The genetic (cytogenetic) criterion of the species, along with others, is used to distinguish between elementary systematic groups and analyze the state of the species. In this article, we consider the characteristics of the criterion, as well as the difficulties that the researcher who uses it may encounter.
What is a view?
In different branches of biological science, the species is determined in its own way. From an evolutionary perspective, one can say that a species is a collection of individuals that have similarities in the external structure and internal organization, physiological and biochemical processes that are capable of unlimited crossbreeding, abandonment of prolific offspring and genetically isolated from similar groups.
A species can be represented by one or several populations and, accordingly, have a whole or dissected habitat (territory / water area)
Type Nomenclature
Each species has its own name. In accordance with the rules of binary nomenclature, it consists of two words: a noun and an adjective. The noun is a generic name, and the adjective is a generic name. For example, in the name "Medicinal Dandelion", the species "medicinal" is one of the representatives of plants of the genus "Dandelion".
Individuals of related species within the genus have some differences in appearance, physiology, and environmental preferences. But if they are too similar, then their species affiliation determines the genetic criterion of the species based on the analysis of karyotypes.
Why does the mind need criteria
Carl Linnaeus, the first to give modern names and describe many species of living organisms, considered them unchanged and non-variable. That is, all individuals correspond to a single species image, and any deviations from it are a mistake in the embodiment of the species idea.
From the first half of the 19th century, Charles Darwin and his followers substantiate a completely different concept of the species. In accordance with it, the species is variable, heterogeneous and includes transitional forms. The constancy of the species is relative, it depends on the variability of environmental conditions. The elementary unit of the existence of a species is a population. It is reproductively isolated and meets the genetic criteria of the species.
Given the heterogeneity of individuals of one species, it is difficult for scientists to determine the species affiliation of organisms or distribute them among systematic groups.
Morphological and genetic criteria of a species, biochemical, physiological, geographical, ecological, behavioral (ethological) - all these are complexes of differences between species. They determine the isolation of systematic groups, their reproductive discreteness. And one can distinguish one species from another, establish the degree of their relationship and position in the biological system.
Characteristic of the genetic criterion of the species
The essence of this trait is that all individuals of the same species have the same karyotype.
A karyotype is a kind of chromosome "passport" of an organism; it is determined by the number of chromosomes present in mature somatic cells of the body, their size and structural features:
- the ratio of the length of the shoulders of the chromosomes;
- the position of the centromeres in them;
- the presence of secondary constrictions and satellites.
Individuals of different species will not be able to interbreed. Even if offspring are possible, as with a donkey and a horse, a tiger and a lion, interspecific hybrids will not be prolific. This is because the halves of the genotype are not the same and the conjugation between the chromosomes cannot occur, therefore gametes are not formed.
In the photo: the mule is a barren hybrid of a donkey and a mare.
Object of study - karyotype
The human karyotype is represented by 46 chromosomes. In most species studied, the number of individual DNA molecules in the nucleus that form the chromosomes falls within the range of 12-50. But there are exceptions. The fruit fly of Drosophila has 8 chromosomes in the nuclei of the cells, and in the small representative of the family Lepidoptera Lysandra the diploid chromosome set is 380.
An electronic micrograph of condensed chromosomes, allowing us to evaluate their shape and size, reflects the karyotype. Analysis of the karyotype as part of the study of the genetic criterion, as well as a comparison of the karyotypes among themselves, helps to determine the species affiliation of organisms.
When two species as one
A common attribute of species criteria is that they are not absolute. This means that using only one of them may not be sufficient for an accurate determination. Organisms that are apparently indistinguishable from each other can be representatives of different species. Here the morphological comes to the aid of the genetic criterion. Examples of doubles:
- To date, two species of black rats are known, which were previously defined as one due to external identity.
- There are at least 15 species of malaria mosquitoes that are distinguishable only through cytogenetic analysis.
- In North America, 17 species of crickets were found that have genetic differences, but are phenotypically related to a single species.
- Among all bird species, it is believed that there are 5% of twins, the identification of which requires the use of a genetic criterion.
- The confusion in the taxonomy of mountain canopies was eliminated thanks to karyological analysis. Three types of karyotypes were distinguished (2n = 54 for mouflons, 56 for argali and argali, and 58 chromosomes for urials).
One of the species of black rats has 42 chromosomes; the karyotype of the other is represented by 38 DNA molecules.
When one kind is like two
For species groups with a large area of distribution and the number of individuals, when geographical isolation acts within them or individuals have a wide ecological valency, the presence of individuals with different karyotypes is characteristic. This phenomenon is another variant of exceptions in the genetic criteria of the species.
Examples of chromosomal and genomic polymorphism are common in fish:
- in rainbow trout, the number of chromosomes varies from 58 to 64;
- two karyomorphs, with 52 and 54 chromosomes, were found in the White Sea herring;
- with a diploid set of 50 chromosomes, representatives of different populations of crucian carp have 100 (tetraploids), 150 (hexaploids), 200 (octaploids) chromosomes.
