THE GENUS COLLINSIA. XVII. A CYTOGENETIC STUDY OF RADIATION-INDUCED RECIPROCAL TRANSLOCATIONS IN C. HETEROPHYLLA

Abstract

Four reciprocal translocations involving two nonhomologous chromosomes and one involving three nonhomologous chromosomes were obtained either from plants of Collinsia heterophylla (n = 7) at the prebud stage or from seed which has been exposed to ionizing radiation from an x-ray source or from a cobalt therapy unit. The interchange complexes occurred either as a ring or chain but rarely as bivalents at metaphase I; the rings and chains displayed a directed orientation at this stage. Although the percentage of stainable pollen grains and the number of seed per capsule were reduced in plants with an interchange complex, the plants were not semisterile; plants with an interchange complex of six chromosomes had a lower-set than plants with an interchange complex of four chromosomes. Five of the seven chromosomes in the complement are involved in the five reciprocal translocations. The gene for white flower is linked with two different reciprocal translocations which have one chromosome in common. The chromosomes of this species are assumed to have a terminal or subterminal chiasma- forming segment in each arm. To account for the different types of interchange complex obtained after colchicine treatment or exposure to ionizing radiation, it is assumed that the former treatmentmore » is responsible for chromosome breaks in the chiasma-forming segments and the latter for breaks in the segment of the chromosome proximal to the centromere. (auth)« less