Chiasma frequency is region specific and chromosome conformation dependent in a rye chromosome added to wheat.

In many plant species synapsis starts at, or close to, the chromosome ends and this has been considered to be related to the distal location of chiasmata. In this regard we have studied the meiotic behavior of rye chromosome pair 5R in a wheat background using fluorescence in situ hybridization. The use of different DNA probes allowed the identification of the 2 rye homologues, their centromeres and subtelomeric heterochromatic chromomeres, and the telomeres of all chromosomes in prophase I and metaphase I. Three types of plants were analyzed: homozygotes for the standard chromosome 5R, homozygotes for a deficient chromosome 5R (del5R) with only the proximal 30% of its long arm (del5RL) and heterozygotes. Synapsis of the deficient chromosome arm pair del5RL was completed in most meiocytes at pachytene but the number of chiasmata formed was much lower than in the intact 5RL arm. Deletion facilitated the migration of the telomere of the accompanying chromosome arm 5RS during bouquet organization. This was followed by an increase of synapsis and chiasma frequency in this arm with regard to its counterpart of the intact chromosome. Results demonstrate that crossover formation depends on the DNA sequence or the chromatin organization of each chromosome region and that homologous alignment, synapsis and chiasma formation may be conditioned by chromosome conformation.