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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Aberrant euploidy

The number of chromosomes in a basic set is called the monoploid number (x). Organisms with multiples of the monoploid number of chromosomes are called euploid. We learned in earlier chapters that eukaryotes normally carry either one chromosome set (haploids) or two sets (diploids). Haploids and diploids, then, are both cases of normal euploidy. Euploid types that have more than two sets of chromosomes are called polyploid. The polyploid types are named triploid (3x), tetraploid (4x), pentaploid (5x), hexaploid (6x), and so forth. Polyploids arise naturally as spontaneous chromosomal mutations and, as such, they must be considered aberrations because they differ from the previous norm. However, many species of plants and animals have clearly arisen through polyploidy, so evidently evolution can take advantage of polyploidy when it arises. It is worth noting that organisms with one chromosome set sometimes arise as variants of diploids; such variants are called monoploid (1x). In some species, monoploid stages are part of the regular life cycle, but other monoploids are spontaneous aberrations.

The haploid number (n), which we have already used extensively, refers strictly to the number of chromosomes in gametes. In most animals and many plants with which we are familiar, the haploid number and monoploid number are the same. Hence, n or x (or 2n or 2x) can be used interchangeably. However, in certain plants, such as modern wheat, n and x are different. Wheat has 42 chromosomes, but careful study reveals that it is hexaploid, with six rather similar but not identical sets of seven chromosomes. Hence, 6x = 42 and x = 7. However, the gametes of wheat contain 21 chromosomes, so n = 21 and 2n = 42.

Monoploids

Male bees, wasps, and ants are monoploid. In the normal life cycles of these insects, males develop parthenogenetically—that is, they develop from unfertilized eggs. However, in most species, monoploid individuals are abnormal, arising in natural populations as rare aberrations. The germ cells of a monoploid cannot proceed through meiosis normally, because the chromosomes have no pairing partners. Thus, monoploids are characteristically sterile. (Male bees, wasps, and ants bypass meiosis in forming gametes; here, mitosis produces the gametes.) If a monoploid cell does undergo meiosis, the single chromosomes segregate randomly, and the probability of all chromosomes going to one pole is (1/2)x−1 where x is the number of chromosomes. This formula estimates the frequency of viable (whole-set) gametes, which is a small number if x is large.

Monoploids play an important role in modern approaches to plant breeding. Diploidy is an inherent nuisance when breeders want to induce and select new gene mutations that are favorable and to find new combinations of favorable alleles at different loci. New recessive mutations must be made homozygous before they can be expressed, and favorable allelic combinations in heterozygotes are broken up by meiosis. Monoploids provide a way around some of these problems. In some plant species, monoploids can be artificially derived from the products of meiosis in a plant’s anthers. A cell destined to become a pollen grain can instead be induced by cold treatment to grow into an embryoid, a small dividing mass of cells. The embryoid can be grown on agar to form a monoploid plantlet, which can then be potted in soil and allowed to mature (Figure 18-1).

Figure 18-1. Generating monoploid plants by tissue culture.

Figure 18-1

Generating monoploid plants by tissue culture. Pollen grains (haploid) are treated so that they will grow and are placed on agar plates containing certain plant hormones. Under these conditions, haploid embryoids will grow into monoploid plantlets. After (more...)

Plant monoploids can be exploited in several ways. In one, they are first examined for favorable traits or allelic combinations, which may arise from heterozygosity already present in the parent or induced in the parent by mutagens. The monoploid can then be subjected to chromosome doubling to achieve a completely homozygous diploid with a normal meiosis, capable of providing seed. How is this achieved? Quite simply, by the application of a compound called colchicine to meristematic tissue. Colchicine—an alkaloid drug extracted from the autumn crocus—inhibits the formation of the mitotic spindle, so cells with two chromosome sets are produced (Figure 18-2). These cells may proliferate to form a sector of diploid tissue that can be identified cytologically.

Figure 18-2. The use of colchicine to generate a diploid from a monoploid.

