The Evolution of Genetic Systems pp. 46-48 (1958)
C. D. Darlington

Chromosome Size

One kind of adaptation which can occur and, in organisms with large chromosomes evidently has to occur, is a reduction of chromosome size. All species of plants and animals have a standard size of chromosome in each tissue and this standard size is usually maintained through most or even the whole of development. In polyploid plants and animals this standard size is often smaller than it is in their diploid relatives. Whether it is smaller than it was in their immediate diploid ancestors does not matter. The point is that polyploidy has been possible only in those species or races of Narcissus or Tulipa or Mantis which have the smaller chromosomes.

How does this reduction of chromosome size come about? By X-ray treatment of root-tips it has been possible to induce mitoses showing chromosomes of a reduced size. Among seedlings of the same parent plant of Lolium perenne evidence of an even greater change has been found: a range of size of perhaps 1:16. In the course of differentiation of one individual changes in chromosome size may occur. This is true of normal differentiation in many plants. In the regeneration of rat liver a fivefold halving (to 1/32) seems to be possible. Evidently prophase can be brought on before the chromosomes have reproduced: chromatids are then formed with half the proper number of nucleo-protein units in their structure.

These changes show us one of many adaptations that must be supposed to underlie and condition the evolution of polyploidy. They also show us something of the physico-chemical properties of the materials underlying all chromosome structure. The ultimate nucleoprotein thread of which the chromosome consists must often, or always, be a multiple thread: the chromosome is polynemic. And a halving of the polynemic thread may compensate physiologically and mechanically for a doubling of the complement in polyploidy. Several interesting physiological questions arise with regard to the relations of polyploidy and polynemy, one of which may be mentioned.

Consider the relative sizes of growth of haploids, diploids and polyploids. Where a haploid or polyploid arises from diploid ancestors, through an error in reproduction, it is different from the diploid, smaller or larger. But where it is a regular part of the sexual cycle it can be adjusted to precisely the same size. Haploidy and diploidy then have no differential effect on growth. In some red and brown algae the two phases can have the same type of growth. They are said to be isomorphic. Similarly in the Hymenoptera the males which are normally haploid need be no smaller than the diploid females. And when a male turns out to be diploid (owing to a breakdown in its system of sex determination) it is no bigger than the haploid. The genes can be adjusted or compensated to produce the same effect in double dose as in single dose. Is this done by a change in polynemy? We do not know.