“CYTOLOGICAL TOOLS FOR LOCATION AND DETERMINATION OF FUNCTION OF GENE”
Cytogenetic stocks are genotypes that carry heritable chromosomal aberrations, which differentiate them from normal and wild types. Linkage maps, in conjunction with cytogenetic and DNA marker-based maps, help in assigning linkage groups to specific chromosomes. Cytogenetics is an integral part of genome analysis. A complete set of monosomics was produced by Ernier Sears, which were considered as genetic stocks and a treasure source for mapping genes at the time when modern techniques were not available. The pioneering work of Sears on the development of wheat (Triticum aestivum) aneuploids greatly facilitated cytogenetic studies.
Genes were first located on specific chromosomes using structural and numerical chromosomal aberrations. Several cytogenetic stocks were developed in wheat, maize, tomato, etc. Based on monosomic analysis, genes were marked on specific chromosomes. Later, genes were located on specific chromosomal arms using monotelodisomic, secondary trisomic, ditelodisomic, etc. Recombination frequency between genes was used for determining distance and order of genes. Availability of aneuploids in bread wheat allowed chromosome manipulations, leading to the development of alien addition/substitution lines and the introgression of alien chromosome segments into the wheat genome.
Compensating nullisomic-tetrasomic lines enabled placing 21 wheat chromosomes into seven groups of three each, such that nullisomic for any one of the three could be compensated by the tetrasomic condition of either of the two. The mode of action of the diploidizing system was established based on the nullisomic condition of the 5B chromosome of wheat. A nullisomic 5B wheat plant showed multivalent formation involving homeologous chromosomes, which enabled localizing the Ph1 gene onto chromosome 5B.
Advancement in cytogenetic techniques could assist in DNA sequence location, sub-genome identification, study of chromosomal pairing, evolution, etc. Considerable progress has been made in alien gene transfer into wheat with the help of cytogenetic techniques.
REFERENCES
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BIKRAM, G.S. (1998). Plant cytogenetics at the dawn of the 21st century. Curr. Opin. Plant. Biol., 1: 109-115.
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GUPTA, P.K. (2022). Cytogenetics. Rastogi Publications, New Delhi.
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RAUPP, W.J., SUKHWINDER, S., BROWN, G.L., GILL, B.S. (2001). Cytogenetic and molecular mapping of the leaf rust resistance gene Lr39 in wheat. Theor. Appl. Genet., 102: 347–352.
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