This is old Lecture 18
Textbook Assignment: Chapter 9, pages 234-243 (to start
of "Variations...")
Also review pages 221-222 for definitions of terms used in this assignment.
Major concepts .
Euploid variation in chromosome number: The term euploid refers to genomes that contain exact multiples of fully normal haploid sets of chromosomes (including one sex chromosome of either type per set). Diploid refers to a genome that has exactly two haploid sets of chromosomes. Polyploid refers to a genome that contains more than two haploid sets of chromosomes. Autopolyploid refers to a polyploid genome in which all of the haploid sets are from the same species. Allopolyploid refers to a polyploid genome that contains haploid genomes from two or more different species.
Aneuploid variation in chromosome number and morphology: The term aneuploid refers to all genomic variations that are not euploid, including increased or decreased numbers of individual chromosomes, as well as changes in chromosome morphology due to insertion, deletion, inversion and translocation, and changes in chromosome number due to centromeric fusion or fission. Cancer cells and permanent cell lines that are maintained in culture for extended periods of time often become grossly aneuploid. This lecture focuses on aneuploidies involving altered numbers of chromosomes and also on polyploidy, particularly in plants. The following lecture will examine aneuploidies in which chromosomal morphology is altered.
Human autosomal aneuploidies: We have already examined human aneuploidies involving altered numbers of sex chromosomes in a previous lecture. Because of dosage compensation, individuals with sex chromosome aneuploidies are usually viable, and sometimes only minimally affected. In sharp contrast to sex chromosome aneuploidies, autosomal monosomies and trisomies in humans are generally not viable beyond infancy. Without dosage compensation, the alteration of the ratio of genes carried on the affected chromosome relative to those in the rest of the genome has severe phenotypic effects, which are usually fatal during embryonic development or soon after birth.
Down syndrome: The one exception is trisomy of chromosome 21 (one of the smallest of the human chromosomes), which is responsible for most cases of Down syndrome (a few cases are caused by partial trisomy resulting from translocation of parts of chromosome 21). Individuals with Down syndrome have low IQ and a characteristic physical appearance. Their life expectancy is substantially reduced and nearly all who survive into their 40's exhibit symptoms similar to Alzheimer disease. The nondisjunction leading to Down syndrome can occur in either parent, but the risk is greatest during oogenesis in older mothers (Fig. 9.13) Simple trisomy of chromosome 21 caused by nondisjunction appears to be a random event, but translocations of portions of chromosome 21 to other chromosomes can also result in familial Down syndrome, as will be explained more fully in the next lecture. .
Edwards syndrome and Patau syndrome: Live births also occur for trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) and rarely for a few others, but the severe developmental defects seen in such individuals invariably result in death in infancy or early childhood. The textbook describes the details of these defects in figures 9.14 and 9.15.
Cri-du-chat syndrome No human monosomies other than Turner's syndrome are viable. However, loss of a portion of one copy of chromosome 5 can result in the live birth of a severely afflicted child with cri-du-chat syndrome (figure 9.9). The name is French for "cry of the cat", based on the cry of the afflicted infants, which has a sound like the a meow of a cat. The severity of the affliction varies greatly with the extent of the deletion of the short arm of chromosome The overall incidence of this syndrome is estimated to be about 1 in 50,000 live births.
Trisomies in plants: Trisomies are frequently viable in plants. In the case of Datura stramonium (Jimson weed), trisomies have been identified for all 12 of its chromosomes, each of which gives the seed capsule a characteristic appearance (Figure 9.10). Such trisomies result in trivalent association of homologous chromosomes during meiosis, which in some cases can give rise to gametes containing a mixture of 1 and 2 copies of the trivalent chromosome, thus making it possible to perpetuate the trisomic condition.
Even-numbered polyploids: Even numbered polyploids are potentially capable of normal meiotic pairing and thus are often fertile. In practice, allopolyploids are more likely to be fully fertile, since their chromosomes are derived from different species and thus are likely to contain minor differences that help to insure correct pairing during meiosis. Such chromosomes are sometimes referred to as homeologous, rather than homologous. Autopolyploids , which contain two diploid sets of chromosomes from the same species, often have reduced fertility due to aberrant pairing of multiple copies of identical chromosomes, but there are usually enough bivalents and tetravalents formed in autotetraploids to achieve some production of functional gametes.
