The monoploidy refers to the number of chromosomes that constitutes a basic chromosome set (x) in an organism; This means that the homologous pairs are not found in the chromosomal set. Monoloidy is characteristic in haploid (n) organisms in which there is only one chromosome for each type.
A monoploid organism carries a single set of chromosomes through most of its life cycle. In nature, whole organisms with this type of euploidy are rare. In contrast, polyploidy is a more widespread type of euploidy in higher organisms such as plants..
Polyploidy is the possession of several sets of homologous chromosomes in the genome. There can then be triploid organisms (3n), tetrapolides (4n) and so on, according to the number of complete sets present in the cell nucleus..
On the other hand, according to the origin of the chromosomes, a polyploid individual can be autopolyploid (autoploid) when the chromosomal endowments are from a single species or allopolyploid (alloploid) when they come from several evolutionarily close species.
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Monoploidy should not be confused with the existence of haploid cells. The haploid number (n) that is used in many occasions to describe the chromosomal load, strictly refers to the number of chromosomes in the gametes which are the female or male reproductive cells.
In most animals and many known plants, the monoploid number coincides with the haploid number, therefore "n" or "x" (or for example 2n and 2x) can be used interchangeably. However, in species such as wheat, which is a hexaploid species, these chromosomal terms are not coincident..
In wheat (Triticum aestivum), the monoploid number (x) does not coincide with the haploid number (n). Wheat has 42 chromosomes and is also a hexaploid species (allopolyploid), since its chromosomal sets do not come from a single parent species); this species has six series of seven quite similar but not the same chromosomes.
Thus 6X = 42, which indicates that the monoploid number is x = 7. On the other hand, wheat gametes contain 21 chromosomes, so 2n = 42 and n = 21 in their chromosome endowment.
In the germ cells of a monoploid organism, meiosis does not occur normally because the chromosomes do not have their counterparts with which to mate. For this reason, monoploids are typically sterile..
Mutations due to errors in the separation of homologous chromosomes during meiosis are the main reason for the existence of monoploids.
Monoloid individuals can occur naturally in populations as rare errors or aberrations. As monoploid individuals, the gametophytic phases of lower plants and males of organisms with sexual determination by haploidy can be considered..
The latter occurs in many orders of insects, including hymenoptera with castes (ants, wasps, and bees), homoptera, thrips, coleoptera, and some groups of arachnids and rotifers..
In most of these organisms the males are normally monoploid, since they come from unfertilized eggs. Commonly, monoploid organisms are prevented from producing fertile offspring, however, in most of these the production of gametes occurs in a normal way (by mitotic division), since they are already adapted.
Monoploidy and diploidy (2n) are found throughout the animal and plant kingdoms, experiencing these conditions during their normal life cycles. In the human species, for example, part of the life cycle is responsible, despite being diploid organisms, of generating monoploid cells (haploid), for the generation of the zygote.
The same occurs in most higher plants where pollen and female gametes have monoploid nuclei.
Haploid individuals, as an abnormal condition, occur more frequently in the plant kingdom than the animal kingdom. In this last group, there are really few references to natural or caused monoploidy.
Even in some organisms so widely studied with Drosophila haploids have never been found. However, diploid individuals have been found with some haploid tissues..
Other cases of monoploidy described in the animal kingdom are salamanders induced by partition of the female gamete in the period of time between the entry of the sperm and the fusion of the two pronuclei.
In addition, there are some aquatic lizards obtained by treatment with low temperatures, in various species of frog such as Rana fusca, R. pipiens, R. japonica, R. nigromaculata and R. rugosa obtained by insemination of females with sperm treated with UV or chemical treatments.
The possibility of a monoploid animal reaching adulthood is very small, which is why this phenomenon may be uninteresting in the animal kingdom. However, to investigate gene action in the early stages of development, monoploidy can be useful, since genes can be manifested when they are in a hemizygous condition..
Monoloids play an important role in current approaches to genetic improvement. Diploidy is an obstacle when it comes to inducing and selecting new mutations in plants and new combinations of genes that are already present..
For recessive mutations to be expressed they have to be made homozygous; favorable gene combinations in heterozygotes are destroyed during meiosis. Monoploids can get around some of these problems.
In some plants, monoploids can be obtained artificially from the products of meiosis in the anthers of the plant. These can undergo cold treatments and assign what would be a pollen grain to an embryo (small mass of dividing cells). This embryo can grow on agar to give rise to a monoploid plant.
One application of monoploids is to search for favorable gene combinations and then, from agents such as colchicine, give rise to a homozygous diploid capable of producing viable seeds through homozygous lines..
Another utility of monoploids is that their cells can be treated as if they were a population of haploid organisms in the processes of mutagenesis and selection..
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