HETEROTYPIC PROPHASES IN THE ABSENCE OF CHROMOSOME PAIRING

Abstract

The prophases of the heterotypic division in a wheat-rye hybrid, in which the chromosomes almost always fail to mate, are described and compared with those in the pure parents in which mating is normal. Up to the end of synizesis the events in the hybrid are in all essentials exactly the same as in the pure parents. Consequently the appearances presented in these stages can have no significance in relation to mating, because mating does not occur in the hybrid. The first striking differences in behavior occur in the stages between synizesis and second contraction when the spireme of the pure species forms loops, the sides of which approximate and twist about each other; in the hybrid spireme the loops are quite irregular. At diakinesis in the pure species three bivalents can be found connected, the mode of attachment being one which could result only from a telosynaptic arrangement of chromosomes.The entire evidence obtained in this study of the wheat-rye hybrid and its parents is in accord with the theory that the spireme is composed of univalent chromosomes "in tandem", and that mating by the formation of loops begins in the post-synizetic period when chromosomes are relatively long and thin. This affords ample opportunity for cross-over phenomena.