Polyploidy

Most eukaryotic organisms are diploid (2n) for most of their life cycles, possessing two sets of chromosomes. Occasionally, whole sets of chromosomes fail to separate in meiosis or mitosis, leading to polyploidy, the presence of more than two genomic sets of chromosomes. Polyploids include triploids (3n) tetraploids (4n), pentaploids (5n), and even higher numbers of chromosome sets.

Polyploidy is common in plants and is a major mechanism by which new plant species have evolved. Approximately 40% of all flowering-plant species and from 70% to 80% of grasses are polyploids. They include a number of agriculturally important plants, such as wheat, oats, cotton, potatoes, and sugar cane. Polyploidy is less common in animals, but is found in some invertebrates, fishes, salamanders, frogs, and lizards. No naturally occurring, viable polyploids are known in birds, but at least one polyploid mammal — a rat from Argentina — has been reported.

We will consider two major types of polyploidy: autopolyploidy, in which all chromosome sets are from a single species; and allopolyploidy, in which chromosome sets are from two or more species.

Autopolyploidy

Autopolyploidy results when accidents of meiosis or mitosis produce extra sets of chromosomes, all derived from a single species. Nondisjunction of all chromosomes in mitosis in an early 2n embryo, for example, doubles the chromosome number and produces an autotetraploid (4n) (FIGURE 9.27a). An autotriploid may arise when nondis-junction in meiosis produces a diploid gamete that then fuses with a normal haploid gamete to produce a triploid zygote (FIGURE 9.27b). Alternatively, triploids may arise from a cross between an autotetraploid that produces 2n gametes and a diploid that produces 1n gametes.

(a) Autopolyploidy through mitosis

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