WHAT IS THE CHROMOSOME ENVELOPE? (WITH EXAMPLES) - BIOLOGY - 2025

The chromosome endowment, chromosome complement or chromosome set defines the total number of chromosomes that represent the genome of each species. Every living organism is made up of cells that have a characteristic number of chromosomes.

Those that contain a double complement of chromosomes are said to be diploid ('2n'). Those that contain a single chromosome set ('n') are said to be haploid.

Chromosomes A from a female human being. Taken from wikimedia.org

The chromosome endowment represents the total number of DNA molecules in which all the genetic information that defines a species is inscribed. In sexually reproducing organisms, somatic '2n' cells have two copies of each somatic chromosome.

If sex is chromosomally defined, they also have a sexual pair. Sex 'n' cells, or gametes, have only one chromosome from each pair.

In humans, for example, the chromosomal complement of each somatic cell is 46. That is, 22 autosomal pairs plus one sexual pair. In the gametes of the species, each one of them therefore has a chromosome set of 23 chromosomes.

When we speak of the chromosome endowment of a species we are referring strictly to the set of chromosomes of a series that we call A. In many species, there is another series of supernumerary chromosomes that is called B.

This should not be confused with ploidy changes, which involve changes in the number of the A-series chromosomes.

The chromosomes that define a species

Since the 20s of the twentieth century it was known that the number of chromosomes per species did not seem to be stable. The stable and standard set of chromosomes of a species was called series A. Supernumerary chromosomes, which were not copies of those of series A, were called series B.

Evolutionarily speaking, a B chromosome is derived from an A chromosome, but it is not its copy. They are not essential for the survival of the species, and only present some individuals of the population.

There may be variations in the number of chromosomes (aneuploidy), or in the full complement of chromosomes (euploidy). But it will always refer to the chromosomes of the A series. This number or chromosome endowment, of the A series, is the one that chromosomally defines the species.

A haploid cell of a certain species contains a chromosomal complement. A diploid contains two, and a triploid contains three. A chromosomal complement contains and represents the genome of the species.

Therefore, two or three more complements do not make a different species: it remains the same. Even in the same organism we can observe haploid, diploid and polyploid cells. In other conditions this can be abnormal and lead to the appearance of defects and diseases.

What defines a species is its genome - distributed in as many A chromosomes as its individuals present. This number is characteristic of the species, which may be, but not its information, identical to that of another.

Changes in chromosome number

We have already seen how in the individuals of certain species some cells may have only one or two chromosomal endowments. That is, the number of chromosomal complements varies, but the genome is always the same.

The set of chromosomes that defines a species and its individuals is analyzed through their karyotypes. The karyotypic traits of organisms, particularly in number, are particularly stable in the evolution and definition of species.

However, in some species, between related species, and in particular individuals, there may be significant changes in chromosome makeup.

We will give some examples here that are not related to the changes in ploidy that are discussed in other articles.

-Changes at the level of evolutionary lineages

The biological rule is that there is chromosomal conservatism that guarantees viable gametes by meiosis, and successful fertilization during fertilization.

Organisms of the same species, species of the same genus, tend to conserve their chromosome endowment. This can be observed even in higher taxonomic ranges.

Lepidoptera

Different species of the order Lepidoptera tend to retain the same chromosome endowment. pixnio.com

However, there are many exceptions. In Lepidoptera, for example, extremes of both cases are observed. This family of insects includes organisms that we collectively call butterflies.

However, Lepidoptera represent one of the most diverse animal groups. There are more than 180,000 species grouped in no less than 126 families.

Most of the families of the order have a modal chromosome set of 30 or 31 chromosomes. In other words, the order, despite the large number of species it includes, is quite conservative in chromosome endowment. However, in some cases, the opposite is also true.

The Hesperiidae family of the order Lepidoptera contains about 4,000 species. But within it we find taxa with modal numbers of, for example, 28, 29, 30 or 31 chromosomes. In some of their tribes, however, variations as great as 5 to 50 chromosomes per species are found.

Within the same species it is also common to find variations in the number of chromosomes between individuals. In some cases, it is attributable to the presence of B chromosomes.

But in others, they are variations of the A chromosomes. In the same species, individuals with haploid numbers that vary between 28 and 53 chromosomes can be found.

-Changes at the cell level of the same individual

Somatic polyploidy

In the world of fungi, it is quite common to find changes in the copy number of chromosomes due to environmental changes. These changes can affect a particular chromosome (aneuploidy) or the entire set of chromosomes (euploidy).

These changes do not involve meiotic cell division. This consideration is important because it shows that the phenomenon is not the product of some recombinational distortion.

On the contrary, the genomic plasticity of fungi in general, thus accounts for their surprising adaptability to the most varied life circumstances.

This heterogeneous mixture of cell types with different ploidies in the same individual has also been observed in other organisms. The human being not only has diploid cells (which are almost all), and haploid gametes. In fact, there is a mixture of diploids and polyploids in the hepatocyte and megakaryocyte populations in a normal manner.

Cancer

One of the defining characteristics of cancer development is chromosomal instability. Cell populations can be found in cancer with complex heterogeneous karyotypic patterns.

That is, an individual has a normal karyotype in their somatic cells during their lifetime. But the development of a particular cancer is associated with a change in the number and / or morphology of its chromosomes.

The numerical changes lead to the aneuploid condition of cells that have lost some chromosome. In the same tumor there may be aneuploid cells for different chromosomes.

Other alterations in number can lead to duplications of a homologous chromosome, but not the other member of the pair.

In addition to contributing to the progression of cancer, these changes complicate therapies aimed at attacking the disease. Cells are no longer, not even genomically speaking, the same.

The information content and its organization is different, and the expression patterns of genes have also changed. Furthermore, in each tumor there may be a mixture of expression patterns, different in identity and magnitude.

References

1. Lukhtanov, VA (2014) Chromosome number evolution in skippers (Lepidoptera, Hesperiidae). Comparative Cytogenetics, 8: 275-291.
2. Rubtsov, NB, Borisov, YM (2018) Sequence composition and evolution of mammalian B chromosomes. Genes 9, doi: 10.3390 / genes9100490.
3. Todd, RT, Forche, A., Selmecki, A. (2017) Ploidy variation in fungi - polyploidy, aneuploidy, and genome evolution. Microbiology Spectrum 5, doi: 10.1128 / microbiolspec.FUNK-0051-2016.
4. Vargas-Rondón, N., Villegas, VE, Rondón-Lagos, M. (2018) The role of chromosomal instability in cancer and therapeutic responses. Cancers, doi: 10.3390 / cancers10010004.
5. Vijay, A., Garg, I., Ashraf, MZ (2018) Perspective: DNA copy number variations in cardiovascular diseases. Epigenetics nsights, 11: 1-9.