Genetic (cytogenetic) species criterion, along with others, is used to distinguish elementary systematic groups, to analyze the state of a species. In this article, we will consider the characteristics of the criterion, as well as the difficulties that a researcher may encounter when applying it.
What is a view
In different branches of biological science, the species is defined in its own way. From an evolutionary perspective, we can say that a species is a collection of individuals that have similarities in external structure and internal organization, physiological and biochemical processes, capable of unlimited interbreeding, leaving fertile offspring and genetically isolated from similar groups.
A species can be represented by one or several populations and, accordingly, have a whole or dissected range (territory/water area)
Species 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 the generic name, and the adjective is the specific name. For example, in the name "Dandelion officinalis", the species "officinalis" is one of the representatives of the plants of the genus "Dandelion".
Individuals of related species within the genus have some differences in appearance, physiology, and ecological preferences. But if they are too similar, then their species affiliation is determined by the genetic criterion of the species based on the analysis of karyotypes.
Why a species needs criteria
Carl Linnaeus, who was the first to give modern names and describe many types 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 an error in the embodiment of the species idea.
Since the first half of the 19th century, Charles Darwin and his followers have been substantiating a completely different concept of species. In accordance with it, the species is changeable, heterogeneous and includes transitional forms. The constancy of the species is relative, it depends on the variability of environmental conditions. The elementary unit of existence of a species is a population. It is reproductively distinct and meets the genetic criteria of the species.
Given the heterogeneity of individuals of the same species, it can be difficult for scientists to determine the species of organisms or distribute them between systematic groups.
Morphological and genetic criteria of the species, biochemical, physiological, geographical, ecological, behavioral (ethological) - all thiscomplexes of differences between species. They determine the isolation of systematic groups, their reproductive discreteness. And they can be used to distinguish one species from another, to establish the degree of their relationship and position in the biological system.
Characterization 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 chromosomal "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:
- ratio of chromosome arm length;
- the position of the centromeres in them;
- the presence of secondary constrictions and satellites.
Individuals belonging to different species will not be able to interbreed. Even if it is possible to obtain offspring, as with a donkey and a horse, a tiger and a lion, then interspecific hybrids will not be prolific. This is because the halves of the genotype are not the same and conjugation between chromosomes cannot occur, so gametes are not formed.
In the photo: a mule - a sterile hybrid of a donkey and a mare.
Object of study - karyotype
Human karyotype is represented by 46 chromosomes. In most species studied, the number of individual DNA molecules in the nucleus that form chromosomes falls within the range of 12–50. But there are exceptions. The fruit fly Drosophila has 8 chromosomes in the nuclei of cells, and in a small representative of the Lepidoptera family Lysandra, the diploid chromosome set is380.
The electron micrograph of condensed chromosomes, which allows 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 comparison of karyotypes with each other, helps to determine the species of organisms.
When two species are one
The common feature of view criteria is that they are not absolute. This means that the use of only one of them may not be sufficient for an accurate determination. Organisms that are outwardly indistinguishable from each other may be representatives of different species. Here the morphological criterion comes to the aid of the genetic criterion. Double examples:
- Today, two species of black rats are known, which were previously identified as one due to their external identity.
- There are at least 15 species of malarial mosquitoes that can only be distinguished by cytogenetic analysis.
- 17 species of crickets found in North America that are genetically different but phenotypically related to the same species.
- It is believed that among all species of birds there are 5% twins, for the identification of which it is necessary to apply a genetic criterion.
- Confusion in the systematics of mountain bovids has been eliminated thanks to karyological analysis. Three varieties of karyotypes have been identified (2n=54 for mouflons, 56 for argali and argali, and 58 chromosomes for urials).
One species of black rat has 42 chromosomes, the karyotype of the other is represented by 38 DNA molecules.
When one view is like two
For species groups with a large area of the range and the number of individuals, when geographic isolation operates within them or individuals have a wide ecological valence, the presence of individuals with different karyotypes is characteristic. Such a phenomenon is another variant of exceptions in the genetic criterion 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, found in White Sea herring;
- with a diploid set of 50 chromosomes, representatives of different populations of silver carp have 100 (tetraploids), 150 (hexaploids), 200 (octaploids) chromosomes.
