Segmental Interchange in Chromosomes of Tradescantia.

Chromosome ring formation has been found in a number of difierent genera. In certain species of Oenothera and of Hypericum and in the monotypic genus Rhoeo, all the chromosomes may be united in a single ring or chain a t meiosis. Ring formation is maintained in these plants by a system of balanced lethals. In Datura, Zea, Pisum and Campanula rings of four or more chromosomes have been described. In most species with either large or small rings there is considerable pollen sterility due to non-disjunction at meiosis. BELLING’S (1927) hypothesis that ring formation is caused by segmental interchange between non-homologous chromosomes has been confirmed by the cytological and genetic evidence in Oenothera hybrids (CLELAND and BLAKESLEE 1931, EMERSON and STURTEVANT 1931) and in Zea (MCCLINTOCK 1930, BURNHAM 1930, 1932, BRINK and COOPER 1931, 1932) and from a study of chromosome arrangement in Rhoeo (SAX 1931). The behavior of reciprocal translocations in Drosophila (STURTEVANT and DOBZHANSKY 1930) is also in accord with BELLING’S hypothesis. The different types of chromosome configurations in the rings or chains is of interest in connection with the mechanism of chromosome segregation a t meiosis. Interlocking of bivalents and rings has been found in several’ ring-forming genera, and the relations of these locked pairs of bivalents have an important bearing on the nature of chiasma formation. Little is known concerning the mechanism involved in producing segmental interchange. Chromosome ring formation in Tradescantia provides an opportunity for a more complete study of these problems.