Somatic Segregation

SOMATIC SEGREGATION When an organism produces several types of offspring the nor mal moment of segregation is at the reduction division, in which case segregation usually obeys Mendel's laws. But other types of segregation may occur. Much less is known about them than about the normal type, but a distinction may be made between three types of event, mutation, break-up of a chimaera, and plastid inheritance. Somatic segregation is more important in plants than in animals, because in the latter the germ cells are usually early differentiated from the soma, and whereas in a plant with white and green branches the gametes will of ten be different on the two branches, an insect whose right and left wings differ will commonly produce only one type of gamete.

Chimaeras.

It will be convenient to begin with a discussion of chimaeras. A chimerical plant may be sectorial, as when one half of a stem, with its leaves, is green, the other white, shoots from the border of the two sectors being of mixed colour. Or it may be periclinal, as when a green core is contained in a white skin, or conversely, a common arrangement in Pelargonium (usually called "geranium"). The outer layer may consist of one, two or more cells. Occasionally as the result of grafting, a peri clinal chimaera may be composed of layers of two different spe cies; as for example Cytisus Adami, which consists of a layer of C. purpureus over a core of C. Laburnum, and bears flowers of both the species together with those of the chimerical type. Though one type of chimaera may occasionally give rise to an other, the periclinal type can generally be propagated vegetatively. Thus many commercial types of potato are periclinal chimaeras. When, however, the vegetative propagation is only from one layer, that layer is alone represented in the offspring. Thus a white-cored geranium can be propagated by an ordinary cutting, but a root-cutting gives only white plants, and when a potato such as Noroton Beauty (a mottled form) has its "eyes" removed the tubers give rise to red potatoes identical with the variety Triumph. It clearly consists of a core of Triumph surrounded by a nearly colourless layer. While periclinal chimaeras are common in such asexually propagated plants as the potato and the geranium, they are rare in the raspberry, because the "suckers" or asexually propagated plants grow from the roots in this case. Occasionally chimaeras are still more complicated, owing to the occurrence of more than two tissue types.

Stable chimaeras cannot be reproduced sexually. Their gametes generally reproduce the characters of the sub-epidermal layer, and are usually of one genotype only. Very often, however, chi maeras are sterile, or only one gender of gamete is produced. Chi maeras may arise from grafting, or more frequently from bud variation, which appears to be simply due to mutation (q.v.) in a somatic cell. If so it is usually found that the mutant portion dif fers by one gene only from the original plant though it may differ in chromosome number. When the chimaera is sectorial the mu tant branches can often be propagated by grafting or by cuttings. In this way, for example, the red Magnum Bonum plum arose as a bud sport from yellow Magnum Bonum. In practice it is often impossible to be sure, without further experiment, whether a given case of somatic segregation is due to the break up of a chimaera, or to mutation in a somatic cell.

Plastid Inheritance and Multimutation.—Plants may exhibit variegation either of the chlorophyll in the leaves, or the pigments of the flowers. In some cases the variegated condition is a simple recessive to the normal. Sometimes, however, as in Mirabilis jalapa, a recessive plant of the var. variegate (here variegated green and yellow), will produce a pure green branch, or, on self-fertilization, a pure green seedling. Such seedlings be have as heterozygotes for the recessive character of variegation. Here it is clear that the recessive gene occasionally mutates back to the dominant. Probably the genetical mutability and the varie gation are different aspects of the same phenomenon. While some geneticists regard these "multimutating genes" as an explanation of all abnormally inherited variegation, many cases seem to call for some other explanation. In Pelargonium zonale, green or green skinned branches give only green seedlings when selfed, and white or white-skinned only white. The cross yields a mixture of green, white and mosaic plants. This kind of inheritance is most readily explained as due to plastids which are not passed on from one cell to another according to any definite law. In such cases inheritance may be through both sexes, as in Pelargonium and Oenothera, or through the female only, as in Mirabilis and Primula. Thus in Mirabilis Jalapa var. albomaculata the leaves are mottled green and white, with occasional green shoots. The condition is not transmitted through the pollen, and the seeds, no matter with what pollen, produce a mixture of green, albomaculata, and white plants. Non-Mendelian variegation occurs even in the prothallia of certain variegated ferns, and is common as a result of species crossing. Non-Mendelian inheritance occurs very rarely.