Autopolyploidy

AUTOPOLYPLOIDY IN CROP PLANTS

1. AUTOPOLYPLOIDY Mr. Naveen kumar B N M.Sc (Horticulture) Department of Biotechnology & Crop improvement College of Horticulture, B engaluru

2. INTRODUCTION Most eukaryotic organisms are diploid (2 n) for most of their life cycles, possessing two sets of chromosomes. Occasionally, whole sets of chromosomes fail to separate in meiosis or mitosis, leading to polyploidy , the presence of more than two genomic sets of chromosomes. Polyploids which originated by doubling the chromosome number of a diploid species, or a hybrid between races of the same species, resulting in two pairs of chromosomes is called A utopolyploid , and the condition is referred to as A utopolyploidy .

3. Autopolyploidy is due to accidents of meiosis or mitosis that produce extra sets of chromosomes, all derived from a single species. Autopolyploids have three(triploid), four( tetraploids ), five( pentaploids ), six( hexaploids ), seven( heptaploids ), eight ( octaploids ) or more copies of same genome.

4. Autopolyploids are produced through chromosome doubling of a species. chromosome doubling may occur in somatic cells giving rise to tetraploid buds. In solanace 6-36percent of shoots are reported to the tetraploids . Some chemicals such as acenapthane , 8-hydroxy quinoline and nitrous oxide induce chromosome doubling. T he most efficient method of chromosome doubling is the treatment of seeds, seedlings or shoot tips with colchicine

5. C olchicine is an alkoloid extracted from the bulbs of C olchicum autumnale . It has the chemical formula of C 22 H 25 O 6 N. It interferes with the development of spindle apparatus as a consequence of which the sister chromatids of chromosomes are unable to migrate to the opposite poles during anaphase. Therefore, all the chromatids(=4) are included in the same restitution nucleus leading to chromosome doubling . Colchicine is generally applied as 0.2% aqueous solution or lanolin paste. The duration may vary from 3-24 hrs in the case of seeds and seedlings. The chromosome doubling effect of colchicine was first described by Blakesle and Avery , and by Nebel in 1937.

6. Occurrence of Autopolyploidy Non disjunction of gametes Autopolyploidy through mitosis Autopolyploidy through meiosis ii. By genome duplication

7. Nondisjunction of all chromosomes in mitosis in an early 2 n embryo, doubles the chromosome number and produces an autotetraploid (4 n).

8. An autotriploid (3n) may arise when nondisjunction in meiosis produces a diploid gamete that then fuses with a normal haploid gamete to produce a triploid zygote. Alternatively, triploids may arise from a cross between an autotetraploid that produces 2n gametes and a diploid that produces 1n gametes.

9. Because all the chromosome sets in autopolyploids are from the same species, they are homologous and attempt to align in prophase I of meiosis, which usually results in sterility. Consider meiosis in an autotriploid . In meiosis in a diploid cell, two chromosome homologs pair and align, but, in autotriploids , three homologs are present. One of the three homologs may fail to align with the other two, and this unaligned chromosome will segregate randomly. Which gamete gets the extra chromosome will be determined by chance and will differ for each homologous group of chromosomes. The resulting gametes will have two copies of some chromosomes and one copy of others. Even if all three chromosomes do align, two chromosomes must segregate to one gamete and one chromosome to the other. Occasionally, the presence of a third chromosome interferes with normal alignment, and all three chromosomes move to the same gamete.

10. In meiosis of an autotriploid , homologous chromosomes can pair or not pair in three ways

11. A gamete produced by meiosis in such an autotriploid might receive, say, two copies of chromosome 1, one copy of chromosome 2, three copies of chromosome 3, and no copies of chromosome 4. When the unbalanced gamete fuses with a normal gamete (or with another unbalanced gamete), the resulting zygote has different numbers of the four types of chromosomes. This difference in number creates unbalanced gene dosage in the zygote, which is often lethal. For this reason, triploids do not usually produce viable offspring.

12. Plant Type of polyploidy Ploidy Chromosome number Potato Autopolyploid 4n 48 Banana Autopolyploid 3n 33 Peanut Autopolyploid 4n 40 Sweet potato Autopolyploid 6n 90 Examples of Autopolyploids

13. Autopolyploidy arises from genome duplication species A species A diploid (fertile) X autotetraploid (fertile) spontaneous genome duplication Cause of genome duplication: a) meiotic non-reduction of gametes (both in egg and sperm)

14. Genetic effects In many species, autopolyploids show an increase in general vigour and size: the phenomenon is known as gaigantism . In general, leaves of autopolyploids are larger and thicker, and their flowers, fruits and seeds are larger. Cells, pollen grains and stomata of autolpolyploids are relatively larger than are those of diploids. All autopolyploids show variable sterility. Their growth rate is generally lower than that of normal plants, and they are later in flowering. Autopolyploids have a relatively higher water content than that of normal plants hence the fresh weight is more.

