosome number is an important feature of living organisms that is called ploidy. Many plants and animals are diploid, which means that the chromosomes occur as two sets or pairs.

The diploid condition occurs because, in sexually reproducing organisms, one set is inherited from the mother while the other set is inherited from the father.

The gametes of organisms that have sexual reproduction contain half the number of chromosomes and are said to be haploid. After fertilization, the diploid number is then restored by fusion of the gametes.

There are, however, quite a few species of plants and even some animals that have more than two sets of chromosomes. These organisms are said to be polyploidy in that there are more than the expected diploid number of chromosomes.

There are different types of polyploidy but in plants, the allopolyploid condition occurs when hybrids are formed from the mating of two different but closely related species.

These hybrids are sometimes stronger than their ancestors and have some useful traits. Scientists have also found this condition in certain species of animals such as some fish, insects, and amphibians.

Allopolyploidy is believed to have been important in the speciation of certain groups of organisms that have occurred during evolutionary time.

In addition, people have found ways to manipulate the ploidy of plants so as to create useful plant cultivars including certain fruits, vegetables, and flowers.

Ploidy

Ploidy is related to the total number of individual chromosomes that occur within the cell of a living creature. The ploidy is specific to the species of organism involved.

A living organism usually has a chromosome number that is said to be diploid, in other words, it occurs in two sets. For example, human body cells have a diploid number of 46, which means there are 46 individual chromosomes in each of the cells because 23 were inherited from each parent.

The gametes or sex cells that are formed during meiosis each contain half the number, which is the haploid number of chromosomes, which in humans is 23 in sperm cells and 23 in egg cells.

It is important to remember that in the first division that occurs, namely meiosis I, the diploid number of chromosomes is usually halved, such that there will be germ cells at the end of meiosis that have only half the number of chromosomes when compared with the parent cell that divided.

When these sex cells fuse during fertilization the chromosome number is once again restored to the diploid number of 46.

Many plant species also have a diploid number of chromosomes; however, there are some species in the plant kingdom which have more than the diploid number. In other words, there are more than two sets of chromosomes in each autosome or body cell.

Chromosome number

Not all organisms can survive a polyploidy state or can survive if the chromosome number differs slightly from the usual diploid state.

Human fetuses, for instance, may not be viable if there are too few or too many chromosomes other than the 46. A fetus with 47 chromosomes can survive if it is trisomy 21, but the individual will be born with Down syndrome.

People who have Down syndrome have significant physical and intellectual disabilities and often have a shorter than usual lifespan.

Polyploidy

Some plant species have polyploidy, this is when there are three or more sets of chromosomes present in each of the cells rather than the two sets as occurs in the diploid condition.

Polyploidy results from nondisjunction during mitosis or meiosis. Nondisjunction is defined as the failure of chromatids or homologous chromosomes to separate during anaphase of cell division.

The polyploidy condition is common among members of the plant kingdom and has resulted in the formation of new species of plants in some cases. Plants that are polyploidy can often be more vigorous than diploid plants.

An allopolyloid is when there is a hybrid individual which has double the number of homologous chromosomes. It is produced from the mating of two different yet closely related species of plants.

Triticum and Aegilops are two plant genera in which allopolyploidy has occurred naturally, and has led to the evolution of different species. Scientists have also recently discovered that allopolyploidy has been a force in the speciation of certain species in the animal kingdom as well.

In the past, it was thought that polyploidy was limited to the plant kingdom but now there is some evidence that this is not the only group of organisms in which it has occurred. In fact, scientists have also found polyploids in insects, amphibians, water fleas, and fish species.

Manipulation of ploidy

Scientists have discovered that they can manipulate the ploidy of organisms by using chemicals such as colchicine. This can result in the production of plants that have useful traits.

Various cultivars of plants, such as roses, have been produced by modification of the ploidy. It has also been used to produce certain types of fruits and vegetables.

Polyploidy can also result in the production of seedless varieties of fruits because it tends to decrease the fertility of the plant. This is advantageous when cultivating plants as food for humans.

References

  1. MC Sattler, CR Carvalho, WR Clarindo (2016). The polyploidy and its key role in plant breeding. Planta.
  2. Crespel, J. Meynet (2017). Manipulation of ploidy level. Retrieved from sciencedirect.com.
  3. G Vogt (2017). Evolution of Epigenetic Mechanisms in Animals and Their Role in Speciation. Retrieved from sciencedirect.com.
  4. Yoshihiro Matsuoka, et al. (2014). Genetic Mechanisms of Allopolyploid Speciation Through Hybrid Genome Doubling. Retrieved from sciencedirect.com.
  5. Editors of Encyclopedia Britannica (2019). Polyploidy. Retrieved from Encyclopedia Britannica.

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