Breeding Methods in Asexually Propagated Crops

After reading this article you will learn about the breeding methods in asexually propagated crops.

Reproduction which does not involve the union of gametes is known as asexual reproduction. Reproduction in which sexual organs or related structures take part but fertilization does not occur, so that the resulting seed is vegetatively produced is known as apomixis.

In those vegetatively propagated crops, in which, flowering occurs and normal fruit and seed setting takes place, the usual breeding procedures applicable to self/cross-pollinated crops can be used. However, in those cases where normal flowering does not occur or fruit/seed setting is poor, other breeding procedures like clonal selection and induced mutations are utilised.

For commercial use, some important cultivated plants such as potato, sugarcane, many fruit species, and ornamental plants are not multiplied by seed, but are vegetatively propagated by tubers, cuttings, grafting’s, etc. All plants of a vegetatively propagated variety are identical in a genetic respect, i.e. they form a clone, which is one individual divided into many parts with the same genetic constitution.

Therefore, no genetic variation is to be expected in a clone variety, unless mutations, i.e. changes of genotype, occur spontaneously within the somatic tissue, resulting in changes of morphological traits or physiological behaviour. Such sports, as these somatic mutations are called by growers, have played an important role in development of new varieties, especially in fruit species and ornamental plants.

All vegetatively propagated species have the great disadvantage of easily spreading pests and diseases, especially viral diseases, from one generation to the next by vegetative propagation. In cases of strong infection, this may then cause complete breakdown of varieties.

The formation of somatic mutations and the versatile possibilities of their use in breeding can only be understood if the structure of the apex of the shoot is known. All organs of a plant originate in the apex. In the flowering plants, the developing vegetative cone consists of three different tissue layers, termed L₁ (dermatogen), L₂ (sub-dermatogen), and L₃ (corpus).

From the L₁ the epidermis is formed, from the L₂, the mesophyll and the gametes and from L₃, the vascular bundle, the flesh, the pith and the adventitious roots. Besides the L₃ layer, the L₂ layer sometimes partially participates in the organisation of the inner corpus.

Usually, the cell division in L₁ and L₂ is anticline, i.e. vertical to the layers, so that these layers show surface expansion only, whereas in the L₃ there are anticline and pericline divisions. Consequently, the cells also divide parallel to the layers.

If a mutation occurs in a cell, it is more or less bound to the layer. The mutated cell can expand by anticline divisions within its layer only, resulting first in the formation of a mericlinical chimera, i.e. a tissue layer consisting of normal and mutated cells. If from such a mericlinal vegetative cone a shoot develops, sectors with an altered feature can be formed.

A tissue layer comprising mutated cells only, e.g., an epidermis, can participate in the formation of a lateral shoot; the mutated L₁ layer then covers the genetically unchanged L₂ and L₃ layers like a coat. A stable mericlinal chimera is then formed, a so-called periclinal chimera, which is characterised by tissue layers genetically differing from one another.

If the lateral shoots are formed without the involvement of tissues of the mutated sector, the mutation in this spot is lost.

The mutation can be of the genic or of the chromosomal type. Since vegetatively propagated plants are, in general, highly heterozygous and gene mutations mainly change an allele from a dominant to a recessive state, the mutation will manifest itself as a recessively conditioned new formation in the morphological or physiological behaviour of the whole plants as periclinal chimera, provided that the original periclinal chimera covers the entire layer affected by the mutation.

The periclinal chimeras mostly remain stable when propagated vegetatively; in the case of sexual propagation, the genetic behaviour of the progeny is solely determined by the genetic constitution of the L₂.

The layer-bound gene mutations can only manifest themselves, however, the features controlled by the mutated genes are also formed from the layer, in which, the mutation takes place. This means that in vegetatively propagated plants, many mutations can be present as non-detectable cryptic mutations. The older a clone, the more crypto-mutations, it will contain; examples of this are, the old wine varieties.

The somatic mutations can be expressed by conspicuous morphological changes such as the colour of the flowers, lack of thorns, and others, or even by small changes of quantitative features that can often only be observed as such, if the clones are examined for differences.

Periclinal chimeras are not only characterised by different genetic constitutions of the single tissue layers, but sometimes also by different chromosomal constitutions.

The apple variety “Winesap”, for example, has a diploid L₁ and L₂ layer, whereas the L₃ is tetraploid. Since the L₂ and L₃ tissues participate in the formation of the fruit pulp, the apples are distinguished by an asymmetrical gigas shape.

