Polyploidy and Aneuploidy

POLYPLOIDY AND ANEUPLOIDY While the above principles apply to inheritance in most plants and animals, they break down to a greater or less extent in many of the most important cultivated plants, because these possess a more complicated mechanism for the reduction division, and therefore a more complex type of segregation.

Tetraploidy.

An organism with double the normal somatic and four times the gametic or haploid number is called a tetra ploid (Gr. TeTpair1ois, fourfold), and in general one with any multiple of this number is called a polyploid (Gr. iroXinrXoi s, manifold). Tetraploids may arise in a species in several ways. The commonest is by a doubling of the chromosome number dur ing vegetative multiplication of a plant. Thus if tomatoes are cut down and allowed to regenerate, about 2% will regener ate a tetraploid branch. Tetraploids can also arise by the union of two gametes in neither of which the reduction division has taken place. They are generally larger than the parent species, and fer tile inter se, but may be nearly sterile when crossed with the parent. They therefore satisfy many of the criteria of a new spe cies. In such a plant there are four chromosomes of each type, and two of each go into a gamete. The laws of heredity in it can be illustrated by the case of style length in the giant tetraploid form of Primula sinensis. The gene S for shortness is almost completely dominant. There are five genotypes whose gametes may be tested by mating with long-styled (ssss) plants.

The first four are all short-styled. The SSSS and SSSs types can only be distinguished by a genetical analysis of their progeny, but SSss and Ssss differ in that the former give one recessive in six when mated with a recessive, and one in thirty-six when mated inter se or self-fertilized, while the latter give the ordinary Men delian ratios of one in two and one in four.

Allopolyploidy.—A different type of tetraploidy arises from species crossing. If Primula floribunda and Primula verticillata (which have the same chromosome number of 18) are crossed, the offspring generally have 18 chromosomes, but are sterile, like mules. Such plants occasionally produce a tetraploid branch with the chromosome number 36. Such a branch is self-fertile, and the seeds from it yield the new species Primula kewensis. In this the nine pairs of chromosomes derived from each species pair with one another before the reduction division, so the gametes receive one haploid set from each parent species and the new plant breeds true. Such a condition is called allopolyploidy, that of polyploids arising with a species, autopolyploidy. Occasionally a chromosome from floribunda will pair with one from verticillata, and a new type of gamete will thus arise. Hence Primula kewensis does not breed quite true. It will also be clear that when two allopolyploid plants are crossed, the inheritance is not necessarily Mendelian. Recessive characters may disappear on crossing if the conditions before the reduction division are such as to prevent two chromosomes bearing the recessive gene from entering the same gamete.

The following cultivated plants are wholly or mainly polyploid: —Wheat, oats, plum, sour cherry, strawberry, apple, pear, rose, dahlia; and polyploid races of raspberries and other Rubi, chrys anthemum and many other plants exist in cultivation. In nature polyploidy is fairly common in certain genera, but inheritance in natural polyploids has been little studied. Polyploidy is rare in animals, but may arise from certain species-crosses.

Aneuploidy.

Besides the polyploids so far described, which have regular reduction divisions, haploid, triploid and other forms exist in which the chromosomes cannot all find mates, and the reduction division is therefore irregular. Such plants cannot there fore be propagated sexually, for the pollen and ovules are often infertile, and their combination practically never reproduces the parent. But they can be and are propagated asexually, and several domestic tulips, hyacinths and ornamental (though not fruit bearing) cherries are triploids. By irregularities in the reduction division plants may be formed with too many or too few chromo somes. The former are often quite vigorous, and may be eco nomically valuable. Thus all cultivated sweet cherries have one, two or three chromosomes more than the wild form. Their game togenesis is irregular, and they never breed true sexually, but can be propagated by grafting. The various types of ovule pro duced by such plants are sometimes equally viable, but the pollen grains with an abnormal chromosome number are always less so than those with the normal number; hence such plants when bred sexually gradually revert to the normal type. Forms with a chromosome number which does not permit of a normal reduc tion division and normal heredity are called aneuploid.

Triploidy is the normal condition in the endosperm of seeds. The second generative nucleus of the pollen-grain fuses with the double secondary nucleus of the embryo-sac, producing a triploid endosperm nucleus. When this receives a dominant gene from the pollen grain the character of the endosperm may be altered, a phenomenon known as xenia.