A form of *polyploidy
that results from the combination of sets of chromosomes from two or more different species. A (diploid) interspecific hybrid is normally sterile because there is only one of each kind of chromosome per cell. Thus at meiosis no bivalents are formed and any resulting gametes are usually inviable as they contain either too many or too few chromosomes. However if some unreduced diploid gametes form and fuse then the resulting tetraploid will usually be fertile as its nuclei will contain pairs of homologous chromosomes. An allotetraploid may cross with a diploid to form a sterile triploid hybrid, which, if it produces unreduced triploid gametes, may give rise to a fertile allohexaploid. If an allohex-aploid crosses with a diploid an allo-octaploid could arise in a similar fashion, and so on to higher levels of allopolyploidy. An allopolyploid is often intermediate in appearance between both Parental species and cannot reproduce with either. Hence it may merit the status of a new species. Allopolyploidy has been described as instant evolution. Many crop plants are believed to have originated in this way. Wheat (Triticum aestivum), for example, is an allohexaploid. It has a chromosome number of 42 and is probably derived from the three species T. monococcum, Aegilops spel-toides, and A. squarrosa, each with 14 chromosomes.
If at meiosis an allopolyploid forms only bivalents, i.e. it acts like a diploid, then it is termed an amphidiploid. Raphanobrassica (see allotetraploid) is an example. However if certain of the chromosomes from the two parent species are sufficiently similar (see homoeology) then multivalents may be seen at meiosis. When this occurs the allopolyploids are termed segmental allopolyploids. An example is the allotetraploid Primula kewensis derived from a cross between P. floribunda and P. verticillata. Sometimes segmental allopolyploids show reduced fertility as compared to amphidiploids and it is clear that in some allopolyploids there are mechanisms that prevent the formation of multivalents. Wheat, for example, normally behaves as an amphidiploid but if chromosome V of the B genome (the genome derived from A. speltoides) is missing then multivalents are formed.