Elements of Genetics p.194-198. (1949) 1969
Darlington and Mather

How the Cytoplasm Acts: Gradients

The genes thus express themselves by what they give to the cytoplasm. But they vary in how quickly they give it; and it varies in how soon it finds something to react with. Genes express themselves consequently at different times in the course of development. We next have to sec what the cytoplasm gives to the nucleus and how this varies with development. The control of the nucleus by the cytoplasm is most readily seen in the synchronization of mitosis (and meiosis) in groups of cells. Such a synchronization is found in all tissues where cell walls are poorly developed or absent. In the cleaving animal egg, and within follicles of the testis or anther, in the endosperm, in the twin diploidized cells of fungi, and even in the feebly-walled pollen grains in the orchids, all the mitoses move in step. They do so, apparently, because the supplies of materials necessary for mitosis, especially proteins and nucleic acid precursors, are uniformly diffused in the cytoplasm of the whole group of cells.

Fig. 46.—Blood precursor cells from the bone marrow of a man with pernicious anaemia, the normal contrast in nucleic acid charge between reds and whites being exaggerated. Left, overcharged red with thick chromosomes and over-developed and multipolar spindle which will give several non-viable hypo-diploid cells. Right, under-charged white with long thin chromosomes and under-developed open spindle which will give a single tetraploid cell by failure of anaphase. x 2,500 (after La Cour, 1944).

How this works has been shown by La Cour in the differentiation of the bone-marrow of mammals. Here two opposite types of cell-lineage are separated; one, with high nucleic acid content in the cytoplasm, has rapid divisions of its nuclei, a stongly developed spindle and chromosomes heavily charged and strongly spiralized. This type gives rise to the red blood corpuscles. The other, with low nucleic acid content, has slow divisions, a poor open spindle scarcely capable of executing the anaphase movement, and chromosomes feebly charged and weakly spiralized. This gives rise to the white corpuscles in their far smaller numbers. Thus two lines of development are set going in the same ancestral nuclei by two conditions of the cytoplasm, arising no doubt in different positions in the tissue (Fig. 46).

The differentiation of the embryo-sacs and pollen grains of flowering plants exactly parallels that of the blood precursors. In the pollen of Angiosperms one of the daughter nuclei is pressed by the first mitotic spindle against the wall, while the other is left in the middle of the cell. The peripheral, or generative, nucleus forms a small cell and is rich in nucleic acid: it divides again to give the two condensed sperm nuclei, often before the germination of the grain. The central, or vegetative, one forms a large cell and is poor in nucleic acid: it becomes large and diffuse and loses first its staining power, and then its coherence. It vanishes without further mitosis. (The nucleus of the ripe red blood corpuscle, similarly drained of nucleic acid, similarly dissolves.) This differentiation must depend on the fact that the distribution of materials in the pollen grain before mitosis is not uniform. There is a gradient and the position of the mitotic spindle is adjusted to lie along this gradient. If the axis of the spindle lies crosswise to the normal (following heat shock) cells and nuclei with similar properties are produced: differentiation fails (Fig. 47).

In a species of Sorghum extra heterochromatic chromosomes have been found to cause the vegetative, as well as the generative, nuclei to divide again. Extra nucleic acid, which the additional chromosomes make available in the cytoplasm, stimulates nuclear division in a way which recalls the plan of development of the gametophyte in the lower plants. In extreme cases the nuclear division may give four or five generative nuclei and thereby kill the pollen grain, much as the excessive mitosis of a tumour may kill an animal.

Pollen grains provide us, both internally and, as we saw earlier, externally, with crucial tests of developmental principles simply on account of their unique property of being independent organisms limited as a rule to two or three cells. The different sequences of their development arise from the reactions of different types of nuclei, symmetrically or asymmetrically placed with respect to the spatially differentiated cytoplasm of a single cell. The immense range of types of embryo-sacs in flowering plants depends on the same principles operating in more elaborate settings, showing a range of behaviour depending, no doubt, on varying gradients in the distribution of nucleic acid in the mother cell (Fig. 50).

Fig. 47.—The four pollen grains of a tetrad it, Scilla sibirica still exceptionally adhering after the first mitosis. As in Ascaris (Fig. 48), a radial spindle has given normal differentiation in three cells; an exceptional tangential spindle has given no differentiation in the fourth. The concentration of generative-nucleus-forming substances, represented by stippling, is evidently central in the pollen mother cell (La Cour, original).

Similar to the differentiation in space in the blood and pollen is that in time, whereby the series of mitoses in the development in a plant or animal is interrupted by meiosis, as a result, perhaps, of flooding of the nucleus with nucleic acid and the charging of the chromosomes before they have reproduced themselves. Differentiation in space as well as in time is responsible for the fragmentation of the chromosomes in particular embryonic cells in Ascaris. In those which arise from one region of the egg, and are predestined not to form the germ cells, the chromosomes break up at the anaphase of mitosis and leave some unessential pieces of heterochromatin (lacking centromeres) on the equator (Fig. 48).

Fig. 48.—Cleavage, blastula and gastrula stages in the development of the egg of the thread worm Ascaris megalocephala (diploid rare with one gamette chromosome). A, the maternal and paternal chromosomes meet on the first cleavage spindle. B, formation of two cells, differentiated in cytoplasmic determinants, the dorsal with the potentiality for more rapid division, leading to fragmentation of its chromosomes at anaphase and loss of the distal heterochromatin, the ventral with the potentiality for regular mitosis without breakage. C1 and C2 views of the second divisions: the differentiation continues with progeny of the cell which divides in the same axis as that of the first mitosis. D, the T-shaped embryo rounded off to form the blastula. E, F, third cleavage division. G, the gastrula stage: 32 cells of which one only has the complete set of chromosomes and will give rise to the germ cells. The cytoplasmic germ-line determinants, which are directly visible in copepods and are here marked only by the presence of fewer vacuoles are shown here by stippling (cf. Bounure, 1939).

NOTE.—The difference in mitotic rates suggests a gradient in nucleic arid content which should be exaggerated by the absorption of lost heterochromatin in the cytoplasm (after Boveri's figures 1910 et al., but conflicting with the diagrams in Wilson, 1900; Boveri, 1904; Morgan, 1913; and Belar, 1928).

In all these instances the cytoplasm of particular cells is telling the chromosomes what to do. But paradoxically enough, it can only be passing on what it has had from the nucleus. For in polymitotic maize, a recessive gene compels all the pollen grain nuclei produced by a plant homozygous for it to divide again as soon as they are formed, and to go on dividing until the whole nuclear organization is destroyed. In this ease it is clear that the gene has acted on the cytoplasm (already in the mother plant, for the effect is not seen in the individual pollen grains of the heterozygote) and the cytoplasm has acted in torn on the nucleus, a different nucleus.

Finally, it should not be lost sight of that the cytoplasm is in immediate control of all the everyday details of mitosis. Each centromere and nucleolar organizer is a gene, simple or compound, and when all of them keep time in nuclear cycle they are illustrating the unity of reaction of any species of gene to the changes in the common substrate, the cytoplasm. Other genes whose activities are less minutely observable evidently behave in the same way to give the disciplined uniformity of nuclear propagation from which the authority of nuclear action derives.