Cytotaxonomy applies cytological data, particularly chromosome numbers, to resolve taxonomic classifications across species and genera. Examples illustrate how chromosome numbers differentiate closely related species and guide family reclassification based on chromosome size and structure. The morphology and behavior of chromosomes aid in plant identification and understanding fertility among populations.

1. Cytotaxonomy
Prepared By-
Dr. Sangeeta Das
Assistant Professor
Department of Botany
Bahona College, Jorhat, Assam, India.

2. Cytotaxonomy
Cytology in Relation to Taxonomy:
It is the application of cytological data in solving
taxonomic problems.

3. Cytological Data:

4. Chromosome number:
The chromosome number is constant and same in
all individuals of a species.
It is used as a confirmative property to distinguish
a species from members of other specie

5. Example 1:
Separation at species level:
Monotropa hypopitys and Monotropa
hypophegea are two different species looks
like morphologically.
But they are separated on the basis of
chromosome numbers.
M. hypopitys is a hexaploid: 2n = 48
M. hypophegea is a diploid: 2n = 16

6. Example 2:
Separation at Interspecific level:
There are two cytotypes of the species
Veronica prostrata
On the basis of chromosome number, these
two cytotypes are raised to subspecies level
and named as
V. prostrata prostrata (n=8) and V. prostrata
scheerei (n=16).

7. Example 3:
Separation at Generic level:
Cicendia filiformis and Microcala pusilla are
placed in genus Cicendia
Separated based on chromosome number C.
filiformis (n =13); M. pusilla (n=10).
All species of the genus Tephrosia in
Fabaceae have 2n = 22, except T. constricta
having 2n = 16.
Therefore, the later is raised to a level of the
genus Sphinctospermum.

8. Example 4:
In the tribe Boranieae of Rutaceae,
the chromosomal base number
varies from 7 to 19.
Therefore, the members are
rearranged to have a unique system
of classification.

9. It is usually seen that closely related
plants, like the different species of a
genus, show chromosome numbers
which reveal an arithmetic relation with
one another
For example,
The different species of Piper show chromosome
numbers in multiples of 26, like 2n = 52 in P.
nigrum, 2n = 78 in P. betle, and 2n = 104 in wild
species of Piper (Mathew, 1958).

10. Separation at Family level:
Families are rearranged on the basis of
number and size of chromosomes.
For example,
Heleboreae and Anemoneae of
Ranunculaceae – chromosome number 7, 8
and 9
Both have large and small chromosome
Therefore, both the genera are rearranged.

11. Aneuploids:
Aneuploids are plants with basic number of
chromosome with some addition and
deletion of few chromosomes.
Monosomics have 2n – 1,eg., Datura.
Nullisomics have 2n -2, eg., Triticum
Trisomics have 2n + 2.
Tetrasomics have 2n + 2 chromosome.

12. Chromosome Size:
It has already been discovered that
evolutionary development involves in
addition to alterations in chromosome
number.
For example,
Cytologically Cyclea and Cissampelos are
seen to be based on 12, while Stephania
shows n = 13. It is seen that the number n =
13 is characteristic of the tribe Cocculeae,
which further shows chromosomes of small
size.

13. Large chromosomes, low
chromosome number and
symmetrical karyotype represent a
primitive status.
While small chromosomes, high
number and extreme asymmetry
indicate advancement.

14. Chromosome morphology:
Karyotype of plants is now very useful for the
classification of some plants.
Monocotyledons have larger chromosome number
than dicot.
Woody plants have smaller chromosome number than
herbs.

15. The position of centromere and satellite is considered
for classification.
For example,

16.  The shape of metaphase chromosome is
considered for classification.

17. Chromosomal behaviour:
It provides clues about the cause of sterility and
fertility among populations.
Structural difference in the parental chromosome
is the main cause for the sterility.
If the two sets of parental chromosomes are
homologous, the plants seem to be fertile.