Figure 18-2

The use of colchicine to generate a diploid from a monoploid. Colchicine added to mitotic cells during metaphase and anaphase disrupts spindle-fiber formation, preventing the migration of chromatids after the centromere is split. A single cell is created (more...)

Another way in which the monoploid may be used is to treat its cells basically like a population of haploid organisms in a mutagenesis-and-selection procedure. A population of cells is isolated, their walls are removed by enzymatic treatment, and they are treated with mutagen. They are then plated on a medium that selects for some desirable phenotype. This approach has been used to select for resistance to toxic compounds produced by one of the plant’s parasites and to select for resistance to herbicides being used by farmers to kill weeds. Resistant plantlets eventually grow into haploid plants, which can then be doubled (with the use of colchicine) into a pure-breeding, diploid, resistant type (Figure 18-3).

Figure 18-3. Using microbial techniques in plant engineering.

Figure 18-3

Using microbial techniques in plant engineering. The cell walls of haploid cells are removed enzymatically. The cells are then exposed to a mutagen and plated on an agar medium containing a selective agent, such as a toxic compound produced by a plant (more...)

These powerful techniques can circumvent the normally slow process of meiosis-based plant breeding. The techniques have been successfully applied to several important crop plants, such as soybeans and tobacco.

The anther technique for producing monoploids does not work in all organisms or in all genotypes of an organism. Another useful technique has been developed in barley, an important crop plant. Diploid barley, Hordeum vulgare, can be fertilized by pollen from a diploid wild relative called Hordeum bulbosum. This fertilization results in zygotes with one chromosome set from each parental species. In the ensuing somatic cell divisions, however, the chromosomes of H. bulbosum are eliminated from the zygote, whereas all the chromosomes of H. vulgare are retained, resulting in a haploid embryo. (The haploidization appears to be caused by a genetic incompatibility between the chromosomes of the different species.) The chromosomes of the resulting haploids can be doubled with colchicine. This approach has led to the rapid production and widespread planting of several new barley varieties, and it is being used successfully in other species too.

MESSAGE

To create new plant lines, geneticists produce monoploids with favorable genotypes and then double the chromosomes to form fertile, homozygous diploids.

Polyploids

In the realm of polyploids, we must distinguish between autopolyploids, which are composed of multiple sets from within one species, and allopolyploids, which are composed of sets from different species. Allopolyploids form only between closely related species; however, the different chromosome sets are homeologous (only partly homologous)—not fully homologous, as they are in autopolyploids.

Triploids

Triploids are usually autopolyploids. They arise spontaneously in nature or are constructed by geneticists from the cross of a 4x (tetraploid) and a 2x (diploid). The 2x and the x gametes unite to form a 3x triploid.

Triploids are characteristically sterile. The problem, like that of monoploids, lies in pairing at meiosis. Synapsis, or true pairing, can take place only between two chromosomes, but one chromosome can pair with one partner along part of its length and with another along the remainder, which gives rise to an association of three chromosomes. Paired chromosomes of the type found in diploids are called bivalents. Associations of three chromosomes are called trivalents, and unpaired chromosomes are called univalents. Hence in triploids there are two pairing possibilities, resulting either in a trivalent or in a bivalent plus a univalent. Paired centromeres segregate to opposite poles, but unpaired centromeres pass to either pole randomly. We see in Figure 18-4 that the net result of both the pairing possibilities is an uneven segregation, with two chromosomes going in one direction and one in the other. This happens for every chromosome threesome.

Figure 18-4. Two possibilities for the pairing of three homologous chromosomes before the first meiotic division in a triploid.

Figure 18-4

Two possibilities for the pairing of three homologous chromosomes before the first meiotic division in a triploid. Notice that the outcome will be the same in both cases: one resulting cell will receive two chromosomes and the other will receive just (more...)

