The Genus Collinsia. Xvii. a Cytogenetic Study of Radiation-Induced Reciprocal Translocations in C. Heterophylla.

RECIPROCAL translocation between nonhomologous chromosomes may be Adetected cytologically by observing an interchange complex at metaphase I. The interchange complex may occur either as an alternate or adjacent configuration, that is, zigzag or open ring or chain. In some species, the frequency of pollen mother cells with one or the other configurations is approximately equal; in other species, the number of pollen mother cells with an alternate configuration is significantly higher than 50 percent (directed orientation) . Reciprocal translocations within a species generally yield interchange complexes exhibiting either a random or directed orientation but not both (BURNHAM 1956). Seventeen colchicine-induced reciprocal translocations in Collinsia heterophy2la Buist (n = 7) gave interchange complexes with a random orientation at metaphase I ( SORIANO 1957). Cytogenetic studies of radiation-induced reciprocal translocations to be reported in this paper indicate a high frequency of directed orientation of the interchange complexes at this stage.