Morbid mitosis and the activity of inert chromosomes in sorghum

Abstract

Sorghum purpureo-sericeum has five pairs of active, A, chromosomes and a variable number of extra, B, chromosomes in equilibrium in the wild population (Janaki-Ammal 1940; and table 7). The B-chromosomes vary in structure within and between plants owing to frequent spontaneous changes, including misdivision of the centromere. One is an iso-chromosome. The B's are sex-limited so far as that is possible in a plant: they are confined to the germ track owing to loss by lagging elsewhere. They are lost in the radicle before seed ripening and in the shoot tissues as they reach maturity. Only in the anthers and ovaries are they regularly maintained. B-chromosomes pair with one another at meiosis when homologous, and the two arms of the iso-chromosome form chiasmata with one another. Pollen grains of plus plants (with extra B's) have extra divisions of the vegetative nucleus rapidly following the primary division. The first pollen grain division is delayed by the presence of B-chromosomes. Its course is always normal. At the second division the B's always pass to the generative pole undivided and so double its dose. When only two generative nuclei are formed, one or both may produce sperm. Three, four or five generative nuclei, however, kill the pollen grain. The extra divisions are thus malignant. The B-chromosomes as usual are heterochromatic. They have an abnormal nucleic acid cycle. Their action on the cells containing them is non-specific and cumulative, and their apparently specific effect in stimulating mitosis in the pollen grains is possibly due to these being the only cells that contain them whose mitosis and growth are normally limited. Spontaneous structural changes in heterochromatic chromosomes are frequent at mitosis in plants and animals. Such changes could evidently establish malignant propensities in somatic cells by stimulating recurrent mitosis.