Cytotaxonomy: Study of Chromosome Structure

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Study of chromosome structure is very important in cytotaxonomic studie.It includes chromosome size, shape and position of centromere (i.e. arm-length ratio of each chromosome in the genome)

Chromosomal size and shape are also important as taxonomic criteria in taxonomy.
For examples,

  • The chromosomal ratio if Mediola and Trillium of the family Liliaceae = 1:100.

Usually chromosomes of monocots are larger than chromosomes of dicots.

  • The genus Paeonia of the family Ranunculaceae was suggested to be separated from this family based on morphological and other characters. The chromosomes of the genus Paeonia are larger than chromosomes of other genera of the same family. Therefore separation of Paeonia from Ranunculaceae is more justified as supported by chromosomal data.

Chromosome types

  • Based on the position of centromere the chromosomes may be metacentric(centromere near the middle – shaped), submetacentric (J-shaped) and acrocentric (centromere near one end – 1 shaped). In addition, telocentrics (truly terminal centromere).
  • Another important useful aspect of chromosomal morphology is the position of secondary constrictions, which delimit the occurrence of satellite.

As I told before that the appearance of the basic chromosome set under light
microscope is termed as Karyotype. For example, the karyotype of Calystegeia (2n =22) consists of 8 shoet metacentrics,2 short submetacentrics and 1 short metacentric within addition a secondary constriction and satellite.

Karyotypes are usually calculated from the means of a substantial number of observations,and often they are represented diagrammatically as idiograms or karyograms.

 

In monocotyledons, the genera Yucca and Agave – both are large plants with long
strong leaves borne in a tight rosette. Yucca has flowers with a superior ovary, while Agave has an inferior ovary. For this reason Yucca were previously placed under the familyHutchinson and Agave under Amaryllidaceae. Because of their great overall similarity in these genera they are now segregated from their respective families and placed in Agavaceae by Hutchinson

Later on, cytological investigation revealed that both Yucca and Agave genera bear 5 long. J-shaped chromosomes and 25,small chromosomes. It confirms that the placement of these two genera under a family Agavaceae is more justifiable.

Variation in chromosome shape is also evident in different species of Aconitum. In Ranunuculaceue chromosomes of Delphinium and Aconitum are advanced because their chromosomes are asymmetric and most chromosomes are J-shaped.