Occasionally, there are spontaneous rearrangements of the tissue layers in periclinal chimeras. Since these can be experimentally induced, they are important aids in the analysis of periclinal chimeras. The rearrangement of genetically differing layers often leads to new phenotypes, mostly by activation of cryptic mutations, and therefore increases the variation of vegetatively propagated species.

A rearrangement of the layers can take place in such a way that, e.g., cells of the L₁ migrate into the L₂ layer by periclinal division or, conversely, cells of the L₂ can break through the L₁ layer. These processes can often be observed if, through external influences – such as X-rays or other mechanical changes – one or the other layer has been destroyed.

Due to their genetically differing layers, periclinal chimeras are also termed heterohistonts. From them, homohistonts can be obtained by induction of lateral buds out of the L₃ layer and by root propagation. For this, it is necessary to decapitate a shoot and to eliminate all lateral buds, which stimulate the formation of side buds from the phloem (L₃).

After cutting out all eyes of potato tubers, adventitious shoots regenerate out of the inner tissues after 4-8 weeks. Using this method, it has been possible to prove that the potato variety “Rote Erstling” originating from “Erstling” is a mono-ecto-chimera. After removing the buds of red tubers the regenerates resulted in the original white-tubered “Erstling”.

The heterohistont has reorganised itself into a homohistont again. A further proof for Rote Erstling being a mono-ecto-chimera could be obtained by crossing “Rote X Weisse Erstling”. All progeny produced white tubers, i.e. the L₂ layer of “Rote Erstling” had remained unchanged; it was genetically the same as that of “Erstling”. The sexual transfer of valuable features from mono-ecto-chimeras is therefore dependent on a preceding change into a meso-chimera.

In apple varieties with a tetraploid L₃ layer, pure tetraploid homohistonts can be obtained by induction of adventitious buds, which also produce a tetraploid progeny after sexual propagation.

The induction and use of periclinal chimeras, especially by methodical induction of layer rearrangements, is of interest in plant breeding for a variety of reasons; it has a potential of breeding possibilities that has not yet been fully exploited, especially in ornamental plants, as has been shown in experiments with Euphorbia, Pelargonium, Dahlia, Rosa, and others.

Today, by application of methods developed in recent years for the regeneration of plants from parts of tissues that can be grown in culture media, it is possible to obtain uniform mutants from chimeras much more systematically. Using somatic mutations in single tissue layers, additional desired changes in certain traits can be induced in valuable varieties, without losing the valuable basic character of the original variety.

The fact that many vegetatively propagated plants are periclinal chimeras to a much higher degree than imagined earlier, and that latent genetic variability can develop by means of layer rearrangement can explain the breeding success of a clonal selection, e.g., in potatoes and vines, which had not been understood before.

Another proof for the relatively high genetic variability of old vegetatively propagated, cultivated varieties lies in the existence of various clones of a vine variety, which are characterised by adaptation to special locations. Furthermore, this explains the success of systematic clonal selection.

Spontaneous mutations in somatic tissues, the bud mutations (sports), have resulted in valuable new varieties, especially in fruit species and diverse ornamental plants.

An increase in the spontaneous mutation rate by mutagenic agents would increase the chance of the occurrence of desired mutations and accelerate the breeding process compared to the conventional method of developing novelties, namely, by cultivation of seedling progenies deriving from crosses or open-pollinated female plants.

In species with long generation intervals, such as kernelled and stone fruit, tulips, and others, this would be a particular advantage. Another advantage of the induction of mutations is seen in the fact that the general character of the original variety is maintained in the sports, and that somatic mutations cannot be eliminated during meiosis by haplontic selection, as is the case especially in chromosomal structure changes and in aneuploids.

For induction of somatic mutations, only varieties of economic importance and with a high degree of heterozygosity should be used. Since mutations mainly change dominant alleles into recessive ones, a recessive mutation in the somatic tissue can only be phenotypically, perceived if the original form is already heterozygous in the corresponding gene, i.e. if Aa mutates to aa.

If A is dominant over a, a mutation from AA to Aa cannot be realised in somatic tissue. The mutagenic treatment takes place on those organs used in practice for propagation, such as scions, tubers, bulbs, rhizomes, and others. It is initiated in the resting stage of the buds or shortly prior to sprouting, at a time, when the primordia consist of only a few, undifferentiated cells.

