Products of human ingenuity often mimic what nature has already produced. This is true of grass evolution, where cytological analyses indicate that natural hybridization and chromosome doubling are widespread and a major cause of plant speciation. In addition, it is apparent that polyploidy, the multiplication of chromosome sets, has figured prominently in the evolution of the grass family (Avdulov, 1931; Stebbins, 1972). Whereas the incidence of polyploidy in flowering plants as a whole has been estimated at 30 to 35%, Carnahan and Hill (1961) stated that approximately 80% of grasses are polyploid.
Many taxonomists believe that there has been an evolutionary trend from an ancestral diploid state to the more recent polyploid species. Multiplication of chromosome sets is proposed to occur through two general mechanisms: simple doubling of chromosome complement within a plant (resulting in autopolyploidy) or hybridization between distinct species in which entire chromosome sets are inherited by the offspring (resulting in allopolyploidy) (Fig. 2-3, 2-4). Experimental doubling of chromosomes generally involves treatment with chemicals such as colchicine. In nature, doubling occurs when chromosomes in cells destined to become pollen or ovules are doubled in preparation for meiosis, but meiosis fails to occur, giving rise to unreduced gametes. Since the generation of an autopolyploid involves creation of homologous chromosome pairs, such grasses frequently are at least partially fertile.
Allopolyploids contain chromosome sets (genomes) derived from two or more distinct ancestral species. Interspecific hybridization would normally result in an infertile progeny, but spontaneous chromosome doubling in the hybrid would restore fertility by generating identical, thus homologous, chromosomes (Gould and Shaw, 1983). Natural allopolyploids are abundant in the Festuca-Lolium complex, where chromosome numbers in naturally occurring species range from diploid (2n = 2x = 14) to decaploid (2n = 10x = 70).
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