Autopolyploidy

Autopolyploidy is caused by accidents of mitosis or meiosis that 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), as depicted in Figure 6.23a. An autotriploid (3n) may arise when nondisjunction in meiosis produces a diploid gamete that then fuses with a normal haploid gamete to produce a triploid zygote (Figure 6.23b). Alternatively, triploids may arise from a cross between an autotetraploid that produces 2n gametes and a diploid that produces 1n gametes. Nondisjunction can be artificially induced by colchicine, a chemical that disrupts spindle formation. Colchicine is often used to induce polyploidy in agriculturally and ornamentally important plants.

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Figure 6.23: Autopolyploidy can arise through nondisjunction in mitosis or meiosis.

Because all the chromosome sets in autopolyploids are from the same species, they are homologous and attempt to align in prophase I of meiosis, which usually results in sterility. Consider meiosis in an autotriploid (Figure 6.24). In meiosis in a diploid cell, two homologous chromosomes pair and align, but in autotriploids, three homologs are present. One of the three homologs may fail to align with the other two, and this unaligned chromosome will segregate randomly (Figure 6.24a). Which gamete gets the extra chromosome will be determined by chance and will differ for each homologous group of chromosomes. The resulting gametes will have two copies of some chromosomes and one copy of others. Even if all three chromosomes do align, two chromosomes must segregate to one gamete and one chromosome to the other (Figure 6.24b). Occasionally, the presence of a third chromosome interferes with normal alignment, and all three chromosomes move to the same gamete (Figure 6.24c).

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Figure 6.24: In meiosis in an autotriploid, homologous chromosomes can pair, or not pair, in three ways. This example illustrates the pairing and segregation of a single set of homologous chromosomes.

No matter how the three homologous chromosomes align, their random segregation will create unbalanced gametes, with various numbers of chromosomes. A gamete produced by meiosis in such an autotriploid might receive, say, two copies of chromosome 1, one copy of chromosome 2, three copies of chromosome 3, and no copies of chromosome 4. When the unbalanced gamete fuses with a normal gamete (or with another unbalanced gamete), the resulting zygote has different numbers of the four types of chromosomes. This difference in number creates unbalanced gene dosage in the zygote, which is often lethal. For this reason, triploids do not usually produce viable offspring.

In even-numbered autopolyploids, such as autotetraploids, the homologous chromosomes can theoretically form pairs and divide equally. However, this event rarely takes place, so these types of autotetraploids also produce unbalanced gametes.

The sterility that usually accompanies autopolyploidy has been exploited in agriculture. As discussed in the introduction to this chapter, triploid bananas (3n = 33) are sterile and seedless. Similarly, seedless triploid watermelons have been created and are now widely sold.