Unusual Events in Seed Development

M. M. RHOADES.

BEFORE CONSIDERING unusual nuclear phenomena in seed formation, let us review briefly the standard sequence of events leading to the formation of the embryo and endosperm.

Normal sexual reproduction in the angiosperms involves the fusion of egg and sperm nuclei to form the zygote. Both have one-half the number of chromosomes found in the sporophytic tissues. Their fusion results in a restoration of the diploid number of chromosomes.

The zygote, or fertilized egg, divides. As a consequence of the numerous mitotic divisions that ensue, the young sporophyte is formed. The embryo of the mature seed contains the future vegetative organs of the sporophyte.

Developmental patterns vary widely in different groups of plants, but the mature embryo invariably consists of a hypocotyl-root axis, with the root meristem at one end and the shoot meristem at the other.

One of the two sperm cells discharged into the embryo sac by the pollen tube unites with the two polar nuclei to form the primary endosperm cell. Secondary fertilization is characteristically and uniquely found in the angiosperms.

The rapidly dividing endosperm plays an important role in most instances a decisive one, in providing nutrition for the young embryo. Nutrients from the enveloping sporophytic tissue are channeled to the embryo by the endosperm.

If the endosperm fails to develop normally, the embryo suffers; it may abort or remain in a rudimentary stage. In such plants as the common cereals, the endosperm continues to grow and forms a major portion of the mature seed. In other plants, the endosperm undergoes degeneration as the developing embryo utilizes its substance and only vestiges of the endosperm are present in the mature seed.

The endosperm in intercrosses is of a hybrid nature and may exhibit certain characteristics of the pollen parent. In corn, for example, when a strain with white endosperm is pollinated by one with yellow endosperm, the F₁ seeds will have the yellow color of the male parent. This direct effect of the pollen parent on the endosperm of the seed is called Xenia.

Failure of normal seed development leading to seed collapse is due to a number of causes. In corn, a large number of simply inherited recessive genes are known which, when homozygous, result in defective or shriveled seeds.

Aneuploidy and genic unbalance also produce aborted seeds. The so-called seedless bananas and watermelons are triploids, which, because of an irregular assortment of the 3N number of chromosomes during meiosis, have unbalanced chromosomal complements in both megaspores and microspores. The aneuploid megaspores may fail to produce functional female gametophytes, but if they do give rise to an embryo sac and fertilization occurs, the ensuing proembryo and endosperm are arrested in early development and shriveled seed result.

seedlessness in grapes results from an inhibited growth of the seed under the influence of a specific maternal genotype or from a lack of functional female gametophytes. In this latter situation, no fertilization is possible, but a parthenocarpic fruit develops.

The seedless fruits of most pineapple varieties are caused by pollen incompatibility the inability of pollen tubes to grow down the style and discharge the sperm cells into the embryo sac. One commercial strain, however, the Cabenza of Florida and the West Indies, is a triploid, and seedlessness here, as in both triploid bananas and watermelons, results from the malfunctioning of unbalanced gametophytes or young sporophytes.

Since aborted seeds are found in the fruits of triploid watermelon, banana, and pineapple, the term "seedlessness" is hardly appropriate, but from a gastronomical point of view this is a matter of no consequence.

Normally one of the two sperm cells discharged into the embryo sac fuses with the egg nucleus to form the zygote, and the second sperm from the same pollen tube unites with the two polar nuclei to give rise to the primary endosperm. cell.

Not uncommonly, however, in certain plants more than one pollen tube enters the embryo sac. When this occurs, a sperm cell from one pollen tube may fertilize the egg and a sperm cell from another pollen tube may fuse with the polar nuclei.

This phenomenon is known as heterofertilization. It results in embryo and endosperm of different genetic constitutions if the male parent is heterozygous and dissimilar sperm fertilize egg and polars.

Other phenomena that may occur when more than one pollen tube is present are the fusion of two sperm cells with the egg to give a triploid zygote and the fertilization of the synergid by a third sperm.

When fertilization of both egg and synergid takes place, the embryo sac will contain two zygotes, which will be diploid-diploid twins or triploid-diploid, depending on the number of sperm involved in fertilization of the egg.

Not all diploid-diploid twins come from fertilization of more than one cell of the same embryo sac. Some arise in ovules with two embryo sacs where both eggs are fertilized. Others stein from a cleavage of the proembryo.

