Nuclear Deoxyribonucleic Acid Content and Endopolyploidy in the Meristem of Onion Roots

Abstract

PHOTOMETRIC analysis of the deoxyribonucleic acid content of nuclei would afford an interesting approach to the problems connected with polyploidy, polyteny and endomitosis, our knowledge of which is still fragmentary. It would also help us to understand the evolutionary processes and trace the relationships of species. If, in accordance with the hypothesis of deoxyribonucleic acid constancy, there is a strict correlation between the deoxyribonucleic acid content and the number of chromosomes, or more exactly the number of chromonemata, measurement of deoxyribonucleic acid would be the easiest way of studying polyploidy, polyteny and endomitosis, particularly in the interphase or prophase nuclei where the chromosome numbers cannot be counted. Evolutionary processes involving polyploidization from one species to another close one can be traced by means of photometric analysis, as the deoxyribonucleic acid content of the sperm or of the normal diploid nucleus appears to be characteristic of a species. The utilization of deoxyribonucleic acid content in problems of evolution is well illustrated by Hughes-Schrader's¹ work on a number of closely related species of mantids the karyotypes of which are not analysable by the usual methods of comparative cytology. The deoxyribonucleic acid content of the spermatids of two species, one having twice the number of chromosomes as the other, was the same, agreeing well with cytological studies which led to the conclusion that redistribution of chromosomal material rather than polyploidy was involved in the evolution of these species. On the other hand, in two other close species the deoxyribonucleic acid ratio of the spermatids was very near 1 : 2, indicating that polyploidy had played a part in their evolution.