Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 82
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These monoploid males are fertile because the gametes are produced by amodified meiosis in which chromosomes do not separate in meiosis I.Monoploids are important in plant breeding because, in the selection of diploid organisms with desired properties,favorable recessive alleles may be masked by heterozygosity. This problem can be avoided by studyingmonoploids, provided that their sterility can be overcome. In many plants, the production of monoploids capable ofreproducing can be stimulated by conditions that yield aberrant cell divisions. Two techniques make this possible.With some diploid plants, monoploids can be derived from cells in the anthers (the pollen-bearing structures).Extreme chilling of the anthers causes some of the haploid cells destined to become pollen grains to begin todivide. These cells are monoploid as well as haploid.
If the cold-shocked cells are placed on an agar surfacecontaining suitable nutrients and certain plant hormones, then a small dividing mass of cells called an embryoidforms. A subsequent change of plant hormones in the growth medium causes the embryoid to form a small plantwith roots and leaves that can be potted in soil and allowed to grow normally. Because monoploid cells have only asingle set of chromosomes, their genotypes can be identified without regard to the dominance or recessiveness ofindividual alleles.
A plant breeder can then select a monoploid plant with the desired traits. In some cases, thedesired genes are present in the original diploid plant and are merely sorted out and selected in the monoploids. Inother cases, the anthers are treated with mutagenic agents in the hope of producing the desired traits.When a desired mutation is isolated in a monoploid, it is necessary to convert the monoploid into a homozygousdiploid because the monoploid plant is sterile andPage 267Figure 7.8Production of a diploid from a monoploid by treatment withcolchicine.
The colchicine disrupts the spindle and thereby preventsseparation of the chromatids after the centromeres (circles) divide.does not produce seeds. Converting the monoploid into a diploid is possible by treatment of the meristematic tissue(the growing point of a stem or branch) with the substance colchicine. This chemical is an inhibitor of theformation of the mitotic spindle. When the treated cells in the monoploid meristem begin mitosis, thechromosomes replicate normally; however, the colchicine blocks metaphase and anaphase, so the result isendoreduplication (doubling of each chromosome in a given cell). Many of the cells are killed by colchicine, but,fortunately for the plant breeder, a few of the monoploid cells are converted into the diploid state (Figure 7.8). Thecolchicine is removed to allow continued cell multiplication, and many of the now-diploid cells multiply to form asmall sector of tissue that can be recognized microscopically.
If placed on a nutrientagar surface, this tissue willdevelop into a complete plant. Such plants, which are completely homozygous, are fertile and produce normalseeds.7.4—Extra or Missing ChromosomesOccasionally, organisms are formed that have extra copies of individual chromosomes, rather than extra entire setsof chromosomes. This situation is called polysomy. In contrast to polyploids, which in plants are often healthy andin some cases are more vigorous than the diploid, polysomics are usually less vigorous than the diploid and haveabnormal phenotypes.
In most plants, a single extra chromosome (or a missing chromosome) has a more severeeffect on phenotype than the presence of a complete extra set of chromosomes. Each chromosome that is extra ormissing results in a characteristic phenotype. For example, Figure 7.9 shows the seed capsule of the Jimson weedDatura stramonium, beneath which is a series of capsules of strains, each having an extra copy of a differentchromosome.
The seed capsule of each of the strains is distinctive.An otherwise diploid organism that has an extra copy of an individual chromosome is called a trisomic. In atrisomic organism, the segregation of chromosomes in meiosis is upset because the trisomic chromosome has twopairing partners instead of one. The behavior of the chromosomes in meiosis depends on the manner in which thehomologous chromosome arms pair and on the chiasmata formed between them.
In some cells, the threechromosomes form a trivalent in which distinct parts of one chromosome are paired with homologous parts ofeach of the others (Figure 7.10A). In metaphase, the trivalent is usually oriented with two centromeres pointingtoward one pole and the other centromere pointing toward the other. The result isPage 268Figure 7.9Seed capsules of the normal diploid Datura stramonium (Jimson weed), which has a haploid numberof 12 chromosomes, and 4 of the 12 possible trisomics. The phenotype of the seed capsulein trisomics differs according to the chromosome that is trisomic.Figure 7.10Meiotic pairing in a trisomic.
