The Evolution of Rosa

Abstract

THE recently published paper of Dr. C. C. Hurst (“Experiments in Genetics,” 38, 1925) on the chromosomes and characters in Rosa may well mark a new epoch in biology. We must await the appearance of his monograph for full details, but as he suggests, we may in the meanwhile try to apply the Rosa principles to other plants and animals. It is suggested that the common ancestor of all roses was a northern decaploid species, at present unknown. The various forms could have arisen by the dropping out of sets of chromosomes, the existing five diploid types being the end-products of this process. We are told, however, that “certain cultivated triploid and tetraploid forms are obviously duplicated forms which have arisen by duplication of the septets of chromosomes.” These maintain the essential specific characters of the original diploid species, instead of showing a complex of the characters of diverse septets. Is it not possible to suppose that this duplication may have been the first step in the production of polyploid species, the latter acquiring the diverse septet characters by successive mutations? The Drosophila work has shown that mutations are likely to be lethal or unfavourable. In a diploid species such mutations should apparently work more havoc than in a polyploid one. Polyploidy might then be a condition favouring the survival and accumulation of mutated genes, resulting after very long periods of time in diversity of the septets. Such diversity might come to have its advantages, as Dr. Hurst indicates, in specific cases. On the other hand, in some cases the shedding of a septet might be ad vantageous, getting rid of some undesirable features, and producing a more uniform or consistent type. Thus the diploid R. rugosa, which I found to be a strictly sea-coast plant in Siberia, is a well-defined type specially adapted to its peculiar habitats but not extending even a few miles inland. Yet it is not to be expected that the diploids will all be successful, and close field study combined with cytological research may be expected to reveal a variety of forms with reduced chromosome numbers, coming into existence only to perish at once or in a few years. Many, probably, may not even be capable of development.