Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 81
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Thus the gametes of a polyploid organism are not always monoploid, asthey are in a diploid organism; for example, a tetraploid organism has diploid gametes.The distinction between the terms monoploid and haploid is subtle:• The monoploid chromosome set is the basic set of chromosomes that is multiplied in a polyploid series of species,such as Chrysanthemum.• The haploid chromosome set is the set of chromosomes present in a gamete, irrespective of the chromosomenumber in the species.The potential confusion arises because of diploid organisms, in which the monoploid chromosome set and thehaploid chromosome set are the same.
Considering the tetraploid helps to clarify the difference: It contains fourmonoploid chromosome sets, and the haploid gametes are diploid.Polyploidy is widespread in certain plant groups. Among flowering plants, from 30 to 35 percent of existingspecies are thought to have originated as some form of polyploid. Valuable agricultural crops that are polyploidinclude wheat, oats, cotton, potatoes, bananas, coffee, and sugar cane. Polyploidy often leads to an increase in thesize of individual cells, and polyploid plants are often larger and more vigorous than their diploid ancestors;however, there are many exceptions to these generalizations. Polyploidy is rare invertebrate animals, but it is foundin a few groups of invertebrates.
One reason why polyploidy is rare in animals is the difficulty in regularsegregation of the sex chromo-Page 263somes. For example, a tetraploid animal with XXXX females and XXYY males would produce XX eggs and XYsperm (if all chromosomes paired to form bivalents), so the progeny would be exclusively XXXY and thus unlikeeither of the parents.Polyploid plants found in nature almost always have an even number of sets of chromosomes, because organismsthat have an odd number have low fertility. Organisms with three monoploid sets of chromosomes are known astriploids. As far as growth is concerned, a triploid is quite normal because the triploid condition does not interferewith mitosis; in mitosis in triploids (or any other type of polyploid), each chromosome replicates and divides just asin a diploid. However, because each chromosome has more than one pairing partner, chromosome segregation isseverely upset in meiosis, and most gametes are defective.
Unless the organism can perpetuate itself by means ofasexual reproduction, it will eventually become extinct.The infertility of triploids is sometimes of commercial benefit. For example, the seeds in commercial bananas aresmall and edible because the plant is triploid and most of the seeds fail to develop to full size.
In oysters, triploidsare produced by treating fertilized diploid eggs with a chemical that causes the second polar body of the egg to beretained. The triploid oysters are sterile and do not spawn, so they remain edible through the hot summer months ofJune, July, and August (the months that lack the letter r), when normal oysters are spawning. In Florida and incertain other states, weed control in waterways is aided by the release of weed-eating fish (the grass carp), whichdo not become overpopulated because the released fish are sterile triploids.Tetraploid organisms can be produced in several ways. The simplest mechanism is a failure of chromosomeseparation in either mitosis or meiosis, which instantly doubles the chromosome number.
Chromosome doublingthrough an abortive cell division is called endoreduplication. In a plant species that can undergo self-fertilization,endoreduplication creates a new, genetically stable species, because the chromosomes in the tetraploid can pair twoby two in meiosis and therefore segregate regularly, each gamete receiving a full diploid set of chromosomes. Selffertilization of the tetraploid restores the chromosome number, so the tetraploid condition can be perpetuated.
Thegenetics of tetraploid species, and that of other polyploids, is more complex than that of diploid species because theorganism carries more than two alleles of any gene. With two alleles in a diploid, only three genotypes arepossible: AA, Aa, and aa. In a tetraploid, by contrast, five genotypes are possible: AAAA, AAAa, AAaa, Aaaa, andaaaa.
Among these genotypes, the middle three represent different types of tetraploid heterozygotes.An octoploid species (eight sets of chromosomes) can be generated by failure of chromosome separation in mitosisin a tetraploid. If only bivalents form in meiosis, then an octoploid organism can be perpetuated sexually by selffertilization or through crosses with other octoploids.
Furthermore, cross-fertilization between an octoploid and atetraploid results in a hexaploid (six sets of chromosomes). Repeated episodes of polyploidization and crossfertilization may ultimately produce an entire polyploid series of closely related organisms that differ inchromosome number, as exemplified in Chrysanthemum.Chrysanthemum represents a type of polyploidy, known as autopolyploidy, in which all chromosomes in thepolyploid species derive from a single diploid ancestral species. In many cases of polyploidy, the polyploid specieshave complete sets of chromosomes from two or more different ancestral species. Such polyploids are known asallopolyploids.
