Chromosome Pairing in Intercrosses between Stocks of Interchanges Involving the Same Two Chromosomes in Maize

Abstract

Cytological observations were made in maize on diakinesis configurations from crosses between interchanges in which the same two chromosomes were involved but their break positions differed. Seventeen combinations produced by intercrossing seven different interchanges between chromosomes 2 and 6, two between 3 and 6, and two between 6 and 9 were used. Diakinesis configurations of the four interchanged chromosomes in the intercrosses of different types were as follows: In type 1a in which both breaks were in opposite arms in the two chromosomes and the interchanged segments were long enough, there was a high frequency of two ‘pairs’ (56, 70 and 76% for the three studied). In type 1a, when an interchanged end segment was a satellite, only associations of four were formed. In type 1b with both breaks in opposite arms in one chromosome and in the same arm in the other chromosome, all or nearly all were associations of four. In group 2 with breaks in both chromosomes in the same arm, a high frequency or all were two ‘pairs’. The type 1b intercrosses can be distinguished from the other two intercross types based on examination at diakinesis alone, provided no interchange segment is extremely short. Decreased sterility will distinguish certain of those in group 2 from those in type 1a. The high frequency of ‘pairs’ in type 1a and group 2 as well as the high frequency of associations of four in type 1b can be ascribed to the preferential initiation of pairing in the homologous ends of chromosomes rather than in the intercalary segments. In these same intercross combinations, pairing may occur simultaneously in intercalary as well as in end segments resulting in an association of the four chromosomes. ‘Pairs’ were never observed in which only the intercalary segments were paired.