EUPLOIDY: ORIGIN, TYPES AND CONSEQUENCES - BIOLOGY - 2025

The euploidía refers to the condition of some cells with the basic haploid chromosome number characteristic of a particular species, or an exact multiple of the haploid number.

Euploidy can also be described as the normal diploid number of chromosomes in a cell or the existence of additional complete sets of chromosomes, calling one member of each pair of homologous chromosomes a set.

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Alterations in the number of chromosomes or sets of chromosomes are closely related to the evolution of many species of plants and of various diseases in the human species.

Origin of euploidy

The life cycles that involve changes between a haploid chromosomal structure and a diploid constitution and vice versa, are those that give rise to euploidy.

Haploid organisms have a single set of chromosomes for most of their life cycle. Diploid organisms, on the other hand, contain a pair of complete sets of chromosomes (homologous chromosomes) throughout most of their life cycle. In the latter case, each set of chromosomes is usually obtained through each parent.

When an organism has more than the diploid number of sets of chromosomes, it is considered polyploid. These cases are especially common in plant species.

Types of euploidy

There are some types of euploidy, which are classified according to the number of sets of chromosomes present in the cells of the body. There are monoploids with one set of chromosomes (n), diploids with two sets of chromosomes (2n), and polyploids with more than two sets of chromosomes.

Monoploidy is the basic chromosomal constitution of organisms. Generally, in animals and plants, the haploid and the monoploid numbers coincide, with haploidy being the exclusive chromosomal endowment of the gametes.

Within the polyploids are triploids with three chromosomal sets (3n), tetraploids (4n), pentaploids (5n), hexaploids (6n), heptaploids (7n) and octaploids (8n).

Haploidy and diploidy

Haploidy and diploidy are found in various species of the plant and animal kingdom, and in most organisms both phases occur in their life cycles. Angiosperm plants (flowering plants) and the human species are examples of organisms that present both phases.

Humans are diploid, since we have a set of maternal and a paternal chromosomes. However, during our life cycle, the production of haploid cells (sperm and eggs) occurs, which are responsible for providing one of the sets of chromosomes to the next generation.

The haploid cells produced in flowering plants are pollen and embryo sac. These cells are responsible for starting a new generation of diploid individuals.

Polyploidy

It is in the plant kingdom where it is more common to find polyploid organisms. Some cultivated species of great economic and social importance for human beings, originated from polyploidy. Some of these species are: cotton, tobacco, oats, potatoes, ornamental flowers, wheat, etc.

In animals we find polyploid cells in some tissues such as the liver. Some hermaphroditic animals, such as the bog (leeches and earthworms), present polyploidism. We also found polyploid nuclei in animals with parthenogenetic reproduction such as some aphids and rotifers.

Polyploidy is very rare in higher animal species. This is due to the high sensitivity of animals to alterations in the number of chromosomes. This low tolerance perhaps corresponds to the fact that sexual determination in animals obeys a fine balance between the number of autosomes and sex chromosomes.

Polyploidy is considered as a mechanism capable of increasing the genetic and phenotypic variability of many species. This is advantageous for species that cannot change their environment and must adapt quickly to changes in it.

Euploidy as a chromosomal abnormality

Among the chromosomal alterations we find numerical alterations and alterations or aberrations in their structures. The deletions or additions of sets of chromosomes are responsible for the appearance of various alterations in the number of chromosomes.

When the change in the number of chromosomes results in exact multiples of the haploid number, euploidy occurs. On the contrary, when the deletion or addition of chromosomes involves only one set of chromosomes (a member or several members of homologous pairs), then it is aneuploidy.

Changes in chromosome numbers in cells can be produced by chromosomal nondisjunction, an anaphasic delay in the movement of chromosomes towards cell poles or by changes in the number of chromosomes of gametes that involve the repetition of various sets chromosomal.

The factors that cause nondisjunction are not well understood. Some viruses of the paramyxovirus (mumps virus) and herpesvirus (herpes simplex virus) families could be involved in nondisjunction.

These viruses have been linked to the achromatic spindle of cells, increasing nondisjunction by breaking the union of the centromeres in the spindle fibers.

Consequences of euploidy

Euploidy carries important biological consequences. The deletion or addition of complete sets of chromosomes have been transcendental evolutionary tools in wild plant species and of agricultural interest.

Polyploidy is an important type of euploidy involved in the specialization of many plants through genetic variability, being more common to find them in them.

Plants are sessile organisms that must tolerate environmental changes, unlike animals, capable of moving from a hostile environment to one that they can tolerate more efficiently.

In animals, euploidy is the cause of various diseases and afflictions. In most cases, the various types of euploidy that occur in an early embryonic state cause the non-viability of said embryo, and therefore early abortions.

For example, some cases of euploidy in placental villi have been associated with conditions such as congenital communicating hydrocephalus (or Chiari type II malformation).

The euploidies found in these cells cause villi with low amounts of fibrin on their surface, a uniform coverage of microvilli on the trophoblast and this with an often cylindrical diameter. These characteristics are related to the development of this type of hydrocephalus.

References

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