Polyploid forms are found in plants (goat willow), and in insects (weevils). House mice and gerbils may have a different number of chromosomes, not a multiple of the diploid set.
Karyotype counterparts
Representatives of different classes and types may have karyotypes with one number of chromosomes. Much more such coincidences among representatives of the same families and clans:
- Gorillas, orangutans, and chimpanzees have a karyotype of 48 chromosomes. In appearance, the differences are not determined, here it is necessary to compare the order of nucleotides.
- Slight differences in karyotypes of the North American bison and European bison. Both have 60 chromosomes in a diploid set. They will be assigned to one species if the analysis is carried out only by genetic criteria.
- Examples of genetic counterparts are found among plants, especially within families. Among willows, it is even possible to obtain interspecific hybrids.
To identify subtle differences in the genetic material of such species, it is necessary to determine the sequence of genes and the order of their inclusion.
The effect of mutations on the analysis of the criterion
The number of chromosomes of the karyotype can be changed as a result of genomic mutations - aneuploidy or euploidy.
With aneuploidy, one or several additional chromosomes appears in the karyotype, and there may also be a smaller number of chromosomes than in a full-fledged individual. The reason for this violation is the non-divergence of chromosomes at the stage of gamete formation.
The figure shows an example of aneuploidy in humans (Down syndrome).
Zygotes with a reduced number of chromosomes, as a rule, do not start crushing. And polysome organisms (with "extra" chromosomes ") may well be viable. In the case of trisomy (2n + 1) or pentasomy (2n + 3), an odd number of chromosomes will indicate an anomaly. Tetrasomy (2n + 2) can lead to an actual error in determining the species by the genetic criterion.
The effect of genomic mutations on karyotype analysis| Mutation | The essence of the mutation | Effect on the genetic criterion of the species |
| Tetrasomy | In the karyotype, there is an additional pair of chromosomes or two non-homologous accessory chromosomes. | When analyzing only by this criterion, an organism can be assigned to a species that has one pair of chromosomes more. |
| Tetraploidy | The karyotype contains four chromosomes from each pair instead of two. | The organism can be assigned to another species instead of a polyploid variety of the same species (in plants). |
Multiplication of the karyotype — polyploidy — can also mislead the researcher when the mutant karyotype represents the sum of several diploid sets of chromosomes.
Criteria complexity: elusive DNA
The diameter of the DNA strand in the untwisted state is 2 nm. The genetic criterion determines the karyotype in the period preceding cell division, when thin DNA molecules repeatedly spiral (condense) and are dense rod-shaped structures - chromosomes. The thickness of the chromosome is on average 700 nm.
School and university laboratories are usually equipped with microscopes with a small increase (from 8 to 100), it is not possible to consider the details of the karyotype in them. The resolution of a light microscope, in addition, allows you to see objects at least half the length of the shortest light wave at any, even the largest magnification. The shortest wavelength is for violet waves (400 nm). This means that the smallest object, distinguishable in a light microscope, will be from 200 nm.
It turns out that the colored decondensed chromatin will look like clouded areas, and the chromosomes will be visible without details. Clearly see and compare different karyotypes among themselves allows an electron microscope with a resolution of 0.5 nm. Given the thickness of the filamentous DNA (2 nm), it will be clearly visible under such a device.
School cytogenetic criterion
For the reasons described above, the use of micropreparations in laboratory work according to the genetic criterion of the species is impractical. In tasks, you can use photos of chromosomes obtained under an electron microscope. For the convenience of working in the photo, individual chromosomes are combined into homologous pairs and arranged in order. Such a scheme is called a karyogram.
Lab job example
The task. Consider the given photographs of karyotypes, compare them and draw a conclusion about the belonging of individuals to one or two species.
Photos of karyotypes for comparison in the laboratory.
Work on the task. Count the total number of chromosomes in each photo of the karyotype. In case of coincidence, compare them in appearance. If not a karyogram is presented, among the chromosomes of medium length, find the shortest and longest in both images, compare them in size and location of centromeres. Make a conclusion about the difference / similarity of karyotypes.
Answers to the task:
- If the number, size and shape of the chromosomes coincide, then two individuals whose genetic material is presented for study belong to the same species.
- If the number of chromosomes is two times different, and the same chromosomes are found in both photos in the same size and shape, then most likely the individuals are representatives of the same species. These will be karyotypes of diploid and tetraploid forms.
- If the number of chromosomes is not the same (differs by one or two), but in general the shape and size of the chromosomes of both karyotypes are the same, we are talking about normal and mutant forms of one species (aneuploidy phenomenon).
- With a different number of chromosomes, as well as a mismatch in the characteristics of size and shape, the criterion will classify the individuals represented as two different species.
In the conclusion, it is required to indicate whether it is possible to determine the species affiliation of individuals on the basis of the genetic criterion (and only it).
Answer: it is impossible, since any species criterion, including genetic, has exceptions and can give an erroneous determination result. Accuracy can only be guaranteed by the application of a set of view criteria.