Chromosome Behaviour in Early and Advanced Generations of Tetraploid Maize

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Abstract
Increased seed set and yield in a tetraploid maize synthetic that was mass selected for 22 years, was not accompanied by a change in chromosome associations during diakinesis. Frequency of both multivalent and bivalent formation remained constant from one generation to the next. Occurrence of trivalente and univalents was rare. Dihaploids obtained from different generations of tetraploids were not different from normal diploid maize in cytological behaviour, thus supporting the conclusion that no structural changes had occurred in the chromosomes. Conflicting data from the literature concerning diploidization mechanisms are discussed. Selection for fertility as a heritable unit and selection against cytologically unbalanced gametes are postulated as possible explanations for the increased seed set.