The Nature of Mutation

Mutation refers to any heritable change in the DNA, but the breeder is generally inter- ested only in those mutations that alter a phenotype. At the molecular level, muta-

tions can alter DNA by base substitution, insertion, deletion or sequence rearrange- ment. Any of these types of changes may cause a phenotypic change. It is important to distinguish the ‘permanent’ change in DNA from types of temporary change that are generally classifi ed as epigenetic. For example, expression of a gene may be silenced by methylation of the DNA. This state may be ‘semi-permanent’ in that daughter cells derived from the altered cells retain the methylation pattern. This means that it may be possible to propagate trees that retain this character. However, regener- ation from tissue culture or meiosis can sometimes restore the non-methylated DNA and corresponding phenotype. It is not clear whether epigenetic changes con- tribute to the diverse phenotypes seen in citrus.

A further complexity is chimerism (Fig. 16.1). When a new mutation is induced in somatic cells, it will affect cells in one of the three histogen layers: L-I which gives rise to the epidermis and most juice vesicle tissue; L-II which produces subepidermal tissues including the fl avedo, nucellus and gametes; and L-III which includes most internal tissues such as the cortex. Generally cells divide to give daugh- ter cells of the same layer, so a mutation

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 345

Fig. 16.1. Types of chimeras in a shoot of citrus. Shaded areas represent mutant tissue.

affecting one cell layer can appear stable during vegetative propagation. Such geno- types are called periclinal chimeras. A ‘layer substitution’ can cause such chimeras to revert to the original type, or become a ‘solid’ or non-chimeric mutant. Layer substitutions can arise when tissues differentiate from callus such as when pruning or freeze damage leads to regenera- tion of new branches.