Ploidy
- Genomatic mutation is a chromosomal mutation in which the numerical changes in chromosomes or variation in chromosomal numbers occurs, known as numerical chromosomal aberration. The variation in chromosome numbers is due to error in meiosis and mitosis. The measure of number of chromosome in a cell is called called ploidy.
- The minimum number of chromosome which function as a harmonious and integrated unit is called basic chromosomal number.
- The chromosome number present in gamete formed by diploid cell is called haploid (n). The chromosome occurs in pair in somatic cell called diploid (2n). For eg. In Human 23 pairs of chromosome is present, which is the basic chromosomal number.
- The chromosomal number is maintained from generation to generation in a species, however, certain mutation causes change in chromosomal number in somatic cell. This condition is called ploidy.
- The change in chromosome number involves- addition or deletion of individual chromosome or of a complete set of chromosome.
Types of ploidy:
Changes in chromosome number are of two basic types: changes in one or more chromosome number in a set, resulting in a condition called aneuploidy and changes in whole chromosome sets, resulting in a condition called aberrant euploidy.
- Aneuploidy
- Euploidy
Aneuploidy:
- It is the condition in which one or more chromosome is added or deleted from basic chromosomal number in a diploid cell.
- In aneuploidy, the chromosome number is not multiple of basic chromosomal number.
- The organism with such abnormal conditions are called aneuploids or heteroploids.
There are two class of aneuploidy
- Hyperploid: addition of one or more chromosome to a diploid set
- Hypoploid: deletion of one or more chromosome from a diploid set
Causes of aneuploidy
- i). Non-disjunction:
- It is the condition in which one or more pairs of chromosome (bivalent chromosome) fails to separate during anaphase of meiosis-I.
- Because of irregular distribution of chromosome at poles, one daughter cell receives one or more extra chromosome whereas other daughter cell lacks one or more chromosome and they form respective gametes.
- When the gametes having extra chromosome fuse with normal haploid gametes, it result in hyperploids.
- And when the gamete lacking one or more chromosome fuse with normal gamete, it result in hypoploids.
- ii). Non orientation of one or more bivalent at metaphase-I of meiosis-I
- iii). Loss of individual chromosome in meiosis or mitosis
- iv). Irregularities in segregation of chromosome during meiosis in polyploidy condition also results in aneuploidy
- v). Multipolar mitosis with irregular distribution of chromosome to daughter cell
Types of aneuploidy
i. Monosomics: 2n-1
- It is the result of loss of one copy of chromosome from a diploid complement set.
- It’s chromosome number is represented by 2n-1
- A diploid cell missing a single chromosome is monosomic. And if a cell misses two nonhomologous chromosomes, it is called double monosomic.
- In most diploid organisms, loss of one chromosome copy from a pair is deleterious.
- In humans, monosomics condition in any autosomes are fatal. And also, monosomic in X-chromosome are fatal, however, few viable cases are present. Eg. Turner syndrome
- Example: Turner’s syndrome: (44+X)
- It occurs when an abnormal egg (O) fuse with normal sperm (X). The individuals have 45 chromosome (44 autosome and one X).
- The affected individual is sterile female with under-developed breasts, reduced ovaries, short stature, and often have a web of skin extending between the neck and shoulders lacks menstrual cycle and few male like characters
ii. Nullisomic: 2n-2
- It is the result of loss of a pair of chromosome from diploid set.
- In this case, a diploid organism lacks a pair of homologous chromosome.
- It’s chromosomal number is represented by 2n-2
- It is generally lethal in an organism.
- Example: In wheat, they can tolerate a nullisomic mutation
iii. Trisomic: 2n+1
- An organism containing one extra chromosome in addition to diploid set.
- In normal meiosis-I, chromosome pair of bivalent separates and goes to each of the daughter nuclei. But very rarely, one pair of chromosome fails to disjoin and finally it moves to one pole, so half of the daughter cell receive extra chromosome and other half of daughter cell lose one. Such that (n+1) and (n-1) gametes are formed.
- When n+1 gamete fuse with normal gamete, it gives trisomic organism.