Polyploidy in food plants: Polyploidy is common among plants, and often selected for in food plants because it increases cell size and thus the size of the fruit or seed grain that is produced. Thus, potatoes, coffee, peanuts, and McIntosh apples are tetraploid, bread wheat and barley are hexaploid, and strawberries are octaploid. Polyploidy also occurs in some types of animal life, but is not well tolerated systemically (in all body tissues) in most complex species. However, hepatocytes (liver cells) in humans and other mammals are generally polyploid. This is often referred to as endopolyploidy because it results from a process of "endomitosis" in which chromosomes are duplicated and separated but then merge into a single nucleus without cell division.
Origins of polyploidy: Polyploidy can occur naturally either through meiotic abnormalities that result in production of diploid gametes or through mitotic abnormalities that result in failure of mitosis to occur after the chromosomes have duplicated. Allotetraploidy is sometimes the result of mitotic duplication of chromosomes in a cross-species hybrid that would otherwise be sterile. The presence of two copies of all of the parental chromosomes from both species makes it possible for homologous pairing to occur, generating gametes that contain one copy of each of the chromosomes from both original parents. Because of this, allotetraploids are also sometimes referred to as amphidiploids.
Radish plus cabbage allopolyploid: A famous historical case is a cross between a radish and a cabbage done in the 1920's in Russia with the goal of producing a plant with the leaves of a cabbage and the root of a radish. . Each parent species had 18 chromosomes. The fertile plant that finally emerged after chromosome duplicaiton had 36 chromosomes. Unfortunately, it had the root of a cabbage and the leaves of a radish, and thus was of no agricultural value. Such a plant cannot breed successfully with either of the parents, and thus fully fits the definition of a new species!
Induced allopolyploidy: With modern technology, one does not have to wait for rare natural events to convert a sterile cross-species hybrid into a fertile allotertaploid. Instead, the process can be induced by drugs such as colchicine that disrupt mitotic spindle function and allow a single nuclear envelope to reform around the duplicated chromosomes. An example is the development in Canada of a cold-resistant allotetraploid for commercial production of rapeseed (Canola) oil. Two species with haploid chromosome numbers of 9 and 10 were used to generate an allotetraploid with a chromosome nubmer of 38.
Trictale: Another example of a man-made allopolyploid is trictale (Fig. 1.10), which was generated from a hexaploid wheat and a diploid rye. In this case, desirable properties of the rye and wheat parents were combined in the fertile hybrid. Trictale is not only considered to be a new species, but because of its differences from both of its parents, it has been assigned to a new genus.
Cell culture hybridization: Modern technology even makes it possible to generate fetile allopolyploids from plants that cannot be hybridized directly. Fusion of cultured cells from the two plants, followed by duplication of chromosome number and hormonal induction of differentiation can sometimes result in the formation of a fully fertile plant despite the incompatibility of cross-fertilization of the parental plants.
Sterile (seedless) odd numbered polyploids: Triploids (and other odd numbered polyploids) are sterile because they are unable to undergo meiotic pairing in a manner that will produce chromosomally balanced gametes. This property has been exploited to generate a number of types of seedless fruits, which are also generally larger than their diploid counterparts. Sterile autotriploids occur spontaneously when a meiotic failure results in a diploid gamete (which in most cases will combine with a normal haploid gamete).
Bananas: Bananas are a classic example of a triploid seedless fruit. The little black dots in commercial bananas are abortive seeds that did not fully develop. A diploid banana is smaller and contains many large hard seeds about the size of coffee beans. Bananas have 22 chromosomes per haploid set. Each trivalent is expected to generate one haploid chromosome and one diploid pair. The chances that a gamete will be all haploid or all diploid is therefore (1/2)22. Thus, the chances of two euploid gametes participating in fertilization event that will lead to development of a seed is extremely small.
Other seedless triploids: Seedless watermelons are also triploids. It is relatively easy to produce large numbers of sterile triploids from a single plant for species that can be propagated vegetatively from cuttings. For those that cannot be grown in that manner, it is necessary to set up special crosses (such as diploid x tetraploid) to obtain seeds that will produce the sterile triploid plants.