If all the single chromosomes pass to the same pole and simultaneously the other two chromosomes pass to the opposite pole, then the gametes formed will be haploid and diploid. The probability of this type of meiosis will be (1/2)x−1, and this proportion is likely to be low. All other possibilities will give gametes with chromosome numbers intermediate between the haploid and diploid number; such genomes are aneuploid—“not euploid.” It is likely that these aneuploid gametes will not lead to viable progeny; in fact, this category is responsible for the almost complete lack of fertility of triploids. The problem is one of genome imbalance, a phenomenon that we shall encounter repeatedly in this chapter. For most organisms, the euploid chromosome set is a finely tuned set of genes in relative proportions that seem to be functionally significant. Multiples of this set are tolerated because there is no change in the relative proportions of genes. However, the addition of one or more extra chromosomes is nearly always deleterious because the proportions of genes in those extra chromosomes are altered. Although the action of some genes can be regulated to compensate for extra gene “dosage,” the overall effect of the extra genetic material seems too great to be overcome by gene regulation. The deleterious effect can be expressed at the level of gametes, making them nonfunctional, or at the level of the zygote, resulting in lethality, sterility, or lowered fitness.

In triploids, it is possible that some haploid or diploid gametes will form, and some may unite to form a euploid zygote, but the likelihood of this possibility is inherently low. Consider bananas. The bananas that are widely available commercially are triploids with 11 chromosomes in each set (3x = 33). The probability of a meiosis in which all univalents pass to the same pole is (1/2)x−1, or (1/2)10 = 1/1024, so bananas are effectively sterile. The most obvious expression of the sterility of bananas is that there are no seeds in the fruit that we eat. Another example of the commercial exploitation of triploidy in plants is the production of triploid watermelons. For the same reasons that bananas are seedless, triploid watermelons are seedless, a phenotype favored by some for its convenience.

MESSAGE

Some types of chromosome mutations are themselves aneuploid; other types produce aneuploid gametes or zygotes. Aneuploidy is nearly always deleterious because of genetic imbalance—the ratio of genes is different from that in euploids and interferes with the normal operation of the genome.

Autotetraploids

Autotetraploids arise naturally by the spontaneous accidental doubling of a 2x genome to a 4x genome, and autotetraploidy can be induced artificially through the use of colchicine. Autotetraploid plants are advantageous as commercial crops because, in plants, the larger number of chromosome sets often leads to increased size. Cell size, fruit size, flower size, stomata size, and so forth, can be larger in the polyploid (Figure 18-5). Here we see another effect that must be explained by gene numbers. Presumably the amount of gene product (protein or RNA) is proportional to the number of genes in the cell, and this number is higher in the cells of polyploids compared with diploids.

Figure 18-5. Epidermal leaf cells of tobacco plants, showing an increase in cell size, particularly evident in stoma size, with an increase in autopolyploidy.

Figure 18-5

Epidermal leaf cells of tobacco plants, showing an increase in cell size, particularly evident in stoma size, with an increase in autopolyploidy. (a) Diploid, (b) tetraploid, (c) octoploid. (From W. Williams, Genetic Principles and Plant Breeding. Blackwell (more...)

MESSAGE

Polyploid plants are often larger and have larger organs than their diploid relatives.

Because 4 is an even number, autotetraploids can have a regular meiosis, although this is by no means always the case. The crucial factor is how the four homologous chromosomes, one from each of the four sets, pair and segregate. There are several possibilities, as shown in Figure 18-6. Pairings between four chromosomes are called quadrivalents. In tetraploids, the two-bivalent and the quadrivalent pairing modes tend to be most regular in segregation, but even here there is no guarantee of a 2:2 segregation. If all chromosome sets segregate 2:2 as they do in some species, then the gametes will be functional and a formal genetic analysis can be developed for such autotetraploids.

Figure 18-6. Meiotic pairing possibilities in tetraploids.

Figure 18-6

Meiotic pairing possibilities in tetraploids. (Each chromosome is really two chromatids.) The four homologous chromosomes may pair as two bivalents or as a quadrivalent. Both possibilities can yield functional gametes. However, the four chromosomes may (more...)