15. Cytology Autopolyploids are charecterized by the presence of multivalents at MI. In triploids the three homologues for each chromosome form either a trivalent or a bivalent and a univalent. During AI, the disjunction of trivalents and the distribution of univalents is irregular producing a range of aneuploid gametes. As a result, triploids produce a range of aneuploid progeny. EX; trisomics , double trisomics etc., most of the gametes of triploids are unbalanced and inviable as a result of which they are highly sterile.

16. In autotetraploids , the four homologues of each chromosome may associate as one quadrivalent , a trivalent and a univalent, or two bivalents at MI. T he disjunction of trivalents and the behaviour of univalents at AI is usually irreguar , while the quadrivalents may often dissociate 2:2. as a result a variable proportion of the gamates produced by the tetraploids are unbalanced and inviable , which accounts for their variable fertility. The fertility of autotetraploids has been considerably improved through selection in several species, e.g., maize, bajra , rice etc.

17. How can we identify autopolyploids ? Autopolyploids typically have multivalent pairing - chromosomes are more or less identical (polysomic inheritance)

18. Role of autopolyploids in Evolution Autopolyploidy has contributed to a limited extenct in evolution of plant species.some of our crops are autopolyploids . E.g., potato(4x), peanut(4x), coffee(4x), lucern (4X), banana(3x) and sweet potato(6x). Autotetraploids appear to have been more successful as crops than other forms of autopolyploidy . In addition, many of forage grasses and several ornamental species are autopolyploids .

19. Applications in crop improvement Triploid water melons are produced by crossing tetraploid (4x, female) and diploid(2x male)lines. Triploid sugarbeets produce larger roots and more sugar per unit area than do diploids. Autotetraploid cabbage and turnip are larger in size and water content than diploids. In ornamental tetraploids the flowers are larger and longer flowering duration than do diploids. The only autotetraploid commercial varieties are of Rye. Examples: Double steel and Tetra petkus .

20. Triploids Triploids are produced by hybridization between tetraploid and diploid strains. They are generally highly sterile, except in a few cases. This feature is useful in the production of seedless watermelons . In certain species, they may be more vigorous than the normal diploids, e.g., in sugarbeets . Tetraploids Autotetraploids have been produced in a large number of crop species and have been extensively studied in several cases . Tetraploids may be useful in one of the following ways: (1) in breeding , (2) improving quality, (3) overcoming self incompatibility, (4) making distant crosses and (5) used directly as varieties . 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 activity than the diploid .

21. Limitations of Autopolyploidy The larger size of autopolyploids is generally accompanied with a higher water content. As a result, autopolyploids of the crop species grown for vegetative parts do not always produce more dry matter than the respective diploids . Triploids cannot be maintained except through clonal propagation . New polyploids (raw polyploids ) are always characterized by a few or more undesirable features, e.g., poor strength of stem in grapes, irregular fruit size in watermelons, etc. Thus new polyploids can rarely be used directly in crop production.

22. Chromosome segregation in autopolyploids Possible genotypes in an autotetraploid AAAA Quadruplex AAAa Triplex AAaa Duplex Aaaa Simplex aaaa Nulliplex A simplex individual has one dominant and 3 recessive alleles( Aaaa ), a duplex has 2 dominant and 2 recessive alleles ( Aaaa ), while a nulliplex has none( aaaa )

23. Random chromosome segregation Segregation in a simplex ( Aaaa ) when the gene A is located close to the centromere, crossing over between the centromere and gene A would not take place. In this case, both the sister chromatids of each chromosomes are attached to the same centromere and would move to the same pole at AI. At AII the sister chromatids carrying the dominant alleles would move to the opposite poles. Therefore, the two dominant alleles do not reach the same gamete,i.e ., AA gamete is not produced. Such segregation is known as Random chromosome segregation Types of Chromosome segregation Random chromosome segregation Random chromatid assortment

24. Chromosome segregation in in a simplex would produce two type of gametes Aa and aa in the ratio 1:1 and genotypes Aaaa , Aaaa and aaa in the ratio 1:2:1. If a single dominant allele A is able to produce phenotype. It is assumed that at MI only quadrivalents are formed and that separation of chromosome is random.

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