Meiotic behavior of rye heterozygous for ditelocentric substitutions.

Meiotic behavior has been investigated in rye plants carrying a heterozygous ditelocentric substitution for chromosomes 1R and 3R. Such a chromosomal rearrangement replaces a biarmed chromosome with two telocentrics, a situation karyotypically comparable to a Robertsonian translocation. As in Robertsonian translocation heterozygotes, delayed pairing characterizes the centromeric region in the rye pachytene heteromorphic trivalents. In addition, telocentric axes appear to be frequently engaged in nonhomologous synapsis (e.g., fold-back loops and two-by-two heterosynapsis) at their centromeric ends. Unlike those in Robertsonian translocation heterozygotes, the synaptic abnormalities detected here cannot be due to lack of partner homology. Rather, the chromosomal rearrangement itself appears to interfere with normal synaptonemal complex (SC) progression. The synaptic irregularities, which often result in complex pachytene multivalents, seem to have no effect on chiasma formation, since the rearranged pairs behave properly at metaphase I. Finally, differences in both SC length and progression patterns between plants and animals could explain the trans configurations of telocentric centromeres in plants, whereas only cis configurations of acrocentric kinetochores have been described in animals.