“Structural Chromosomal Aberrations and Their Utility in Plant Breeding”

UNIVERSITY OF AGRICULTURAL SCIENCES, BANGALORE

DEPARTMENT OF GENETICS AND PLANT BREEDING

M.Sc. Seminar - GPB 582 (0+1)

“Structural Chromosomal Aberrations and Their Utility in Plant Breeding”

Structural chromosomal aberrations alter the chromosome structure, i.e., the number, sequence, or type of genes present on chromosomes. The four common types of structural aberrations include deletion, duplication, inversion, and translocation. Deletions and duplications alter the number of genes present, inversions change the gene sequence, while translocations cause a change in the genetic map.³

In the case of terminal deficiency, the terminal region of the normal chromosome remains unpaired. In intercalary deficiency, a loop formation occurs in the normal chromosome at the site of deletion. Deficiency can be utilized for locating a gene on a particular chromosome.¹

The duplicated segment forms a loop during pachytene in duplication heterozygotes. True breeding heterosis may be established in self-fertilized crops by the technique of duplication breeding. Gene duplication is a major mechanism in the evolution of genomes and genetic novelty. These duplicated genes can follow different evolutionary paths, including neofunctionalization, sub-functionalization, redundancy, and pseudogenization.⁴

When a portion of the gene sequence of a chromosome is rearranged in reverse order, the aberration is known as inversion. In the case of pericentric inversion, when the inverted segment is large, a loop is formed during synapsis and the centromere is included in the loop. Crossing over in the loop causes the formation of deficiency–duplication chromatids. In paracentric inversions, chromosome pairing occurs by loop formation in the inverted region while the centromere remains outside the loop. Crossing over in the loop causes the formation of dicentric chromatids, which produce bridges at anaphase. Asymmetrical inversion breakpoints on the two sides of the centromere in a pericentric inversion can change the karyotype of the chromosome. A metacentric chromosome may become submetacentric or acrocentric depending on the break positions in the two arms.¹

McClintock reported in maize that interchange heterozygotes involving two pairs of nonhomologous chromosomes formed a “cross-shaped” configuration at pachytene and a ring of four chromosomes at metaphase I, due to the pairing of homologous segments of the chromosomes involved in the translocation. Interchanges have been used to produce duplications, evolve allopolyploids, facilitate karyotype evolution, and serve as a source of trisomics.¹

According to cytological investigations on meiotic cells of barley plants produced after seed treatment, all five pesticides were capable of generating chromosome abnormalities and, in certain cases, abnormal cellular behavior, such as cytoplasmic furrowing. Some of the chromosomal abnormalities observed included stickiness, coalescence, chromosome bridges, fragments, and micronuclei. Additionally, parallel nuclear and cellular divisions were also observed.²

REFERENCES

1. Ganesh Prasad, 2024. Introduction to Cytogenetics, 1st ed., pp. 255–318.

2. Grover, I.S. and Tyagi, P.S., 1980. Chromosomal aberrations induced by pesticides in meiotic cells of barley. Caryologia, 33(2): 251–259.

3. Singh, B.D., 2023. Fundamentals of Genetics, 6th ed., pp. 77–93.

4. Ohno, S., 1970. Evolution by Gene Duplication, 1st ed., pp. 71–80.