Spontaneous Chromosome Aberrations in Allium.

ROSS chromosome aberrations or rearrangements have recently G come to occupy an important position in the study of problems of evolution and speciation. Such aberrations are known to occur in many plants and animals. However, the spontaneous occurrence of these aberrations is rather difficult to study, since their frequency is usually very low. In an endeavor to obtain a further understanding of the nature and causes of these rearrangements many investigators have resorted to a study of similar rearrangements artificially induced by means of certain external agents such as irradiation, heat, chemicals, and aging. Such studies have revealed much regarding the different types of aberrations and how they occur. It is quite likely that with further investigation it may be possible to determine why such aberrations occur in nature. One of the first steps necessary is the accumulation of more data on the spontaneous occurrence of structural chromosome rearrangements. Very little data of this sort are available a t the present time. GILES (1940) has been able to estimate the number of spontaneous chromosome aberrations in certain Tradescantia hybrids. He also has shown that natural radiation is not sufficient to account for the spontaneous rate of chromosome alterations. It has been suggested by STUBBE (193513) that possibly the spontaneous chromosome mutation rate in plants is somewhat higher than has been found for certain animals. However, the spontaneous chromosome alteration rate has been investigated in only a few organisms.