Chromosomal Aberration (Part III)

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EUPLOIDS

•  The changes in the number of chromosomes are usually described as variations in the ploidy of the organism.

• Organisms with one or more complete sets of chromosomes are said to be euploid.

• Organisms that carry more than two sets of chromosomes are said to be polyploid and the level of polyploidy is described by referring to a basic chromosome number, usually denoted as n.

•      Haploids: carry a single set of chromosomes (n)

•      Diploids: carry two chromosome sets (2n)

•      Triploids: carry three chromosome sets (3n)

•      Tetraploids: carry four chromosome sets (4n)

• An individual of a normally diploid species that has only one chromosome set is called a monoploid to distinguish it from an individual of a normally haploid species (n).

MONOPLOIDS

•  An individual of a normally diploid species that have only one chromosome set is called a monoploid.

•  Male bees, wasps, and ants are monoploid. They develop by parthenogenesis (the development of an unfertilized egg into an embryo).

•   In most other species, monoploid zygotes fail to develop.

•  The reason is that virtually all individuals in a diploid species carry a number of deleterious recessive mutations, together called a “genetic load.”

•  The deleterious recessive alleles are masked by wild-type alleles in the diploid condition but are automatically expressed in a monoploid derived from a diploid.

•  Monoploids that do develop to advanced stages are abnormal. If they survive to adulthood, their germ cells cannot proceed through meiosis normally because the chromosomes have no pairing partners. Thus, monoploids are characteristically sterile.

POLYPLOIDS

•  Organisms that carry more than two sets of chromosomes are said to be polyploid.

•  Polyploid plants are often larger and have larger component parts than their diploid relatives.

• Polyploids with odd numbers of chromosome sets, such as triploids, are sterile or highly infertile because their gametes and offspring are aneuploid.

AUTOPOLYPLOIDS AND ALLOPOLYPLOIDS

• Autopolyploids: They have multiple chromosome sets originating from within one species

•  Allopolyploids: They have sets of chromosomes from two or more different species.

• Allopolyploids form only between closely related species; however, the different chromosome sets are only homeologous (partially homologous), not fully homologous as they are in autopolyploids.

•  Generally plants seem to be much more tolerant of polyploidy than animals.

AUTOPOLYPLOIDS: TRIPLOIDS

•  Triploids are usually autopolyploids.

•  They may arise spontaneously in nature.

•  They can also be constructed by geneticists from the cross of a 4n (tetraploid) and a 2n (diploid). The 2n and the n gametes produced by the tetraploid and the diploid, respectively, unite to form a 3n triploid.

•  Triploids are characteristically sterile because of their problem in synapsis during meiosis.

•  The three homologous chromosomes of a triploid may pair in two ways at meiosis:

a trivalent 

a bivalent plus a univalent

• It is unlikely that a gamete will receive two for every chromosomal type, or that it will receive one for every chromosomal type. Hence, they will be aneuploids.

AUTOPOLYPLOIDS: TETRAPLOIDS

•  Autotetraploids arise by the doubling of a 2n complement to 4n.

• This doubling can occur spontaneously, but it can also be induced artificially by applying chemical agents that disrupt microtubule polymerization (ex: colchicine).

• In colchicine-treated cells, the S phase of the cell cycle occurs, but chromosome segregation or cell division does not. As the treated cell enters telophase, a nuclear membrane forms around the entire doubled set of chromosomes.

•  Therefore, if a diploid cell of genotype A/a; B/b is doubled, the resulting autotetraploid will be of genotype A/A/a/a ; B/B/b/b.

•  Because 4 is an even number, autotetraploids can mostly have regular meiosis.

• Chromosomes may pair as:

a pair of bivalents (chromosomes segregate normally producing diploid gametes)

a quadrivalent (chromosomes segregate normally producing diploid gametes)

a trivalent plus a univalent (nonfunctional aneuploid gametes)

ALLOPOLYPLOIDS

• An allopolyploid is a plant that is a hybrid of two or more species, containing two or more copies of each of the input genomes.

• Allopolyploid plants can be synthesized by crossing related species and doubling the chromosomes of the hybrid or by fusing diploid cells.

Raphanobrassica

•  An allotetraploid 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 two species has 18 chromosomes.

                                           2n₁ = 2n₂ = 18

•  The species are related closely enough to allow intercrossing.

• Fusion of an n₁ and an n₂ gamete produced a viable hybrid progeny but it was sterile.

•  Eventually, one part of the hybrid plant produced some seeds due to accidental chromosome doubling which enabled synapsis during meiosis.

•  This resulted in an allotetraploid individual (2n₁ + 2n₂) which would produce n₁ + n₂  gametes.  These gametes fuse to form 2n₁ + 2n₂ progeny again.

•  This kind of allopolyploid is sometimes called an amphidiploid or doubled diploid.

•  Unfortunately, the hybrid exhibited leaves like radish and roots like cabbage!

Bread wheat

• A particularly interesting natural allopolyploid is bread wheat, Triticum aestivum (6n = 42). 

• Bread wheat is an allohexaploid (6x), which regularly forms 21 pairs of chromosomes (2n = 42) during meiosis.

• These chromosomes are subdivided into 3 closely related (homoeologous) groups of chromosomes, the A, B, and D genomes.

• In the first hybridization event, the A genome progenitor combined with the B genome progenitor to form primitive tetraploid wheat (2n=28, AABB). This hybrid occurred in the cytoplasm of the B genome.

• The second event involved hybridization between the tetraploid (AABB) form and the D genome progenitor to form the basic hexaploid configuration, AABBDD, again in the B genome cytoplasm.

• At meiosis, pairing is always between homologs from the same ancestral genome.

• Hence, in bread wheat meiosis, there are always 21 bivalents.

• The domestication of diploid and tetraploid wheat is thought to have occurred in the fertile crescent of the Middle East.

• Domestication of the diploid and tetraploid wheat is thought to have occurred at least nine thousand years ago with the hybridization event producing hexaploid wheat occurring more than six thousand years ago.

APPLICATION OF POLYPLOIDY IN AGRICULTURE

• Monoploids: Monoploids help breeders to select recessive characters in hybrids.

•  Autotriploids: Commercially available seedless bananas are sterile triploids (3n = 33). Seedless watermelons are another example.

• Autotetraploids: Many autotetraploid plants have been developed as commercial crops to take advantage of their increased size. Large fruits and flowers are particularly favored.

• Allopolyploids: Best example is bread wheat. New World cotton is a natural allopolyploid that occurred spontaneously. A synthetic amphidiploid, Triticale, has been widely used commercially. This is an amphidiploid between wheat (Triticum, 6n=42) and rye (Secale, 2n=14). Hence, for Triticale, 2n=2x(21+7)  56. This novel plant combines the high yields of wheat with the ruggedness of rye.