Let’s consider the genetics of a fertile tetraploid. We can consider an experiment in which colchicine is used to double the chromosomes of an A/a plant to form an A/A/a/a auto-tetraploid, which we will assume shows 2:2 segregation. We now have a further concern because polyploids such as tetraploids give different phenotypic ratios in their progeny, depending on whether the locus in question is tightly linked to the centromere. First, we consider a centromere-linked gene. The three possible pairing and segregation patterns are presented in Figure 18-7; these patterns occur by chance and with equal frequency. As Figure 18-7 shows, the 2x gametes produced are A/a, A/A, or a/a, in a ratio of 8:2:2, or 4:1:1. If such a plant is selfed, the probability of an a/a/a/a phenotype in the offspring is 1/6 × 1/6 =1/36. In other words, a 35:1 phenotypic ratio of A/–/–/–:a/a/a/a will be observed if A is fully dominant over three a alleles.

Figure 18-7. Gene segregation in a tetraploid showing orderly pairing by bivalents.

Figure 18-7

Gene segregation in a tetraploid showing orderly pairing by bivalents. (Each chromosome is really two chromatids.) The locus is assumed to be close to the centromere. Self-fertilization could yield a variety of genotypes, including a/a/a/a.

If, in the same kind of plant, a genetic locus having the alleles B and b is very far removed from the centromere, crossing-over must be considered. This example forces us to think in terms of chromatids instead of chromosomes; there are four B chromatids and four b chromatids (Figure 18-8). Because the number of crossovers in such a long region will be large, the genes will become effectively unlinked from their original centromeres. The packaging of genes two at a time into gametes is very much like grabbing two balls at random from a bag of eight balls: four of one kind and four of another. The probability of picking two b genes is then

Image ch18e1.jpg

Figure 18-8. Highly diagrammatic representation of a tetraploid meiosis in which a heterozygous locus is distant from the centromere.

Figure 18-8

Highly diagrammatic representation of a tetraploid meiosis in which a heterozygous locus is distant from the centromere. The net effect of multiple crossovers in such a long region will be that the genes become effectively unlinked from their original (more...)

So, in a selfing, the probability of a b/b/b/b phenotype is 2/4 × 3/14 = 9/196, which is approximately 1/22. Hence, there will be a 21:1 phenotypic ratio of B/–/–/–:b/b/b/b. For genetic loci of intermediate position, intermediate ratios will result.

Allopolyploids

The “classic” allopolyploid was synthesized by G. Karpechenko in 1928. He wanted to make a fertile hybrid that would have the leaves of the cabbage (Brassica) and the roots of the radish (Raphanus). Each of these species has 18 chromosomes, and they are related closely enough to allow intercrossing. A viable hybrid progeny individual was produced from seed. However, this hybrid was functionally sterile because the nine chromosomes from the cabbage parent were different enough from the radish chromosomes that pairs did not synapse and disjoin normally:

Image ch18e2.jpg

However, one day a few seeds were in fact produced by this (almost) sterile hybrid. On planting, these seeds produced fertile individuals with 36 chromosomes. All these individuals were allopolyploids. They had apparently been derived from spontaneous, accidental chromosome doubling to 2n1 + 2n2 in the sterile hybrid, presumably in tissue that eventually became germinal and underwent meiosis. Thus, in 2n1 + 2n2 tissue, there is a pairing partner for each chromosome and balanced gametes of the type n1 + n2 are produced. These gametes fuse to give 2n1 + 2n2 allopolyploid progeny, which also are fertile. This kind of allopolyploid is sometimes called an amphidiploid, which means “doubled diploid” (Figure 18-9). (Unfortunately for Karpechenko, his amphidiploid had the roots of a cabbage and the leaves of a radish.)

Figure 18-9. The origin of the amphidiploid (Raphanobrassica) formed from cabbage (Brassica) and radish (Raphanus).

Figure 18-9

The origin of the amphidiploid (Raphanobrassica) formed from cabbage (Brassica) and radish (Raphanus). The fertile amphidiploid arose in this case from spontaneous doubling in the 2n = 18 sterile hybrid. Colchicine can be used to promote (more...)

When the allopolyploid was crossed with either parental species, sterile offspring resulted. The offspring of the cross with radish were 2n1 + n2 constituted from an n1 + n2 gamete from the allopolyploid and an n1 gamete from the radish. The n2 chromosomes had no pairing partners, so sterility resulted. Consequently, Karpechenko had effectively created a new species, with no possibility of gene exchange with its parents. He called his new species Raphanobrassica.

