Allopolyploids appear to offer greater promise than autopolyploids in plant breeding. In general, they are much more fertile and more nearly true-breeding. Allopolyploids provides a means of conferring fertility upon hybrids between different species, while retaining all of the hybrid vigor. It thereby provides for the synthesis of new species almost at will, within the limits of cross-fertility. Some of the new species thus produced will certainly have practical value.

Perhaps the best known allopolyploid involving crop plants is wheat X rye ( that is, wheat crossed with rye). This new species ( called Triticale, from Triticum for wheat and Secale for rye) has been produced by a number of investigators, and has several desirable characteristics.

Triticale is relatively infertile, however, and thus has not proved useful. Other allopolyploids involving wheat have been produced, such as macaroni ( durum) wheats X Timopheevi (a recently discovered wheat highly resistant to various diseases), which are quite fertile. Some of these are highly regarded, particularly in Russia. The perennial wheats reported by Russian breeders are not allopolyploids, but are segregates from crosses of wheat with Agropyrons. It is doubtful that perennial wheats would be acceptable to American farmers, because growing wheat for two or more successive years on the same land is contrary to good crop-rotation practice.

Aside from their possible value as new species, many allopolyploids are useful as breeding material. Turning to the wheats again, we find certain diploid species of the related group Aegilops that possess desirable characteristics, particularly resistance to disease. The direct transfer of these characteristics to hexaploid wheats is difficult or impossible because of the failure of the crosses to set seed, or because of extreme sterility of the hybrids. But by first adding the diploid Aegilops species to a tetraploid wheat through allopolyploidy, we can obtain a plant with 21 pairs of chromosomes that can be crossed readily with wheat. Furthermore, the resulting hybrid has a fair degree of fertility.

The problem of polyploidy involves much more than just the artificial production of new types. Many of the common crop plants are polyploid—wheat, oats, tobacco, and cotton, to name a few.

The genetic analysis of polyploids is difficult, because the effects of a particular pair of genetic factors may often be concealed by other similar or identical genes located on a different pair of chromosomes. In wheat,

Here is a potentially useful allopolyploid in the wheat group: Heads of A, Triticum dicoccoides, one parent; D, Aegilops speltoides, the other parent; B, the sterile hybrid between A and D; and C, the same hybrid with chromosome number doubled by colchicine treatment and now allopolyploid and fertile. The use of this or a similar allopolyploid would seem to furnish the best chance of transferring to wheat the high resistance of Ae. speltoides to the wheat rusts. The allopolyploid, itself highly resistant, can readily be crossed to common wheat, whereas Ae. speltoides crosses with common wheat with difficulty and sometimes not at all. although many genetic studies have been carried out, the total genetic situation is much less clearly understood than in many nonpolyploid plants that have received considerably less attention. Hence, breeding of wheat has not proceeded on so precise a basis as plant breeders would like.

Recent research with tobacco and wheat shows the possibility of obtaining whole-chromosome deficiencies and using them in genetic studies. In tobacco, an allotetraploid with 24 pairs of chromosomes, 24 different types of monosomic plants have been obtained, each deficient for a different chromosome. In common wheat, a still more extreme type of deficiency has been obtained—namely, nullisomics. These completely lack one pair of chromosomes; that is, they have only 20 pairs of chromosomes instead of 21. Of the 21 different nullisomics possible in wheat, each deficient for a different pair of chromosomes, 18 have been obtained.