Plastid inheritance : variegation in plants

Most firmly established cases of true cytoplasmic inheritance are those that involve plastids, which are minute cytoplasmic organelles in plant cells. Most important of these plastids are chloroplasts carrying green pigment, the chlorophyll. These chloroplasts duplicate themselves independently of other parts of the cell, and carry some genetic information in the form of DNA. As obvious, usually plastids are transmitted through egg cytoplasm, and only very rarely few, if any plastids are transmitted through pollen. Consequently characters depending on plastids exhibit cytoplasmic inheritance sometimes independent of nuclear genes as in Mirabilis jalapa. Plastids also carry DNA which may contain genetic information. Variations due to mutations are also known in these plastids. However, it does not mean that plastid characters are not controlled by nuclear genes, because in a variety of crop plants, albino seeding characters have been found to be under the control of nuclear genes which have also been mapped in some cases. Such cases of nuclear inheritance involving lack of green colour in albino seedlings will not be discussed in this section, since these do not concern cytoplasmic inheritance.

It is believed that variation in colour of leaves, branches or whole plants is due to two kinds of plastids (normal and mutant albino). These two types of plastids will faithfully multiply and give rise to their own types due to cell division. But these daughter plastids may not equally distribute themselves to daughter cells. Further, in rapidly dividing cells, division of plastids may not keep pace with cell division, so that distribution of plastids to daughter cells may become a chance phenomenon as illustrated in Figure 18.7. A cell having both kinds of plastids may give rise to three types of cells namely (i) those with mainly normal green plastids; (ii) those with mainly mutant albino plastids and (iii) those with both kinds of plastids. These three kinds of cells when present as eggs will give three kinds of progenies. However, these three kinds of cells will not be distinguished as sperms and, therefore, variegated plant when used only as male will give only one kind of progeny (Fig. 18.8).

Plastid, inheritance in four o'clock (Mirabilis jalapa).
Although a number of cases are now known in plants, where inheritance of plastids is controlled by genes, there are also cases where this character is transmitted through the agency of cytoplasm alone. Since major part of cytoplasm in zygote is derived from egg, inheritance in such cases will be maternal.

In four o'clock plant (Mirabilis jalapa), three kinds of branches with respect to occurrence of plastids may be found. These are (i) completely green, (ii) completely pale green or (iii) variegated. In such cases, phenotype of progeny will depend upon phenotype of branch on which flowers are pollinated (Fig. 18.9). Results from three kinds of branches used as female parents are diagrammatically shown in Figure 18.10.

As would be evident from Figure 18.9 (c) and Figure 18.10, when variegated branches are used as female source, both green and pale plastids are present in cells of female parent. Therefore, female gametes may carry either green or pale plastids or both. Consequently, three kinds of plants namely green, pale and variegated plants would be obtained.

Iojap inheritance in corn.
Iojap is characterized by contrasting stripes of green and white colours on leaves. This character is controlled by a gene (y) located on chromosome 7 in corn. The dominant allele Ij is responsible for normal green plastids and its recessive allele ij for iojap (striped) character, so that Ijlj is green and ijij is iojap. Plants with genotype ijij would carry normal green plastids as well as abnormal white plastids, while plants with genotype Ijij would carry only green normal plastids. Female gametes from ijij can, therefore, carry either only green plastids or only abnormal white plastids or both. Consequently, if ijij is used as female parent and IjIj as male parent, the F₁ individuals (Ijij)will be green, white or striped. From these F₁ individuals, if Ijij (striped) is again used as a female parent, and crossed with IjIj (green) as male parent, two genotypes, namely Ijij and Ijij would be obtained in equal proportions. Each of these two genotypes would have three phenotypes (green, white and striped). It should be noticed that, in such a cross, IjIj (striped) would also be obtained (Fig. 18.11).

It is, therefore, obvious that iojap character once inherited through female parent will be able to maintain itself in the cytoplasm irrespective of genotype of plant. Therefore, as discussed above, iojap or striped phenotype may have any of the three possible genotypes, IjIj, Ijij and ijij.