The Relationship between Chiasmata and Crossing over in TRITICUM AESTIVUM.

Telocentrics for the beta arm of chromosome 4A and the long arm of 6B were used as cytological markers for the determination of chiasma frequency. In concomitant studies of recombination, terminal segments of rye and T. umbellulatum chromatin carrying Hp (Hairy peduncle) and Lr9 (Leaf-rust resistance), respectively, marked 4A and 6B. Two temperatures, 21 degrees and 32 degrees , were used for both the 4A and 6B experiments.-Only one chiasma was observed in each heteromorphic bivalent. Because there was a substantial reduction in pairing between diakinesis and metaphase I, all determinations of chiasma frequency were made at diakinesis. In the 21 degrees experiments, agreement was good between genetic recombination and cytological prediction on the basis of the partial chiasmatypy hypothesis that each chiasma represents a crossover. At 32 degrees both chiasma frequency and crossing over, but particularly the latter, were strongly reduced. The fewer crossovers than expected are explained in part by stickiness of chromosomes at the high temperature, sometimes resulting in adjacent chromosomes being wrongly scored as having a chiasma, and in part by premetaphase disjunction of some recombined bivalents and subsequent independent behavior of the two resulting univalents.-Male transmission of the 4A telocentric from the heteromorphic bivalent was unusually high: 51% at 21 degrees and 31% at 32 degrees .