In this article we will discuss about the classification of polypoidy.
Individual carrying chromosome numbers other than the diploid (2x & not 2n) number are known as heteroploids and such situation is known as heteroploidy.
x = basic chromosome no. or genomic number
n = Somatic chromosome no. of the species whether diploid or polypoid.
In a diploid species, n = x.
The change in chromosome number may be either in an exact multiple of the basic number (i.e. Euploidy) or in not an exact multiple of the basic number.
1. Euploidy:
Individuals having exact multiple of the basic or genomic number are known as euploids and the situation is known as euploidy. Euploidy is more commonly known as polyploidy. Polypoid is an individual having more than two basic or monoploid sets of chromosomes and the condition is known as polypoidy.
When all the genomes present in a polyploid species are identical i.e., chromosomes derived from parents with similar genomes, it is known as autopolyploid. The genetic composition of autotetraploid may be given as AAAA. Autopolyploid plants are generally larger than their diploids but their fertility is somewhat reduced. Cytologically autopolyploid is identified by the presence of multivalents formed at metaphase of meiosis I.
Whereas individuals having two or more distinct genomes are called allopolypoids means polypoids arising from hybridization of two species or genera with subsequent duplication of each chromosome complement are allopolypoids. Here chromosomes are derived from parents with different genomes. The genetic composition of allotetraploids may be AABB.
The doubling of chromosome number in a hybrid is the basis of allopolyploid.
Pairing between two or more homologous chromosomes (e.g. Raphanobrassica- cross between Raphanus satiuus and Brassica oleracea) is called autosyndesis.
Pairing between two or more non-homologous chromosomes whether it be complete or partial is called allosyndesis.
Amphidiploid is an allopolypoid that has two copies of each genome and consequently behaves as a diploid during meiosis. A segmental allopolypoid contains 2 or more genomes which are identical with each other except for some minor differences.
Autopolypoids:
Polypoids have larger cell size than diploids, larger guard cells of stomata, larger pollen grains, slower in growth and later in flowering, larger and thicker leaves, larger flowers & fruits but lesser in number than of diploids, reduced fertility due to meiotic irregularity and genotypic imbalances.
Autotriploids are generally highly sterile e.g. water melons, banana etc. but in some cases they are highly fertile e.g., spinach.
In autotetraploid, 4 chromosomes are homologous to each other hence each gene has 4 copies.
Autopolypoid played a limited role in the evolution of plant species. Autopolypoid crops are potato (4x), alfalfa (4x), banana (3x), and sweet-potato (6x).
Triploids:
Produced by hybridization between tetraploid and diploid strains; generally highly sterile, useful in production of seedless watermelons. Seedless watermelons are produced by crossing tetraploid (4x, ♀) and diploid (2x, ♂) lines and are grown commercially in Japan. They do not produce true seeds.
Triploid sugarbeets have larger roots and produce more sugar per unit area than diploids. Triploid sugarbeets were grown commercially in Europe and Japan but their popularity is declining. Seed production is difficult because beet flower is small.
Triploids can’t be maintained except through clonal propagation.
Tetraploids:
Some autotetraploids may be superior in some quality characters to their respective diploids e.g., tetraploid maize has 43% more carotenoid pigment and vitamin A.
Certain distant crosses are not successful at the diploid level, but are relatively successful at the autotetraploid level e.g. 4x Brassica oleracea x B. chinensis is successful.
Autotetraploid varieties of forage crops are proved successful e.g. tetraploid red clover (Trifolium pratense), rye grass (Lolium perenne), aliska clover (Trifolium hybridum variety Tetra) and Berseem (Trifolium alexandrium) variety Pusa Giant have been proved most successful. Autotetraploid red clover and ryegrass are more vigorous, more digestible and palatable and have greater resistance to nematodes as compared to dipoids.
Autotetraploid turnips (B. rapa) and cabbage (B. oleracea) are larger in size and have more water content than diploids. Many ornamental plants are autotetraploids which have increased flower size and longer flowering duration.
Pusa Giant Berseem is the 1st autopolypoid variety released for general cultivation in India. It yields 20-30% more green fodder than diploid varieties. Sugandha is an autotetraploid variety of vetiver (Vetiueria zizanoides) which gives 11% more oil yield. Variety HMT-1 of Hyoscyamus niger is an autotetraploid which gives 15% more biomass and 36% more crude drug yield than diploid parent.
Allopolypoids:
The present day allopolypoids were most likely produced by chromosome doubling in F1 hybrids between two distinct species belonging to the same genera or to different genera.
