Cytological Evidence for Segmental Interchange between Non-Homologous Chromosomes in Maize

Abstract

In the first meiotic division in the pollen mother cells of 2 strains of partially sterile maize, semisterile-1 and semisterile-5, a ring of 4 chromosomes and 8 bivalents are found. Semisterile-5 is a particularly favorable object for study in that one of the 2 pairs of chromosomes involved in the ring is regularly attached to the nucleole at the end which bears satellites. This attachment makes it comparatively easy to follow the ring complex in the spireme stage when it is recognizable as a cross-shaped figure. Such a figure would be expected, if one accepts Belling''s hypothesis that ring formation is a consequence of interchange of terminal segments between non-homologous chromosomes, because the homologous portions of the chromosomes of the ring would be paired at this stage. The length of the arms of several cross-shaped figures in semisterile-5 material was determined and from them it appears that the interchange involved 2/3 of the satellite chromosome and 1/3 of the other chromosome entering the ring. The individual chromosomes of the ring are distributed in 3 ways at anaphase: (1) adjacent chromosomes of approximately the same size may go to the same pole, (2) adjacent chromosomes of different sizes may go to the same pole or (3) alternate chromosomes may go to the same pole. On the basis of the segmental interchange hypothesis, only the spores resulting from the last method of distribution would be functional.