Genetic and Evolutionary Aspects of Allopolyploidy in Wheat

The polyploid species of the weat (Triticum and Aegilops) group constitute a classical example of evolutionary success through allopolyploidy. The evolutionary advantage of these polyploids over their diploid progenitors reflects a successful genètic system based on allopolyploidy, diploid-like cytological behavior and predominant self-pollination. The different genomes of the newly formed allopolyploids, derived recurrently from diverging diploid species, differ by numerous allelic variations of homoeologous loci. The permanent heterozygosity of the different homoeoalleles facilitated enzyme multiplicity and thereby, wider and greater adaptability. Whereas this genetic multiplication has an evolutionary advantage for loci coding for functional proteins, it may be redundant for others, e.g., multigene families such as rRNA genes or storage protein genes. Activity of all loci in such genes might result in over-production and inefficiency. One should expect, therefore, to find in polyploid wheat regulatory processes involved, on one hand, in the preservation of the activity of favorable gene loci and, on the other hand, in the reduction of the number and activity of the redundant ones. The latter include diploidization (inactivation) and gene dosage compensation (reduced gene expression) processes. The presented evidence indicates that diploidization is a non-random process achieved through mutations or intergenomic suppression. In contrast, gene dosage compensation is a non-specific process determined by several post-transcriptional rate-limiting factors. The evolutionary significance of these regulatory processes is discussed.