They derive from occasional hybridization between different diploid species when pollen from onespecies germinates on the stigma of another species and sexually fertilizes the ovule, followed byendoreduplication in the zygote to yield a hybrid plant in which each chromosome has a pairing partner in meiosis.The pollen may be carried to the wrong flower by wind, insects, or other pollinators. Figure 7.4 illustrateshybridization between species A and B in which endoreduplication leads to the formation of an allopolyploid (inthis case, an allotetraploid), which carries a complete diploid genome from each of its two ancestral species.
Theformation of allopolyploids through hybridization and endoreduplication is an extremely important process inPage 264Figure 7.4Endoreduplication (the doubling of the chromosome complement) plays a key role in theformation of polyploid species. When it takes place in a diploid species, endoreduplication resultsin the formation of an autotetraploid species. When it takes place in the hybrid formed bycross-fertilization between distinct species, endoreduplication results in the formation ofan allotetraploid species that has a complete diploid set of chromosomes from each ofthe parental species.plant evolution and plant breeding.
At least half of all naturally occurring polyploids are allopolyploids. Cultivatedwheat provides an excellent example of allopolyploidy. Cultivated bread wheat is a hexaploid with 42chromosomes constituting a complete diploid genome of 14 chromosomes from each of three ancestral species.The 42-chromosome allopolyploid is thought to have originated by the series of hybridizations andendoreduplications outlined in Figure 7.5.The ancestral origin of the chromosome sets in an allopolyploid can often be revealed by the technique ofchromosome painting, in which chromosomes are "painted" different colors by hybridization with DNA strandslabeled with fluorescent dyes.
DNA from each of the putative ancestral species is isolated, denatured, and labeledwith a different fluorescent dye. Then the labeled single strands are spread on a microscope slide and allowed torenature with homologous strands present in the chromosomes of the allopolyploid species.An example of chromosome painting is shown in Figure 7.6. The flower is from a variety of crocus called GoldenYellow. Its genome contains seven pairs of chromosomes, which are shown painted in yellow and green. GoldenYellow was thought to be an allopolyploid formed by hybridization of two closely related species followed byendoreduplication of the chromosomes in the hybrid.
The putative ancestral species are Crocus flavus, which hasfour pairs of chromosomes, and Crocus angustifolius, which has three pairs of chromosomes. To paint thechromosomes of Golden Yellow, DNA from C. flavus was isolated and labeled with a fluorescent green dye, andthat from C. angustifolius was isolated and labeled with a fluorescent yellowPage 265Figure 7.5(A) Repeated hybridization and endoreduplication in the ancestry of cultivated bread wheat (Triticum aestivum),which is an allohexaploid containing complete diploid genomes (AA, BB, DD) from threeancestral species. (B) The spike of T.
turgidum in the photograph is an allotetraploid. The large grainsof this species made it attractive to early hunter-gatherer societies in the Middle East One of theearliest cultivated wheats, T. turgidum, is the progenitor of commercial macaroni wheat[Photograph courtesy of Gordon Kimber.]Figure 7.6Flower of the crocus variety Golden Yellow and chromosome painting that reveals its origin as anallopolyploid. Its seven pairs of chromosomes are shown at the right.
The chromosomes in greenhybridized with DNA from C. angustifolius, which has three pairs of chromosomes, and those inyellow hybridized with DNA from C. flavus, which has four pairs of chromosomes.[Courtesy of J. S. Heslop-Harrison, John Innes Centre, Norwich, UK. With permission of theAnnals of Botany.]Page 266Figure 7.7Allopolyploidy in the evolutionary origin ofthe Golden Yellow crocus. The original hybridbetween C. flavus and C. angustifolius produced asterile monoploid with one copy of eachchromosome from each species. Chromosomeendoreduplication in the monoploid results ina fertile allotetraploid with a complete diploidgenome from each species.dye. The result of the chromosome painting is very clear: three pairs of chromosomes hybridize with the greenlabeled DNA from C. flavus, and four pairs of chromosomes hybridize with the yellow-labeled DNA from C.angustifolius.
This pattern of hybridization strongly supports the hypothesis of the autopolyploid origin of GoldenYellow diagrammed in Figure 7.7.7.3—Monoploid OrganismsAs noted, the monoploid chromosome set is the set of chromosomes multiplied in polyploid species. An organismis monoploid if it develops from a monoploid cell. Meiosis cannot take place normally in the germ cells of amonoploid, because each chromosome lacks a pairing partner, and hence monoploids are usually sterile.Monoploid organisms are quite rare, but they occur naturally in certain insect species (ants, bees) in which malesare derived from unfertilized eggs.
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