Allopolyploidy arises from hybridization between two species; the resulting polyploid carries chromosome sets derived from two or more species. Figure 6.25 shows how allopolyploidy can arise from two species that are sufficiently related that hybridization takes place between them. Species 1 (AABBCC, 2n = 6) produces haploid gametes with chromosomes ABC, and species 2 (GGHHII, 2n = 6) produces haploid gametes with chromosomes GHI. If gametes from species 1 and 2 fuse, a hybrid with six chromosomes (ABCGHI) is created. The hybrid has the same chromosome number as that of both diploid species, so the hybrid is considered diploid. However, because the hybrid chromosomes are not homologous, they will not pair and segregate properly in meiosis; this hybrid is functionally haploid and sterile.

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The sterile hybrid is unable to produce viable gametes through meiosis, but it may be able to perpetuate itself through mitosis (asexual reproduction). On rare occasions, nondisjunction takes place in a mitotic division, which leads to a doubling of chromosome number and an allotetraploid with chromosomes AABBCCGGHHII. This type of allopolyploid, consisting of two combined diploid genomes, is sometimes called an amphidiploid. Although the chromosome number has doubled compared with what was present in each of the parental species, the amphidiploid is functionally diploid: every chromosome has one and only one homologous partner, which is exactly what meiosis requires for proper segregation. The amphidiploid can now undergo normal meiosis to produce balanced gametes with six chromosomes each.

George Karpechenko created polyploids experimentally in the 1920s. Cabbage (Brassica oleracea, 2n = 18) and radishes (Raphanus sativa, 2n = 18) are agriculturally important plants now, as they were then, but only the leaves of the cabbage and the roots of the radish are normally consumed. Karpechenko wanted to produce a plant that had cabbage leaves and radish roots so that no part of the plant would go to waste. Because both cabbage and radish possess 18 chromosomes, Karpechenko was able to cross them successfully, producing a hybrid with 2n = 18, but, unfortunately, the hybrid was sterile. After several crosses, Karpechenko noticed that one of his hybrid plants produced a few seeds. When planted, these seeds grew into plants that were viable and fertile. Analysis of their chromosomes revealed that the plants were allotetraploids, with 2n = 36 chromosomes. To Karpechencko’s great disappointment, however, the new plants possessed the roots of a cabbage and the leaves of a radish. TRY PROBLEM 27