Homoeologous Chromosome:
Homoeologous chromosome (Partially homologous chromosome). The partially similar chromosomes but not exact identical, are called homoeologous chromosome. Chromosomes of related species are partially similar to each other and are called homoeologous chromosomes. It means interspecific hybrids have two or more homoeologous chromosomes.
Amphidiploid:
An allopolypoid having two copies of each of the two or more different genomes present. Thus an amphidiploid has the somatic chromosome complement of two or more diploid series.
Diploidization:
The process due to which allopolypoids show diploid like behaviour i.e., only bivalent formation, is known as diploidization.
Synthesis of Triticale & Rephanobrassica:
Hardiness of rye (Secale cereale) and yielding ability of wheat (Triticum turgidum) were combined in Triticale hexaploide. But in Raphanobrassica, the aim was not fulfilled. The aim in producing Raphanobrassica was to synthesize a crop species that would combine the root of radish (R. sativus) with the leaves of cabbage (B. oleracea). Raphanobrassica combined the leaves of radish and roots of cabbage.
Triticale varieties are mainly grown in Polland, Germany & France.
Many of allopolypoids are apomictic. Apomictic species are common in grasses e.g., Poa, Taraxacum, Parthenium, Rubus etc.
Allopolypoids have been more successful as crop species than autopolypoids. About 1/3rd of the angiosperms are polypoids and by far the vast majority of them are allopolypoids.
The synthetic allopolypoid does often resemble in many ways with natural allopolypoids.
Evolution of Bread wheat (Triticum aestiuum) Bread wheat appeared about 8000 years ago as a hybrid between T. turgidum (♀) and T. tauschii (♂) possible in the regions of northern Iran and Armenia, where natural hybrids between T. turgidum & T. tauschii can be found in farmer’s field.
Previously considered that the source of ‘B’ genome was Aegilops speltoides and ‘D’ genome was Aegilops squarrosa.
Evolution of Amphidiploid Brassica Species:
The origin of amphidiploid Brassica species is based on the famous U’s triangle proposed by N.U. in 1935.
Evolution of Nicotiana Tabacum:
Evolution of Gossypium Hirsutum:
2. Aneuploidy:
An aneuploid has a chromosome no. that is not an exact multiple of the basis chromosome number (x). In other words the addition or deletion of one or more chromosomes or pairs of chromosomes but not the whole complement is known as aneuploidy.
The aneuploidy is the resultant of nondisjunction of the chromosomes during cell division.
The 1st variation in chromosome number (heteroploidy) discovered in an experimental population was gigas mutant in Oenothera described by Lutz in 1907. The gigas was an autotetraploid (4n). In 1910 Blakeslee discovered the globe mutant of Datura stramonium (dhatura) which was subsequently demonstrated by belling (1920) to be a trisomic and this was the 1st reported case of aneuploidy.
1st autotetraploid was experimentally induced by Winkler in 1916 in Solarium nigrum.
1st synthesized species- Triticale by Rimpau (but with no full information like chromosome no.) in 1890 from a cross between Triticum & Secale.
1st synthesized species with full information:
Nicotiana digluta obtained by spontaneous chromosome doubling of F1 hybrid from interspecific cross between-
The chromosome doubling action of colchicine was 1st described by Blakeslee and Nebel in 1937 but the effect of colchicine on mitosis was discovered in 1955.
Colchicine is a poisonous chemical isolated from seeds (0.2- 0.8%) and bulbs (0.1-0.5%) of autumn crocus (Colchicum autumnale). Pure colchicine is C22O6 N. It blocks spindle formation and thus inhibits movement of sister chromatids to the opposite poles.
A trisomic (2n + 1) is known as primary trisomic when the extra chromosome is the same as one of the haploid genome means it is not modified. But when additional chromosome is an iso- chromosome (two arms of the chromosome are identical), it is called secondary trisomic and when the extra chromosome is translocated chromosome, it is called tertiary trisomic.
The best source of aneuploids are triploid plants. Meiotic irregularities lead to formation of n + 1 and n-1 gametes.
Aneuploids are generally weaker than diploids. Monosomies don’t survive in diploid species. Only trisomies survive in diploid species.
Aneuploids are useful in locating a linkage group and a gene to a particular chromosome.
Aneuploids are useful in production of substitution lines.
Chromosome substitution means the transfer of a pair of chromosomes from one strain into a different strain. Chromosome substitution is easily done using nullisomics of the variety into which the chromosome is to be transferred (recurrent parent) but monosomies are used more commonly in place of nullisomics.
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