CYTOTAXONOMY

Sanjeet Kumar
sanjeet.biotech@gmail.com
Department of Botany
Ravenshaw University, Cuttack

CYTOTAXONOMY

Taxonomy is a multifacet interdisciplinary subject of plant science and its phylogenetic problems are duly solved by applying the importance of cytology. Nature of pollen grains and a large number of morphological characters in numerical taxonomy for the cause of classification. Cytology has made an important contribution to taxonomy during last few decades. Actually cytological characters such as chromosome number, morphology, their behavior and structure at meiotic phase of cell-division use to bear considerable taxonomic values which create a branch of biology dealing with the relationships and classification of organism is known as Cytotaxonomy. It is comparative studies of chromosomes. As the cytologic data are directly derived from nucleus, the seat of hereditary material, they may be used for understanding the evolution and relationships of population. The chromosome number is usually constant in a species which makes it as an important taxonomic character. Chromosomes also very in forms, size, volume and in the amount of distribution of heterochromatin. These characteristics of  Karyotypes are taxonomically useful where the individual chromosomes are large enough for detailed microscopic observation. The chromosome number can be a plesiomorphic characteristics of a large clade or a recurrent trait which arose independently in two or more clades.  Chromosome numbers are usually determined at mitosis and quoted as the diploid number (2n), unless dealing with a polyploidy series in which case the base number or number of chromosomes in the genome of the original haploid is quoted. In this branch another useful taxonomic character is the position of the centromere. Meiotic behavior may show heterozygosity of inversions. This may be constant for a taxon, offering further taxonomic evidence. Cytological data is regarded as having more significance than other taxonomic evidence. The class monocotyledons have usually got large sized chromosome than the dicotyledonous. In general, woody plants have got smaller chromosomes than in their herbaceous relatives.

Examples:

Ø  In family Ranunculaceae, the chromosome number and chromosome morphology have keenly provided prime basis for more natural arrangement of genera and tribes. Two major tribes of family Ranunculaceae namely Helleboreae and Anemoneae have got genera base chromosome numbers of 7, 8 and 9 and both the tribes have got genera with large and small chromosome types.

Ø  The genera Aquilegia and Isopyrum of Heeboreae, Thalictrum and Anemonella of Anemoneae have got base number 7 with small type chromosomes. These four genera have thus been segregated in a separate tribe called Thalictrea.

Ø  The genera Agave and Yucca placed in two separate families, Amaryllidace & Liliaceae respectively, with 3 long and 25 short chromosomes. Their Karyotype similarity justifies the cause of inclusion in a single family Agavaceae as has been done by Hutchinson.

Ø  The two genera of family Brassicaceae, Physaria and Lesquerella were recognized by many as a single genus.

Palynotaxonomy

According to G. Endtman (1952) pollen grains morphology pay a pivotal role in solving taxonomic problems. Latter Erdtman (1969) also published an excellent review on the application of palynology in taxonomy. Investigation in this line become more aggravated by the use of Electron Microscope in pollen studies. Following pollen characters are used in taxonomic studies:

1)      Size and shape.

2)      Exine sculpturing.

3)      Apertures

4)      Ultrafine structure

5)      Pollen associations

6)      Nuclear number in pollen.

Size and Shape

The size are minute, small, medium, large, very large and gigantic. The shapes also varies with different views i.e. circular, prolate, perprolate, plano convex or biconvex.

Exine sculpturing

The surface of the pollen grains may be smooth or they may be variously sculptured which consist of two types:

a)      Excrescences Type :

1)      Spinulose

2)      Granulose

3)      Gemmate

4)      Tuberculate

5)      Clavate

6)      Baculate (rod-shaped)

b)      Depression Type:

1)      Striate (Lumina parallel)

2)      Regulate (Lumina anastomosing)

3)      Reticulate

4)      Foveolate (Lumina circular with closely-set)

5)      Scrobiculate (Lumina circular & distanthyplaced)

6)      Fossulate (Lumina elongated)

 Aperture

Based on the number, position and character (NCP) of aperture, the pollens are consisted of different types:

1.      Number: 1 -- Many.

2.      Position: Proximal, distal, intercalary, lateral, zonal or global.

3.      Characteristics: Colpate (furrow), Porate (Circular), Inaperturate.

Ultrafine structure

The observation of pollen grain under Transmission Electron Microscope provides a lot of considerable taxonomic valueat species, generic and family level.

Species Level

The pollen characteristics help in differentiating the species within a single genus.

Examples:

In Anemone: Based on germinal aperture, the different species of these genera can be distinguished.

Ø  Anemone obtusiloba : The pollens are 3-zonocolpate

Ø  Anemone rivularis : Pantocolpate

Ø  Anemone alchemillaefolia : Pantopovate

Ø  Anemone fulgens: Spiraperturate

In Bauhinia : Based on the exine pattern, the different species of this genus can be assigned as such.

Ø  Bauhinia acuminata: Pilate

Ø  Bauhinia vacemosa : Reticulate

Ø  Bauhinia krungii: Striate

Ø  Bauhinia retusa : Vervucate

Ø  Bauhinia malabarica : Spinulate

Ø  Bauhinia purpurea : Reticulate / tuberculate

Genus Level

Family Salicaceae consists of two genera, Salix and Populus that can be distinguished on the basis of  pollen characteristics.  The genus Populus is provided with spherical pollen grains without distinct aperture whereas Salix contains 3-furrowed aperture.

Family Phytolaccaeae consists of 22 genera. The pollen studies on two genera Rivinia and Phytolacca indicate that they can be distinguished easily on pollen characteristics. The pollen of Rivinia is Pantocolpate while Phytolacca is provided with 3 zonocolpate.

Family Level

In Araceae and Lamnaceae : Hutchinson in his Arales included both Araceae and Lamnaceae. The family Araceae is europalynous, with 1-2-4- colpate, 3-potate or inaperturate, with pollen grains provided with exine sculpturing while Lemnaceae is stenopalynous with 1-porate and spinous exine.

In Bombacaceae: Bentham and hooker divided this family into four subfamilies

Ø  Malvaceae

Ø  Ureneae

Ø  Hibisceae

Ø  Bombaceae

Later, Engler treated Bombacaceae as a seprate family. The exine of most of the members pf Malvaceae  is spinous but that of Bombacaceae is reticulate. This study supporting the separation of Bombacaceae as a separate family is justified.

In Berberidaceae : Family Berberidaceae includes 12 genera. Modern taxonomist removed the genus Podophyllum from Berberidaceae and placed it in a separate family, Podophyllaceae. The pollen grains in Podophyllum remains united, but they are free in other genera, supporting the removal of Podophyllum.

In Orchidaceae: According to Vij and Kashyap (1978), there are three groups of pollen grains:

1.      Single pollen grain: Cypripedium cordigeratum

2.      Tetrads : in tribe Neottieae and Epidendrieae.

3.      Perfect massulae: Orchidaceae and some members of Neottieae.