- Example: Down syndrome; trisomy 21
- It is due to an extra chromosome number 21.
- The individual with Down syndrome have 47 chromosome.
- The disorder is characterized by mental retardation, short body stature, swollen tongue, eyelid folds resembling Mongolian race.
- Example: Klinefelter’s syndrome:
- It is characterized by 2n+1 (44+XXY) genotype.
- It occurs when an abnormal egg (XX) fuse with normal sperm (Y)
- The affected individual is sterile male and is characterized by unusually long body, obese and female like characteristics
iv. Tetrasomic: 2n+2
- An organism having one extra pair of chromosome in addition to its diploid set.
- It is represented as 2n+2
- Examples: tetrasomy 9p, tetrasomy 18p, tetrasomy 12p (Pallister-Killian syndrome), tetrasomy 22 (Cat eye syndrome)
Euploidy:
It is the condition of addition or loss of complete one set or more than one set of chromosome in diploid organism.
Types of euploidy:
- Monoploidy or Haploidy
- Diploidy
- Polyploidy
Monoploidy or haploidy:
- The total amount of genetic material in a haploid cell or in prokaryotic organism is called genome.
- Monoploidy or haploidy involves loss of complete one set of chromosome from a diploid cell.
- Monoploid or haploid organism contains single genome (n) in their cell.
- They contains one member of each kind of chromosome.
- Haploid cell is formed during gametogenesis is diploid organism.
- Viruses and bacteria contains single genome and are haploid
- Majority of lower plants particularly thallophyta and bryophyte exists in monoploid form.
- In higher plants, haploidy develops as a result of parthenogenesis.
- In some animals, like honey bees and wasps, male drone are haploid.
Characteristics of haploids
- Haploid plants are usually weaker and smaller than diploid, but in pepper the haploid are as healthy as normal diploid plant
- Leaves of haploid plants are generally small and Plants have low viability.
- In monoploid male honey bees, during spermatogenesis the meiosis is bypassed by
mitosis. As a result, their sperms are haploid and viable.
Diploidy:
- Normally all higher plants and animals occurs in diploid form.
- They contains two copy of each chromosome
Polyploidy:
- Polyploidy is a condition of addition of one or more complete set of chromosome in diploid cell.
- Polyploidy results due to failure of separation of chromosomal sets during mitosis or meiosis such that more than two chromosomal sets are present in a cell.
- Polyploidy is more common and sometimes resulting in evolution of new plant species with better yield
- A number of agriculturally important crops, such as wheat, oats, cotton, potatoes, and sugar cane are polyploids.
- Generally, Organism possess two sets of chromosome (2n) called diploid.
- The organism with three sets of chromosome called triploid, 4 sets; tetrapolid, 5 sets; pentaploid and so on. These all are polyploids and the condition is known as polyploidy.
Artificial method of polyploidy:
i. Radiation:
- X rays, gamma rays induces rate of cell division in seeds, buds, flowers and also causes multiplication of chromosome number (somatic chromosome doubling)
ii. Injury:
- Injured part of plants forms callus. Callus growth is enhanced by a chemical Coumerine which also brings chromosomal doubling.
- Eg. Tetraploids tomato is developed from injured part
iii. Chemical treatment:
- Chemicals such as colchicine, 8-hydroxyquinolin, acetophenon, nitrous oxide, granosan, chloroform, choral hydrate, some alkaloids induce chromosome doubling.
- Colchicine is an alkaloid drug obtained from the corms of plants–Colchicum autmunale and C. luteum) and its aqueous solution is found to prevent the formation and organization of spindle fibres. So, when the cell is treated with colchicine, it prevent metaphasic plate formation of chromosomes and cell division do not proceed further. Colchicine even prevent cytokinesis. Thus, cell contains double chromosomes number in each treatment for diving cells.
Types of polyploidy
- Autopolyploidy:
- Allopolyploidy
Autopolyploidy:
- It is the condition in which an individual organism contains more than two sets of same genome (homologous chromosome).
