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What is cytogenetics?

Cytogenetics aims to study nuclear cell DNA, and specially, during asexual cell division, called mitosis. During this step, nuclear chromatin, thinly reticulated, condenses and folds to form individualized bodies, the chromosomes. Chromosomes can be spread onto microscope slides, stained and observed, before photographs and analyses.

Which data are obtained?

Like any other structure in living organisms, chromosomes display a wide diversity. At individual scale, all cells possess identical chromosomes (except reproductive cells and cell showing chromosomal abnormalities, frequently observed in cancers). But between species, karyotypes (the whole chromosome set in a cell) are often different.

  • Different by the number

The karyotypes of two Antarctic Acanthomorpha are shown below.

Notothenia cyanobrancha
Notothenia cyanobrancha has a diploïd number of 48 chromosomes per cell.
Left: chromosomes are classified by pairs and by decreasing sizes;
right: photographs of the metaphase plates they come from (scale bars 10 µm). Standard Giemsa staining.
Notothenia rossii
Notothenia rossii has a diploïd number of 24 chromosomes per cell.

  • Different by their morphology

Chromosomes can show two main shapes:

 A "X" shape, with two arms of more or less equal size: forme en « X » Such chromosomes are called "metacentric". They have two arms linked by a centromere, situated here at the very centre of the chromosome.
 A "inverted V" shape with two short arms non present or scarcely visible: forme en « V inversé » The centromere is subterminal or terminal. Such chromosomes are called "acrocentric".

N. cyanobrancha  shows 4 metacentric chromosomes and 44 acrocentric chromosomes. It's chromosomal formula is 4m + 44a.
N. rossii shows 24 metacentric chromosomes and no acrocentric one. It's chromosomal formula is 24m.

The first type of analysis, which  consists of determining chromosome number and formula concerns the chromosomal macrostructure. But chromosome number and formula are not always exclusive to a species. For instance, the two Antarctic Acanthomorpha species genus Artedidraco, well differentiated at morphological level, both have 46 chromosomes (like human and potatoes!) and even chromosomal formulae are identical between the two species.

Artedidraco orianae
Artedidraco orianae.
Artedidraco orianae karyotype photo
Artedidraco orianae karyotype photo.
Artedidraco shackletoni
Artedidraco shackletoni.

Here is an example of C-banding (centromeric bands) obtained in Notothenia rossii: chromosomes are heated, and then stained and the areas that are rich in repeated DNA sequences appear darker (the centromeric regions and a few terminal bands). This type of study concerns the chromosomal microstructure. Among Acanthomorpha, chromosomes are often small, numerous and difficult to characterize by banding techniques currently used in mammals.

Photo of C-banding in the chromosomes of N. rossii
Photo of C-banding in the chromosomes of N. rossii.

So then, how can we differentiate and separate species at chromosomal level ?

Molecular cytogenetics (or cytogenomics) allows to describe the chromosomal ultrastructure by localizing genes or particular sequences in the DNA (molecular probes) by the fluorescent in situ hybridization technique (FISH). Chromsomes can be thus compared on the base of their genetic stucture.

Principe de la FISH
FISH Principle.

For instance, localization of  DNA probes coding for 28S ribosomal RNA  by FISH  helps to go deeper in the chromosomal analysis (below, this technique applied to the chromosomes of Notothenia rossii).

28S ribosomal RNA gene mapping on the chromosomes of N. rossii
28S ribosomal RNA gene mapping on the chromosomes of N. rossii.

Once a sufficient batch of molecular probes is at disposal, it is possible to identify identical (homologous) chromosomal structures among species or to detect differences arising from chromosome changes, corresponding to the re-arrangements which occured during evolution. Tracking these similarities and differences contributes to the work (alpha systematics, morphology, molecular biology) that aims to identify species, rebuild and understand their biological evolution using characters that be identified in living organisms. Similarities in the number, morphology and the chromosomal ultra-structure among several species allow to postulate that they share a common ancestor and are thus parents. However, these homologies can sometimes arise from chromosomal rearrangements that occured several times independently during the evolution of a species group. Thus, as usual, these hopotheses of parental relationships must be compared and confronted to results from other approaches (comparative studies of morphological and molecular characters).