Today, allopolyploids are routinely synthesized in plant breeding. Instead of waiting for spontaneous doubling to occur in the sterile hybrid, the plant breeder adds colchicine to induce doubling. The goal of the breeder is to combine some of the useful features of both parental species into one type. This kind of endeavor is very unpredictable, as Karpechenko learned. In fact, only one synthetic amphidiploid has ever been widely used. This amphidiploid is Triticale, an amphidiploid between wheat (Triticum, 2n = 6x = 42 and rye (Secale, 2n = 2x = 14 Triticale combines the high yields of wheat with the ruggedness of rye. Figure 18-10 shows the procedure for synthesizing Triticale.

Figure 18-10. Techniques for the production of the amphidiploid Triticale.

Figure 18-10

Techniques for the production of the amphidiploid Triticale. If the hybrid seed does not germinate, then tissue culture (lower path) may be used to obtain a hybrid plant. (From Joseph H. Hulse and David Spurgeon, “Triticale.” Copyright (more...)

In nature, allopolyploidy seems to have been a major force in speciation of plants. There are many different examples. One particularly satisfying one is shown by the genus Brassica, as illustrated in Figure 18-11. Here three different parent species have hybridized in all possible pair combinations to form new amphidiploid species.

Figure 18-11. A species triangle, showing how amphidiploidy has been important in the production of new species of Brassica.

Figure 18-11

A species triangle, showing how amphidiploidy has been important in the production of new species of Brassica.

A particularly interesting natural allopolyploid is bread wheat, Triticum aestivum(2n = 6x = 42). By studying various wild relatives, geneticists have reconstructed a probable evolutionary history of bread wheat (Figure 18-12). In a bread wheat meiosis, there are always 21 pairs of chromosomes. Furthermore, it has been possible to establish that any given chromosome has only one specific pairing partner (homologous pairing)—not five other potential partners (homeologous pairing). The suppression of such homeologous pairing (which would make the species more unstable) is maintained by an allele, Ph, on the long arm of chromosome 5 of the B set. Thus, Ph ensures a diploidlike meiotic behavior for this hexaploid species. Without Ph, bread wheat could probably never have arisen. It is interesting to speculate about whether Western civilization could have begun or progressed without this species—in other words, without the Ph mutation.

Figure 18-12. Diagram of the proposed evolution of modern hexaploid wheat, in which amphidiploids are produced at two points.

Figure 18-12

Diagram of the proposed evolution of modern hexaploid wheat, in which amphidiploids are produced at two points. A, B, and D are different chromosome sets.

Somatic allopolyploids from cell hybridization

Another innovative approach to plant breeding is to try to make allopolyploid-like hybrids by fusing asexual cells. Theoretically, such a technique would allow us to combine widely differing parental species. The technique does indeed work, but the only allopolyploids that have been produced so far can also be made by the sexual methods that we have considered already. In the cell-fusion procedure, cell suspensions of the two parental species are prepared and stripped of their cell walls by special enzyme treatments. The stripped cells are called protoplasts. The two protoplast suspensions are then combined with polyethylene glycol, which enhances protoplast fusion. The parental cells and the fused cells proliferate on an agar medium to form colonies (in much the same way as microbes). If these colonies, or calluses as they are called, are examined, a fair percentage of them are found to be allopolyploid-like hybrids with chromosome numbers equal to the sum of the parental numbers. Thus, not only do the protoplast cell membranes fuse to form a kind of heterokaryon, but the nuclei fuse, too, to give rise to a 2n1 + 2n2 amphidiploid.