Polyploid forms are found in both plants (goat willow) and insects (weevils). House mice and gerbils can have different numbers of chromosomes, not a multiple of the diploid set.
Karyotype twins
Representatives of different classes and types may have karyotypes with the same number of chromosomes. There are much more such coincidences among representatives of the same families and genera:
- Gorillas, orangutans and chimpanzees have a 48-chromosome karyotype. In appearance, the differences are not determined, here you need to compare the order of nucleotides.
- Slight differences in the karyotypes of North American bison and European bison. Both have 60 chromosomes in a diploid set. They will be assigned to the same species if analyzed only by genetic criteria.
- Examples of genetic twins are also found among plants, especially within families. Among the willowsit is even possible to obtain interspecific hybrids.
To reveal subtle differences in the genetic material in such species, it is necessary to determine the sequence of genes and the order in which they are included.
Influence of mutations on the analysis of the criterion
The number of karyotype chromosomes can be changed as a result of genomic mutations - aneuploidy or euploidy.
When aneuploidy occurs in the karyotype, one or more additional chromosomes appear, and the number of chromosomes may also be less than that of a full-fledged individual. The reason for this violation is the non-disjunction of chromosomes at the stage of gamete formation.
The picture shows an example of human aneuploidy (Down syndrome).
Zygotes with a reduced number of chromosomes, as a rule, do not start crushing. And polysomic 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 genetic criteria.
| Mutation | Essence of mutation | Influence on the genetic criterion of the species |
| Tetrasomy | An extra pair of chromosomes or two non-homologous extra chromosomes are present in the karyotype. | When analyzed by this criterion alone, an organism can be classified as having one more pair of chromosomes. |
| Tetraploidy | In the karyotypethere are four chromosomes from each pair instead of two. | An organism can be assigned to another species instead of a polyploid cultivar of the same species (in plants). |
Karyotype multiplication - polyploidy - can also mislead the researcher when the mutant karyotype is the sum of several diploid sets of chromosomes.
Criterion 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 spiralize (condense) and represent dense rod-shaped structures - chromosomes. The average thickness of a chromosome is 700 nm.
School and university laboratories are usually equipped with microscopes with a low magnification (from 8 to 100), it is not possible to see the details of the karyotype in them. The resolving power of a light microscope, in addition, allows at any, even the highest magnification, to see objects not less than half the length of the shortest light wave. The smallest wavelength is for violet waves (400 nm). This means that the smallest object visible in a light microscope will be from 200 nm.
It turns out that the stained decondensed chromatin will look like cloudy areas, and the chromosomes will be visible without details. An electron microscope with a resolution of 0.5 nm allows you to clearly see and compare different karyotypes. Considering the thickness of filamentous DNA (2 nm), it will be clearly visible under such a device.
Cytogenetic criterion at school
For the reasons described above, the use of micropreparations in laboratory work according to the genetic criterion of the species is inappropriate. In tasks, you can use photographs 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.
Example lab assignment
Assignment. Consider the given photographs of karyotypes, compare them and make a conclusion about the belonging of individuals to one or two species.
Photos of karyotypes for lab comparison.
Working on a task. Count the total number of chromosomes in each karyotype photo. If they match, compare them in appearance. If not a karyogram is presented, find the shortest and longest among the chromosomes of medium length in both images, compare them according to the size and location of the centromeres. Make a conclusion about the difference / similarity of karyotypes.
Answers to the task:
- If the number, size and shape of chromosomes match, then the two individuals whose genetic material is presented for study belong to the same species.
- If the number of chromosomes is two times different, and in both photographs there are chromosomes of the same size and shape, then most likely the individuals are representatives of the same species. These will be diploid and tetraploid karyotypes.form.
- If the number of chromosomes is not the same (it 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 the same species (phenomenon of aneuploidy).
- With a different number of chromosomes, as well as a mismatch in the characteristics of size and shape, the criterion will classify the presented individuals as two different species.
In the output, it is required to indicate whether it is possible to determine the species of individuals based on the genetic criterion (and only it).
Answer: it is impossible, because any species criterion, including genetic one, has exceptions and can give an erroneous result of the determination. Accuracy can only be guaranteed by applying a set of criteria of the form.