- For examples: if an organism has two set of chromosome (homologous chromosome ie AA) then the autotriploid (an autopolyploidy condition) will have similar three chromosome AAA.
- Autopolyploidy condition is multiplication of same basic set of chromosome within same species.
Cause of Autoployploidy:
- Autopolyploidy
arises due to the failure of disjunction of chromosome during anaphase
resulting in duplication of genomes and or failure to separate cell during
cytokinesis resulting in tetraploid cell.
- The failure of all chromosomes to segregate during meiosis gives rise to a diploid gametes which when fertilized by a haploid gamete, the resulting zygote has three sets of chromosomes.
- Fertilization of an ovum by two sperms results in triploid zygote
- Triploids can be generated experimentally by crossing diploids with tetraploids
Characteristics of autopolyploids:
- Autopolypolids are not very common and have of a little evolutionary consequences.
- Autopolyploids plants are more fertile, vigour and large size, resistant to disease. Eg. Banana, grapes, sugar beet, tomato, watermelon, marigold, corn, snapdragon etc
- Examples:
- Autotriploid plants (3n); developed by fertilization of diploid (2n) and haploid (n) gametes
- Autotetraploid (4n); developed by fertilization of two diploid gametes
- Autotetraploids (4n) are more common in nature than autotriploid becauses they have an even number of chromosomes, and produces genetically balanced plants.
- Autotetraploids can be produced experimentally from diploid cells by Heat shock or cold shock treatment and also by colchicine treatment to somatic cell. If the chromosome of the diploid cell undergo replication, but the cell did not divide, it results in the doubling of the chromosome number.
- Colchicine prevents separation of chromosome during anaphase by interfering with spindle fiber formation. Upon removal of colchicine the cell can re-enter into interphase during which the paired sister chromatids separate and uncoil. Now, the nucleus has four sets of chromosomes giving rise to a condition called autotetraploids (4n).
Significance of Autopolyploidy:
- Generally Autopolyploidy leads to increase in size, vigour and strength and often larger than their diploid counterparts. In some cases autopolyploids are smaller and weaker then diploid
- Pollen grains, stomatal guard cell and xylem parenchyma are larger in size in autoployploids than diploids
- Autopolyploids generally show reduced fertility due to high irregularities during meiosis which causes genotypic imbalance leading to physiological disturbances.
- Generally autopolyploids reproduce by vegetative propagation.
- The flower and fruits per plant in autopolyploids are usually less in number than diploids
- Autopolyploidy is much successful in species with low chromosome number and in cross pollinated species.
- Autopolyploidy is used in horticulture for ornamental plants like roses, dahlias and also in production of seedless plants. Examples; apples, pears, banana, grapes, orange etc
Allopolyploidy:
- Allopolyploidy is a condition developed by hybridization between two genetically distinct species followed by doubling of chromosomes.
- For examples; hybridization between species X with AA set of chromosomes and species Y with BB set of chromosomes results in hybrid species XY with AB set of chromosomes. On doubling the hybrid chromosomes set, resulting individuals have AABB set of chromosome, condition known as Allopolyploidy
- Generally the hybrid with AB set of chromosome are sterile but when the chromosomes is doubled (AABB), then resulting allopolyploids (amphidiploids) are fertile as they can produce gametes.
Examples of Alloploidy:
- George karpechenko (1927) performed a polyploidy experiment on Raphanus sativus (Raddish; 2n=18) and Brasssica oleracea (cabbage; 2n=18) by inter-generic crossing.
- The hybrid (Raphanobrassica) had 18 chromosome, 9 from raddish and 9 from cabbage but were sterile
- When the chromosome of sterile Raphanabrassica were doubled by artificial means (colchicine), a fertile Allopolyploids Raphanabrassica with 36 chromosome is produced.
- * Raphanabrassica has rooting system of cabbage and fruiting body of raddish.
Significance of Allopolyploidy:
- Used in crop breeding
- Used as a bridge species in transfer of desired characters from one species to another. For eg. Modern wheat
- For production of new crop species; Raphanabrassica
- Played vital role in evolution of species: 1/3 rd of flowering plants are polyploids and most are allopolyploids