Another good example of an allopolyploid-like hybrid is commercial tobacco, Nicotiana tabacum, which has 48 chromosomes. This species of tobacco was originally found in nature as a spontaneous amphidiploid. The two probable parents are N. sylvestris and N. tomentosiformis, each of which has 24 chromosomes. A sexual cross between N. tabacum and either of these two probable parents give a 36-chromosome hybrid containing 12 chromosome pairs plus 12 unpaired chromosomes. A cross between N. sylvestris and N. tomentosiformis yields a 24-chromosome hybrid in which there is no pairing at all. Hence, it appears that part of the N. tabacum genome is from N. sylvestris and part is from N. tomentosiformis. This amphidiploid can be recreated either sexually, by using colchicine as described previously, or somatically by cell fusion. When cells of the prospective parental species are fused, a 48-chromosome hybrid cell line is produced, from which plants identical in behavior to N. tabacum may be grown. (Note that, in the latter method, colchicine is not required, because the fusion product is already amphidiploid.)

The recovery of somatic hybrids may be enhanced if a selective system is available. In one example in N. tabacum, two monoploid lines were fused to form a diploid hybrid culture by using complementation as the selection system. The first line had whitish, light-sensitive leaves due to a recessive mutation w. The other had yellowish, light-sensitive leaves due to a mutation y at a separate locus. When the cells were combined in a petri dish, the diploid w+/w · y+/y calluses could be selected by their resistance to light and their normal green color. The calluses can be grown into plantlets, which then are either grafted onto a mature plant to develop or potted themselves. The protocol for this experiment is illustrated in Figure 18-13.

Figure 18-13. Creating a hybrid of two monoploid lines of Nicotiana tabacum by cell fusion.

Figure 18-13

Creating a hybrid of two monoploid lines of Nicotiana tabacum by cell fusion. One line has light-sensitive, yellowish leaves, and the other has light-sensitive, whitish leaves. Protoplasts are produced by enzymatically stripping the cell walls from the leaf (more...)

Two allotetraploids of Petunia, one produced by sexual hybridization and the other by somatic hybridization, are compared in Figure 18-14. Note that the two are identical in appearance and produce the same range of progeny types.

Figure 18-14. Sexual hybridization and somatic hybridization produce identical Petunia allotetraploids.

Figure 18-14

Sexual hybridization and somatic hybridization produce identical Petunia allotetraploids. The two parental lines are illustrated at the top, the red P. hybrida and the white P. parodii. The F1 allotetraploid in the second row resulted from crossing the (more...)

MESSAGE

Allopolyploids can be synthesized either by crossing related species and doubling the chromosomes of the hybrid or by asexually fusing the cells of different species.

Polyploidy in animals

You may have noticed that most of the discussion of polyploidy so far has concerned plants. Indeed, polyploidy is more common in plants than in animals; nevertheless there are many cases of polyploid animals. Examples are found in flatworms, leeches, and brine shrimp. In these cases, reproduction is by parthenogenesis, the development of a special type of unfertilized egg into an embryo, without the need for fertilization. However, examples are not confined to these so-called lower forms. Polyploid amphibians and reptiles are surprisingly common. They show several modes of reproduction. The males and females of polyploid frogs and toads participate in their sexual cycles, whereas polyploid salamanders and lizards are parthenogenetic.

Some fish also are polyploid; in two cases, it appears that a single polyploid event gave rise to an entire taxonomic family in evolution. This situation contrasts with that of amphibians and reptiles because in those cases the polyploids all have closely related diploid species, and, hence, the polyploid events do not seem to have been important in the evolution of the group as a whole. The Salmonidae family of fishes (containing salmon and trout) is a familiar example of a group that appears to have originated through polyploidy. Salmonids have twice as much DNA as have related fish. Different salmonid species have different chromosome numbers, but the group has an almost invariant number of chromosome arms (some fused in some species) that is twice the number of arms in related groups. Hence, the evidence points to the salmonids’ having evolved from a single event that gave rise to a tetraploid.

You might also be interested to know that the sterility of triploids has been commercially exploited in animals as well as plants. Triploid oysters have been developed, and they have a commercial advantage over their diploid relatives. The diploids go through a spawning season, during which they are unpalatable. Triploids, however, because of their sterility, do not spawn and are palatable the whole year round.

Human polyploid zygotes do arise through various kinds of mistakes in cell division. Most die in utero. Occasionally, triploid babies are born, but none survive.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK22089

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