Chromosome Mutation II: Changes in Chromosome Number
Organisms with multiple chromosome sets (polyploids) are generally larger than diploid organisms, but meiotic pairing anomalies make some polyploid organisms sterile.
An even number of polyploid sets is generally more likely to result in fertility. Then the single-locus segregation ratios are different from those of diploids.
Crosses between two different species followed by the doubling of the chromosome number in the hybrid produces a special kind of fertile interspecific polyploid.
Variants in which a single chromosome has been gained or lost generally arise by nondisjunction (abnormal chromosome segregation at meiosis or mitosis).
Such variants tend to be sterile and show the abnormalities attributable to gene imbalance.
When fertile, such variants show abnormal gene segregation ratios for the misrepresented chromosome only.
The second major type of chromosome mutation is change in chromosome number. Few aspects of genetics impinge on human affairs quite so directly as this one. Chromosome numbers change spontaneously as accidents within cells, and this process has been going on for as long as life has existed on the planet. In fact, changes in chromosome number have been instrumental in molding genomes during evolution. For examples of such changes, we have to look no farther than the food on our dining tables because many of the plants (and some of the animals) that we eat arose through spontaneous changes in chromosome number in the course of the evolution of those species. Today, breeders emulate this process by manipulating chromosome number to improve productivity or some other useful feature of the organism. But perhaps the main relevance of chromosome numbers is for members of our own species in that a large proportion of genetically determined ill health in humans is caused by abnormal chromosome numbers.
In this chapter, we shall investigate the processes that produce new chromosome numbers, the diagnostic tests for detecting such changes, and the properties of cells and individual organisms carrying the different kinds of variant chromosomal complements. As with any area of cytogenetics, the techniques are a combination of genetics and microscopy. Changes in chromosome number are usually classified into two types—changes in whole chromosome sets and changes in parts of chromosome sets—and these two types are dealt with in the next two sections.
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.
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).
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.
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).
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.
To create new plant lines, geneticists produce monoploids with favorable genotypes and then double the chromosomes to form fertile, homozygous diploids.
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 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.
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.
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 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.
Polyploid plants are often larger and have larger organs than their diploid relatives.