Chromosome constitution of polyploid wheats: Introduction of diploid wheat chromosome 4.

Chromosome 4 of diploid wheat (chromosome d4) is not present in hexaploid wheat. This chromosome has been added to hexaploid wheat and observed not to pair meiotically with its 21 chromosomes. Also, chromosome d4 compensates for Cornerstone male sterility, which involves a recessive mutation in chromosome arm 4AS. Chromosome d4 has been separately substituted for chromosomes 4A and 4B. These two substituted hexaploid chromotypes have the entire genome of diploid wheat and may have agricultural significance. An alternative hypothesis of the evolution of polyploid wheats is proposed that involves the loss of chromosome d4 and the retention of two versions of chromosome 4B at the early tetraploid stage.