Autonomous and interrelated formation of chiasmata in Delphinium chromosomes

Abstract

1. Chiasma frequency observations in Delphinium showed an interchromosome effect which occurs consistently in a large number of plants. 2. An interplant analysis indicated that significant differences in mean chiasma frequency do not affect the direction or the degree of interchromosome correlation which in the present material was invariably found to be of the negative type. When the analysis was extended to intraplant level however, a positive relationship was revealed between the strength of the interchromosome correlation and the number of chiasmata in a cell, the relation tending to breakdown towards the higher extreme of the chiasma range. 3. It has been concluded from these observations that each chromosome pair is adapted to have a minimum number of chiasmata independently of others but the different pairs show a competition effect beyond this threshold frequency. The competition is relaxed after a saturation point is reached. 4. Evidence has also been presented which supports the conclusion that the threshold beyond which interchromosome effects occur may vary in different individuals showing genetically determined differences in their chiasma frequencies. This variation partly accounts for the extremely variable behaviour of different organisms both at inter and intraspecies level with regard to interchromosome effects.