Mutations are only induced in one or in a few cells of a tissue, they are bound to one of the three layers forming the vegetative cone, and from which, the individual organs are differentiated.

From the mutated cell within a tissue layer first a mericlinal chimera is formed by anticline division and later on in the process a periclinal chimera, that is, development proceeds from a labile state, in which the mutated tissue only appears as a sector, to the stable state of a periclinal chimera, in which a uniformly mutated tissue layer is present.

Thus, periclinal chimeras are characterised by layers of different genetic constitution; they remain constant in vegetative propagation, in contrast to the mericlinal chimeras.

This histo-genetic structure, combined with the competition between mutated and non-mutated cells in the primordia and already differentiated tissue layers, hinders the occurrence of somatic mutations. Small sectors with changed characters developing from an irradiated bud cannot easily be recognised, and the distinction between a mutation and primary damage due to irradiation is often very difficult.

The induction of somatic mutations in the breeding of vegetatively propagated ornamental plants has been successful. Reports on the accumulated induction of mutations in certain varieties of chrysanthemums, roses, and others should be taken with a degree of scepticism, however.

Mutations can only be recognised if the gene in question is located within a layer, from which also the character controlled by this gene is formed. Thus, for example, mutation of an allele changing the colour of an apple skin is only effective if this mutation occurs in the L₁, or if it reaches the L₁ by rearrangement of the layers.

The new phenotypes, deriving from rearrangement of the layers in a periclinal chimera, can therefore, not be regarded as the result of induced mutations, even if the layer rearrangement was due to mutagenic agents.

Mutation breeding is a useful complementary method in present-day plant breeding. For vegetatively propagated crops enough relevant knowledge has been generated and adequate methods developed to make mutation breeding programmes efficient.

In such crops numerous cases have been reported where mutations have led to new cultivars without much additional breeding. In vegetatively propagated crops, however, a single mutated cell may long remain unnoticed among the non-mutated surrounding cells.

Handling of this issue still remains a problem. According to various published reports, a total of 311 mutant cultivars induced either chemically or by radiation treatments are available in vegetatively propagated crops (Table 6.1).

Mutation are single cell events and can be induced in any cell of a plant or tissue. Very often, such a mutation will not be expressed due to the position of the mutated cells or intra-somatic selection (a specific competition between mutated and non-mutated cells).

Thus, vegetatively propagated crops may accumulate many mutations which may not be expressed for a long time or even not at all or, if expressed, may be seen as chimeras. It is therefore essential to trace this hidden or chimeric variation and develop methods to utilise this in a mutation breeding programme to be feasible for vegetatively propagated crops.

Mutations induced within shoot apices (apical or axillary buds) are relatively easy to manipulate as shoot apex structure is stable. A single mutated primordial cell may give rise to large mutated areas by the low number of active so called initial cells that are the ultimate source of organogenesis.

These may produce a lineage of daughter cells. A mutated lineage may contribute to newly formed shoots, side branches, fruits, or leaves. Occasionally, side branches may be entirely derived from one mutated lineage.

Chimeras are plants consisting of 2 or more genetically different somatic tissues. This is a common situation occurring when one cell of a meristematic tissue is mutated, survives and is able to compete successfully, with surrounding non-mutated cells. These mutants could be solid non-chimeric or periclinal chimeras (sports).

Practical methods have been developed to make proper use of induced chimeric mutations. In black currant it has been demonstrated by R. Baver in 1957 that by repeated back cutting of shoots after X-irradiation and thus stimulating the basal buds to develop, solid mutants can be obtained.

The induction of adventitious buds greatly increases the chances of utilizing mutations which may be induced anywhere on the plant. Adventitious buds may trace back to one or very few cells. If these are mutant cells, the plantlets may contain a very large mutated area or even be chimera free. In this way stable mutants can be obtained.

In vegetatively propagated crops, the use of X-and gamma-irradiation is the most practical method of inducing mutations in all kinds of starting materials such as whole plants, tubers, bulbs, rhizomes, cuttings, and detached leaves.

Low dosages of irradiation are preferred if the objective is to change only one gene in an otherwise undisturbed genetic background. With increasing dosages of radiation, the chromosomal aberrations are increased and may mask the more interesting gene changes. The chemical mutagens are not very specific and effective for these crops.