Some theoretical aspects of selective segregation in interchange complexes

Abstract

1. A population of Allium triquetrum heterozygous for an interchange has been discovered. The meiotic complex of a chain of four is oriented in an alternate fashion in about 75% of cells. 2. The suggestion is put forward that adjacent centromeres of a complex, if linked by at least one chiasma, behave as in a bivalent. Because there are four centromeres in a complex of four chromosomes, the basic orientation frequency is 50% alternate, 50% adjacent. Adjacent orientations are not basically the result of rigidity caused by chiasmata, and an interaction between homologous centromeres or other chromosome parts is not involved in the orientation mechanism. 3. Chiasmata may impose rigidity on the complex and so affect orientation frequencies. 4. Two (necessarily homologous) centromeres linked by an interstitial chiasma will be co-oriented as in a bivalent and adjacent 2 orientations are eliminated. 5. In chain forming configurations without interstitial chiasmata. one type of adjacent orientation is eliminated, depending on whether the centromeres are situated on one or the other pachytene axis; the configuration eliminated is replaced by alternate orientations, the basic frequency of which is thus increased from 50% to 75%. Interstitial chiasmata in a chain configuration alter the orientation frequencies. 6. Ring complexes may orientate in an alternate fashion in 75% of cells. 7. Relative lenghts of pachytene axes have no effect on orientation frequencies. 8. Multiple interchange heterozygotes such as Oenothera have probably evolved a special feature that, together with the typical orientation mechanism found in most organisms, enables a very high frequency of alternate segregation to be attained. 9. Apparent genotypic control of segregation may be the result of genotypic control of chiasma position.