Chromosome Number: A Great Tool in Taxonomic Studies

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Taxonomy – is the branch of biology that deals with identification, nomenclature and classification of an object.

Cytology – is the branch of biology that deals with origin, structure and function of cell. So Cytotaxonomy presumably means the application of cytological data to taxonomy. More precisely we can say Cytotaxonomy is the branch of biology dealing with the relationships and classification of organisms using comparative studies of chromosomes.

A wonderful story is given here to have a postmortem of cytotaxonomy.

A  Story

“Once upon a time, during the early decades of the 20th century, a famous old man, Sir Taxonomy, casually met a nice lady called Cytogenetics. They fell in love immediately, as they were meant for each other. They married and soon had a son, named Cytotaxonomy . Since the beginning, he stood out as a prodigious boy and had an extreme ability to discover the relationship between related species, Sir Taxonomy proud of his clever and fashionable young son sent him around the world, but it was in the United States that he became a strong and lovely boy. Trom there, he travelled to many other countries across the world. People from very different nations, from France to India, from Austria to Brazil, listened to him with great interest and respect”.

The basic principle of cytotaxonomy is the same used for other taxonomical characters: the more similar two groups are the more closely related they are supposed to be. Therefore, species with more similar karyotypes are more closely related than those with less similar ones.

Chromosomal characters used in Cytotaxonomy

  1. Chromosome number
  2. Chromosome structure
  3. Chromosomes behaviour

1. CHROMOSOME NUMBER

Since the early days of cytotaxonomy and up until today, chromosome number has been the most common karyotype character employed.

  • Karyotype – The appearance of the basic chromosome set (genome) under the light microscope is known as karyotype.
  • Idiogrami or Karyogram – The graphical representation of karyotypeUsually the number of chromosome in each cell of all individuals of a single species is constant.
Karyotype and Idiogram of Pisum sativum
https://www.semanticscholar.org/paper/Identification-of-the-Pea-(Pisum-sativum-L.)-Genome-Samatadze-Muravenko/a7c8878b0bfd11f3a588a0dfd9e189fbfd0086ca
  • The more closely related species are, the more likely they are to have the same chromosome number, and more distantly related more likely they are to have a different number.
  • This relative conservativeness renders chromosome number an important and much-used taxonomic character.

Chromosome numbers are usually determined in the form of diploid number (2n). In reporting new chromosome counts it is customary to quote diploid number (2n) when the count is based on mitosis in sporophytic tiisue, and the haploid number (n) when it is based on mitosis in gametophytic material or in meiosis .

Chromosome numbers of angiosperms published in “Index to  Plant Chromosome Numbers”-issued by International Association  of Plan Taxonomists (IAPT).
If we look at the constant number of chromosomes of a species we can cite Onion (Allium cepa) as one of the best example. In Onion, hundreds of thousands of root tips analyzed around the world always display 2n=16 and very similar individual chromosomes.

However, in some case the number of a species can vary and this variation might be due to the following reasons:

  1. Applying wrong methodology for determining chromosome numbers
  2. Wrong identification of the plant chromosome number of which would be
    determined
  3. All taxonomists do not have expertise in cytology and all cytologists are not
    expert in taxonomy
  4. Presence of B-chromosome
  5. Phenomenon of polyploidy

It is most important that voucher specimens of the plants used for cytological study are deposited in herbaria where their identity and characteristics can be checked by future workers.

POLYPLOIDY

  • Polyploidy, the most important source of chromosome number variation in flowering plant evolution, is a highly recurrent state. Even within a single species, polyploid individuals or populations may be independently formed (Tate et al. 2005).
  • Polyploids are always derived from diploids or from plants with lower ploidy levels
  • It is evident that closely related specis often differ in chromosome number, the most frequent variations based upon the phenomenon of polyploidy. For example, in the genus Festuca, there are species with 2n = 14, 28,42,56 and 70; such species are known as diploids, tetraploids, hexaploids, octoploids and decaploids respectively.Definitely, these numbers are based upon 7, the gametophytic chromosome number of the diploid species. This number is known as base number or basic chromosome number (x).
  •  In a diploid species x = n, but in a polyploidy species, n is a multiple of x; hence, in the case of hexaploid Festuca mentioned above 2n = 6x = 42 (or n=3x =21)
  • Raven aven (1975) states that the original base number for angiosperms is x =7, and that comparisons at family level are valid only when the base number rather than chromosome number) is used.
  • Chromosome numbers count so far
    Bryophytes = 6%
    Pteridophytes  =20%
    Angiosperms = 15-20%
  • Highest chromosome number 2n = 1260 in Pteridophyte (Ophiglossum reticulatum)
  • Highest chromosome number 2n =526-530 in Angiosperm (Poa literosa -Poaceae
  • Lowest chromosome number 2n = 4 in Angiosperm (Haplopappus gracilis –Asteraceae)

HOW KARYOTYPE CHARACTERS HELP TO SOLVE TAXONOMIC PROBLEMS?

Chromosome numbers are frequently proved useful within the family at tribal and generic level, and to solve taxonomic problems.

Al tribal level

1. In case of the family Ranunculaceae most genera with x = 8 but some genera with x =7 segregated into separate tribe. In fact one other genus has X=9 and another x = 13, and in present classifications these two are often placed in separate monogenetic tribes.

2. On the basis of chromosome size and basic chromosome number (x = 5) the Paeonia has been separated from the family Ranunculaceae and placed under the family Paeonaceae.

3. In the Poaceae, different subfamilies. tribes and genera can be characterized to varying extents by their base numbers. For example the primitive subfamily Bambusoideae has x = 12, while the subfamily Pooideae has mostly x = 7.

4. In case of Asteracec, some unidentified genera have been placed under different tribes based on basic chromosome number.

Genus level and below

Interspecific variation in chromosome numbers has proved to be one of the richest sources of cytological data of value to taxonomists. At genus level and below karyotype information is much more effective because in this case the number of taxa is not so many and therefore data could be analyzed easily.

1. The genus Crepis is one of the best example at genus level. Based on chromosome number, their size and shape Crepis has been distinguished from its closely relatedg genera Cymboseries and Youngia.

2. In the grass genus Vulpia diploids (2n = 14), tetraploids (2n = 28) and hexaploids (2n= 42) occur. The genus is divided into 5 sections of which three contains only diploids, one (Monachne) contains diplods and retraploids, and the other (Vulpia) has all three levels of ploidy (ie, diploid. tetraploids and hexaploids). In section Monachne there are only 3 species, 2 diploids and I tetraploid, and one of the diploids and the tetraploid resemble each other very closely, so that in the past they often been considered as a single species. The diploid and telraploid do, however, differ in the pattern of ovary pubescence and since they also show different ecological preferences and geographical distribution, it is considered better to recognize them as distinct species.

Species level/identification

Chromosome number also helps to identity species. This can be explained by the following examples:

Nasturtium officinale = 2n = 2x = 32
Nmlerophyllum = 2n =4x = 64
Cardamine hirsuta = 2n = 2x= 16
C. flexuosa= 2n = 4x = 32

Salix viminalis = 2n = 2x = 38
S. atrocinerea= 2n = 4x = 76
S. phylicifolia= 2n = 6x = 114
S. myrsinites= 2n = 8x = 152

So it can be said that species can be identified by determining the chromosome numbers even in absence of morphological characteristics.

Chromosome structure  and Chromosome behaviour of Cytotaxonomy will be described in another article.