Variegation in biological tissue

• Variegation is the existence of difference looking sectors of somatic tissue.
• Geneticists gave another name mosaic useful in relation to this phenomenon.
• Thus, mosaic is an individual composed of tissue of two or more different genotype, often recognizable because of their phenotype.

Sectoring in biological tissue

• Generally, sectoring in biological tissue can occur by following ways:

1. Mitotic crossing over
2. Mitotic chromosome loss
3. Mitotic chromosome non disjunction
4. Position effects variegation
5. Fusion of different zygote
6. Somatic mutation
7. Cytoplasmic mutation and segregation

Mitotic crossing over

• It may be defined as any mitotic process that generates a diploid daughter cells with a combination of gene different from diploid parental cell in which mitosis occur.
• It was first observed in Drosophila by Evert stem.
• He made a cross in drosophila having yellow (Y) and Singed (Sn) bristle.
• In F1 some females had sector of yellow and singed tissue and some showed twins spots.

Mitotic chromosome loss

• In this, chromosome bearing dominant allele somehow gets left behind when the daughter nuclei reconstitute after mitotic division. This is merely called mitotic chromosome loss.
• A specific example of it could be seen in embryonic development of Drosophila because of which mosaic of xx (female) and xo (male) cell is formed.
• Other such phenotypic expression are one eye white and other red.

Mitotic chromosome non disjunction

• This leads for phenotypic segregation thereby causing variegation.
• Best example of this could be Drosophila female that were genotypically M, M being dominant.
• Thus, some female had been found with a sector of wild type bristle on the body “M’ phenotype.
• Thus, allele of heterozygous showed segregation resulted from mitotic non-disjunction.

Position effect variegation

• A further causes of variegation is associated with translocation because the expression of gene can be affected by its position in genome and this will cause variegation.
• Both translocation and inversion result in rearrangement of gene in chromosome and hence form new association.

Fusion of different zygote

• By fusion of different zygote, the cell line show peculiar mosaic.
• This could be seen in mouse, the blastoderm cell.
• When fused in embryo from a mouse, this strain is variegated.

Somatic mutation

• Mutation in somatic cell is known as somatic mutation which leads to a sector of identical mutant cell.
• There is little chance of translocation of such mutation to progeny unless germinal cell is involved.
• Eg: bud mutation

Cytoplasmic mutation and segregation

• Variegation in 4 O’ clock plant is maternal and by self-perpetuating bodies located in cytoplasm
• These are known as plastome and transmitted generation after generation through cytoplasm carried by ovule.