(A) Formation of a trivalent. (B) Formation of a univalentand a bivalent. Both types of pairing result in one pair of gametes containing two copies of thetrisomic chromosome and the other pair of gametes containing one copyof the trisomic chromosomePage 269that, at the end of both meiotic divisions, one pair of gametes contains two copies of the trisomic chromosome, andthe other pair of gametes contains only a single copy. Alternatively, the trisomic chromosome can form one normalbivalent and one univalent, or unpaired chromosome, as shown in Figure 7.10B. In anaphase I, the bivalentdisjoins normally and the univalent usually proceeds randomly to one pole or the other. Again, the end result is theformation of two products of meiosis that contain two copies of the trisomic chromosome and two products ofmeiosis that contain one copy.
To state the matter in another way, a trisomic organism with three copies of achromosome (say, C C C) will produce gametes among which half contain C C and half contain C. The unequalsegregation of trisomic chromosomes is also the cause of the infertility in triploids, because in a triploid, eachchromosome behaves independently as though it were trisomic. For each of the homologous chromosomes in atriploid, a gamete can receive either one copy or two copies.Polysomy generally results in more severe phenotypic effects than does polyploidy. The usual explanation is thatthe greater harmful phenotypic effects in trisomics are related to the imbalance in the number of copies of differentgenes.
A polyploid organism has a ''balanced" genome in the sense that the ratio of the numbers of copies of anypair of genes is the same as in the diploid. For example, in a tetraploid, each gene is present in twice as manycopies as in a diploid, so no gene or group of genes is out of balance with the others.
Balanced chromosomeabnormalities, which retain equality in the number of copies of each gene, are said to be euploid. In contrast, geneequality is upset in a trisomic because three copies of the genes located in the trisomic chromosome are present,whereas two copies of the genes in the other chromosomes are present. Such unbalanced chromosomecomplements are said to be aneuploid. Aneuploid abnormalities are usually more severe than euploidabnormalities.
For example, in Drosophila, triploid females are viable, fertile, and nearly normal in morphology,whereas trisomy for either of the two large autosomes is invariably lethal (the larvae die at an early stage).Just as an occasional organism may have an extra chromosome, a chromosome may be missing. Such an individualis said to be monosomic.
A missing copy of a chromosome results in more harmful effects than an extra copy ofthe same chromosome, and monosomy is often lethal. If the monosomic organism can survive to sexual maturity,then chromosomal segregation in meiosis is unequal because the monosomic chromosome forms a univalent. Halfof the gametes will carry one copy of the monosomic chromosome, and the other half will contain no copies.7.5—Human ChromosomesThe chromosome complement of a normal human male is illustrated in Figure 7.11.
The chromosomes have beentreated with a staining reagent called Giemsa, which causes the chromosomes to exhibit transverse bands that arespecific for each pair of homologs. These bands permit the chromosome pairs to be identified individually. Byconvention, the autosome pairs are arranged and numbered from longest to shortest and, on the basis of size andcentromere position, are separated into seven groups designated by the letters A through G. This conventionalrepresentation of chromosomes is called a karyotype; it is obtained by cutting each chromosome out of aphotograph taken at metaphase and pasting it into place.
In a karyotype of a normal human female, the autosomeswould not differ from those of a male and hence would be identical to those in Figure 7.11 but there would be twoX chromosomes instead of an X and a Y. Abnormalities in chromosome number and morphology are made evidentby a karyotype.The nomenclature of the banding patterns in human chromosomes is shown in Figure 7.12. For each chromosome,the short arm is designated with the letter p, which stands for "petite," and the long arm by the letter q, whichstands for "not-p." Within each arm the regions are numbered. The first digit is the major region, numberedconsecutively proceeding from the centromere toward the telomere;Page 270Figure 7.11A karyotype of a normal human male.
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