The role of U-type exchanges in the differentiation of karyotypes in Najas marina

Abstract

Spontaneous bridge and fragment formation was observed at the first and second meiotic division, in root tip, premeiotic and pollen mitoses of both chromosomal races of Najas murina, a dioecious and annual species. The frequency of disturbed divisions ranged from 0.9 to 6.5 per cent. Since no reverse loop pairing was observed at pachytene, and the aberrations observed were incompatible with inversion polymorphism, the most probable source of the disturbances was considered to be U‐type exchanges. Aberrations in mitotic divisions were also presumed to be formed by U‐type reunion of sister chromatid exchanges. All Ihe fragment size classes can be explained on the basis of a breakage and reunion in the centromeric heterochromatin and in the allocyclic regions. However. some break points outside these regions were actually seen. Hence additional 'weak points' are believed to be present in the brightly fluorescent distal parts of the chromosomes. A model is presented of the rearrangements leading from karyotype B to A. Spontaneous breakage and reunion produce continuously new combinations of chromosome segments in N. marina, and hence have been of evolutionary significance in the differentiation of the two karyotypes.