Allopolyploidy is caused by the presence of several genomes of different species in a cell, a tissue or a whole organism. Most common are allotetraploids (= amphidiploids). Hybrids of different species are much more common in the plant than in the animal kingdom. As has been mentioned before , plant cultivators of the 18th and 19th century have worked nearly solely with such combinations. We know today that they are rather common in nature, too. Such hybrids are usually sterile since no bivalents can be formed during meiosis. The chromosomes stay univalent and are distributed by chance so that nearly only pollen and egg cells without function are developed. If the genome of one parental species is termed AA and that of the other BB than the hybrid's genotype would be AB. In very rare cases, all chromosomes are distributed onto just one daughter cell during meiosis (AB). A combination of two such cells (pollen and egg cells) leads to allotetraploidy (AABB) and restores fertility. In contrast to the autotetraploids, allotetraploids always develop bivalents. They do accordingly behave like normal diploids. At the same time, a new species with a fertility barrier between itself and its parental species is generated. The F1 of the bastards AABB x AA or AABB x BB are mostly sterile since not only bivalents but also univalents are formed during meiosis.
Not all new combinations that have developed by allopolyploidy have a real chance to survive. Many of them do not keep up to natural selection and do therefore never appear as new species.
Genomes of species not closely related can be combined by fusion of their protoplasts. This causes new problems. In the hybrids of soy bean and barley or those of soy bean and peas, for example, the different sets of chromosomes stay separated. Separate metaphase plates form during mitosis and the fusion product perishes subsequently (K. N. KAO et al., 1974).
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