- Polyploidy
- How does autopolyploidy occur?
- What is autotriploidy?
- Allopolipolides and autopolyploids
- References
The autopoliploidía is a type of polyploidy (cells having more than two sets of chromosomes in the nucleus), in which an organism or species has two or more identical sets of chromosomes. Therefore, it is the result of the duplication of a group of chromosomes of the same species.
From studies carried out with plants, it has been decided that the criteria to be used to classify polyploids should start from their mode of origin. The variety of mechanisms that occur in both plants and animals allows us to structure two large classes of polyploidy: autopolyploidy and allopolyploidy.
Tigridia pavonia. By Rjcastillo, from Wikimedia Commons
In the case of autopolyploidy, more than two groups of identical chromosomes are combined, so the cell has more than two groups of haploid chromosomes inherited from the parents. These two sets of chromosomes of the parents are duplicated in the children, being able to originate a new species.
There are several types of chromosomes: haploid (single), diploid (double), triploid (triple), and tetraploid (quadruple). Triples and quadruples are, then, examples of polyploidy.
Living beings that have cells with a nucleus (eukaryotes) are diploid, which means that they have two groups of chromosomes, each group coming from a parent. However, in some living beings (mainly plants) it is common to find polyploidy.
Polyploidy
Polyploidy is the condition of cells that have more than two sets of chromosomes in their nucleus, which form pairs called homologues.
Polyploidy can appear due to an abnormality in cell division. This can happen during mitosis (cell division of somatic cells) or during metaphase I of meiosis (cell division of sex cells).
This condition can also be stimulated in cell cultures and plants, using chemical inducers. The best known are colchicine, which could produce a chromosome duplication, like oryzalin.
Furthermore, polyploidy is a mechanism of sympatric speciation, that is, the formation of a species without prior establishment of a geographical barrier between two populations. This occurs because polyploid organisms cannot interbreed with other members of their species that are diploid, most of the time.
An example of polyploidy is the Erythranthe peregrina plant: the chromosomal sequence of this plant confirmed that the species originated from Erythranthe robertsii, a sterile triploid hybrid from the cross between Erythranthe guttata and E. Erythranthe lutea. These species were brought to the UK from another habitat.
Upon naturalization in the new ecosystem, new populations of Erythranthe peregrina appeared in Scotland and the Orkney Islands due to duplication of the genome of local populations of Erythranthe robertsii.
How does autopolyploidy occur?
Autopolyploidy can occur as a result of different processes experienced by a species:
- Simple genomic duplication due to defects in germinal cell division, after mitotic division
- Production and fertilization of gametes not reduced by error in cell division, after meiosis (in animals it basically occurs in eggs)
- Polyspermia, which is when an egg is fertilized by more than one sperm
In addition, there are external factors such as the way of reproduction and the environmental temperature, which can increase the frequency and quantity of autopolyploid production.
Sometimes autopolyploids appear by spontaneous duplication of the somatic genome, as in the case of apple sprouts (Malus domesticus).
This is the most common form of artificially induced polyploidy, where methods such as protoplast fusion or treatment with colchicine, oryzalin, or mitotic inhibitors are applied to disrupt normal mitotic division.
This process activates the production of polyploid cells and can be very useful in the improvement of plants, especially when you want to apply introgression (movement of genes from one species to another by hybridization followed by backcrossing) in the case of oak and birch in plants. and, case of wolves and coyotes in animals.
What is autotriploidy?
Autotriploidy is a condition in which cells contain triple numbers of chromosomes, from the same species, presenting three identical genomes. In plants, autotriploidy is related to apomictic mating (reproduction by means of seeds).
In agriculture, autotriploidy can lead to lack of seeds, as in the case of bananas and watermelons. Triploidy is also applied in salmon and trout culture to induce sterility.
Triploid pups are sterile ("triploid block" phenomenon), but can occasionally contribute to tetraploid formation. This path to tetraploidy is known as the "triploid bridge."
Allopolipolides and autopolyploids
Allopolyploids are species that have more than three sets of chromosomes in their cells, and are more common than autopolyploids, but autopolyploids are given more relevance
Autopolyploids are polyploids with several groups of chromosomes derived from the same taxon (scientific classification group). Examples of natural autopolyploids are the piggyback plant (Tolmiea menzisii) and the white sturgeon (Acipenser transmontanum).
Autopolyploids have at least three groups of homologous chromosomes, this causes high percentages of mating during meiosis and decreased fertility by association.
In natural autopolyploids, the pairing of irregular chromosomes during meiosis causes sterility because multivalent formation takes place.
A species originates from autopolyploidy if the eggs and sperm of the organisms in the population have an accidentally doubled number of chromosomes and when they reproduce with each other they generate tetraploid offspring.
If these offspring mate with each other, a fertile tetraploid progeny genetically isolated from the rest of the population is generated. Thus, single-generation autopolyploidy creates a barrier to gene flow between maturing species and their parent species.
References
- Campbell, NA and Reece, JB (2007). Biology. Madrid: Editorial Médica Panamericana.
- Gregory, T. (2005). The evolution of the genome. San Diego: Elservier Academic Press.
- Hassan Dar, T. and Rehman, R. (2017). Polyploidy: Recen Trends and Future Perspectives. New Delhi: Springer.
- Jenkins, J. (1986). Genetics. Barcelona: Editorial Reverté.
- Niklas, K. (1997). The evolutionary biology of plants. Chicago: The University of Chicago Press.