Cytotaxonomy: Study of Chromosome Behaviour

Chromosome behaviour is an important factor that determine the pattern of variation and taxonomic distinctness. Pairing behavior and subsequent separation of chromosome occurs at meiosis.

In reporting results of pairing studies, univalents are represented by Is, bivalents by IIs, etc.

Some taxonomic information can be gained from the study of mechanism of meiosis. In the genus Oenothera (family Onagraceae) all species are diploid with 2n = 14. During pairing in meiosis, it is seen that –

• O. biennis – exhibit two rings, a ring of six chromosome and a ring of eight chromosome.
• O. erythrosepala– exhibits a ring of 12 chromosomes and a bivalent
• O. strigosa – presents a single ring of 14 chromosomes.

So in meiosis, species can be identified by observing the pattern of pairing ring.

Chromosome Banding

Chromosome banding is as a modern technique in cytology. These days application of heterochromatin is very significant in phylogeny and evolution.

• Fluorochrome banding –
  Chromomycin A3 (CMA) [preferentially stain GC-rich heterochromatin] 4‘,6-Diamidino-2-phenylindole (DAPT) [preferentially stain AT-rich heterochromatin]
• FISH(Fluorescence in situ hybridization)
• GISH (Genomic in situ hybridization)

All these features together make of cytological data an excellent and irreplaceable source of information for taxonomical and evolutionary studies.

• Detailed comparative karyotype analyses have proved to be an important tool for taxonomy and for understanding chromosome evolution.
• Chromosome numbers  and nuclear DNA content are among the most reliable and practical characters for cytotaxonomical analyses involving a large number of species. In this case, a robust phylogenetic tree is necessary to understand the relationships between species and their karyotype evolution

Examples

• Many works devoted to cytotaxonomy have demonstrated the unique contribution of karyotype information to taxonomy and evolutionary studies. Correctly performed comparative karyotype studies provide an independent source of data that strongly supports the taxonomical arrangement of several recently reviewed taxa, such as the sections of Aristolochia (Ohi-Toma et al. 2006), the subgenera of Passiflora (Ilansen et al. 2006), or the genus Ipheion (Souza et al. 2010).
• Still today, the relationships among Triticeae species, one of the best investigated plant tribes, are largely supported by chromosome numbers and genome identification based on meiotic analyses of hybrids( reviewed by Goncharov 2010).
• Likewise, the identification of the true species of Citrus, a genus mainly represented by apomictic hybrids, has been revealed by careful fluorochrome banding analysis further supported by in situ localization of rDNA sites (reviewed by Guerra 2009).

Advancement

1. In the last 20 years, the advent of molecular cytogenetics, phylogenetics, and genomics has brought new impulse to plant cytotaxonomy and chromosome evolutionary studies, thereby shedding more light on chromosome structure and on the mechanismus generating chromosomal changes (Lysak et al. 2006; Abrouk et al. 2010; Wu & Tanksley 2010; Heslop-Harrison & Schwarzacher 2011).
2. Analyses of chromosomal changes combined with phylogenetic trees (reviewed by Murray 2002) revealed that some chromosome characteristics are more recurrent than previously thought, and that the interpretation of chromosome data should be supported by more precise cytological evidence (Dobigny et al. 2004).

Conclusion
Appropriate karyotype analyses, comparative genomics, and DNA-based phylogenies are different ways to investigate the genetic divergence between species at very distinct levels, each with its own methodological advantages and constraints. The lessons we have learned from this combination of approaches have already paved the way towards a more precise cytotaxonomy, and a better understanding of chromosome evolution.

(This article is completely based on the Lecture sheet provided by Dr. Md Oliur Rahman, Professor, Department of Botany, University of Dhaka.
Some pictures and info have been added by the author. Any mistake, error, misinformation and other related things found in this article is only author’s to blame)

Revised by

1. Md. Siddiq Hasan on July 22, 2020