In certain apoinictic plants, such as Citrus, a number of sporophytes in a single seed are produced by sporophytic budding of the cells of the nucellus or integuments. Haploid-diploid twins in nonapomictic plants originate when the fertilized egg forms a diploid embryo and a synergid or antipodal is stimulated to divide and give rise to a haploid sporophyte.

Infrequently a haploid sperm cell which has entered the embryo sac will, under some unknown stimulus, divide to form a haploid embryo. This is the phenomenon of androgenesis. It occurs much less frequently than does the parthenogenetic development of the egg cell into a haploid sporophyte. As expected, an androgenetic haploid sporophyte has the characteristics of the pollen parent.

Fertilization of antipodals by accessory sperm has also been reported, but presumably this is a rarer event than fertilization of synergids, which are more like the egg cell in size and position.

Another kind of anomalous nuclear behavior due to polyspermy, which, however, is not well substantiated, is the separate fertilization of the polar nuclei by two different sperm cells. If one or both developed into the endosperm, it would be diploid rather than triploid. Reports exist in the literature that in plants with sexual reproduction the unfertilized polars may exceptionally give rise to the endosperm.

There is no question but that the haploid egg can occasionally develop into a haploid sporophyte, although the associated endosperm arising from fertilization of the polars is essential for continued growth of these haploid embryos.

It is possible that unfertilized polar nuclei may also be able to undergo division. This is certainly true in plants with autonomous apomixis where both embryo and endosperm arise from unfertilized, but unreduced, egg and polar nuclei, respectively.

The seeds produced in crosses between polyploid and diploid races are defective because of an upset in the 2:3 ratio of chromosome sets usually found in the embryo and endosperm.

In the cross of a tetraploid (4N) by a diploid (2N) pollen parent, a triploid (3N) zygote and a pentaploid (5N) endosperm, result. Endosperm development is greatly restricted, and the young embryo consequently does not undergo full development, since it is dependent upon the meristematic endosperm for the transfer of nutrients from the mother plant.

In the reciprocal cross of 2N X 4N, the embryo is again 3N in constitution, while the endosperm is 4N. Poorly developed seed are formed. There is a departure in both kinds of crosses from the usual 2:3 ratio found in the embryo and endosperm of diploids, and this is reflected in a drastic impairment of seed development.

It is generally true, however, that there is a less drastic effect in the 4N X 2N cross than in the reciprocal.

The suggestion has been made that the genomic constitution of the maternal parent is a factor affecting seed development and that the production of normal seeds depends upon a specific ratio of chromosome sets in maternal tissue, embryo, and endosperm.

AN EXCEEDINGLY important technique for the plant breeder who wishes to obtain hybrids from crosses that produce aborted or nongerminable seed is that of embryo culturing.

Embryos removed from mature but nongerminable seed and placed on an artificial medium have given rise to viable seedlings. More spectacular is the finding that, when young embryos abort during development, seedlings have been obtained by excision of the young embryos and growth on a sterile nutrient medium.

Through the use of embryo culturing, it has been possible to obtain desired hybrids after repeated failures with conventional methods.

The F, hybrids between different species of the same genus or between species of different genera often are highly sterile and do not produce viable seed. The sterility may be caused by an irregular assortment of the chromosomes at meiosis, leading to aneuploid spores which abort, or to disharmonious (lethal) constellations of genes in the haploid spores if pairing of the chromosomes in meiosis is fairly regular.

These sterile hybrids in many instances have been converted into fertile amphidiploids by a doubling of the chromosome number. This occurs either by formation of unreduced gametes or in a somatic cell. Doubling of the chromosome number may readily be induced experimentally by chemical compounds, such as colchicine. The sterility of the hybrids has been overcome by a duplication of the chromosomes.

Some of our most valuable crop plants have arisen from sporadic crosses of different species followed by doubling of the chromosomes.

For example, the cultivated tobacco (Nicotiana tabacum) has been shown experimentally to be a tetraploid derived from a cross of two diploid species of Nicotiana.

The cultivated cotton (Gossypium hirsutum) races have arisen in a like manner from a hybrid of two related diploid species, and it has been demonstrated that the hexaploid (6N) common bread wheats (Triticum aestivum) have two sets of chromosomes from a diploid species of wheat and two sets each from two different species of the related genus Aegilops.