Chromosome Pairing in Intercrosses Between Stocks of Interchanges Involving the Same Two Chromosomes in Maize

Abstract

Cytological observations on intercrosses between stocks of maize interchanges involving the same chromosomes with different break points showed that at pachytene homologous terminal segments usually were paired whether the configurations were “pairs” or associations of four chromosomes. The intercalary segments showed extensive non-homologous association or asynapsis. In the crosses studied in which the break points were in opposite arms in both chromosomes, “pairs” in which the homologous intercalary segments including the centromeres were paired were never found. Hence, pairing usually began at the ends of the chromosomes, although it started, or occurred at the same time occasionally at other points along them. The intercalary segments were at a great disadvantage in pairing. The centromere played no apparent significant role in initiating pairing.In certain intercrosses the frequent failure of pairing in the short terminal segment translocated to another chromosome was correlated with complete pairing in the adjacent region, indicating the importance of regions near but not at the end in controlling the pairing behavior.Association initiated between two homologous regions continues to proceed along the chromosomes in a zipper-like manner, often bringing about an association of non-homologous parts, until it is prevented by a counter association from the opposite direction.The results of limited genetic linkage tests and spore quartet analyses of crossing-over frequencies showed that regions in which there was usually non-homologous association or asynapsis were characterized by reduced or no detectable crossing-over, whereas in a region that usually was paired, crossing over was essentially normal.