The following definitions will rely on these chromosomal descriptions. Two species will be considered, A and B. The zygote would be 3n. The difficulty arises when autotriploids try to mate because unbalanced gametes are produced because of pairing problems with the additional chromosome set.
Thus, these are invariably sterile. Autotetraploids occur from a doubling of the chromosomal composition. This can occur naturally by doubling sometime during the life cycle or artificially through the application of heat, cold or the chemical colchicine. Because an additional set of chromosomes exists, autotetraploids can but not necessarily in all cases undergo normal meiosis. One generalization that has been made is that autopolyploids are larger than their diploid counterpart.
For example, their flowers and fruits are larger in size which appears to be the result of larger cell size than cell number. This increased size does offer some commercial advantages. The increase in nuclear ploidy affects the structural and anatomical characteristics of the plant.
In general, polyploidy results in increased leaf and flower size Fig. These phenomena are collectively referred to as the gigas effect Acquaah, Its application in forage and ornamental breeding is described later in this chapter.
Physiological changes are also known to accompany genome duplication. These mainly result from change of metabolism resulting in a general increase in secondary metabolites Levin, This property has found application in the breeding of medicinal herbs in the production of pharmaceuticals. Hybrid vigor resulting from interspecific crosses in allopolyploids is one of the most exploited advantages of polyploid in plant breeding. Its application will be described in the next section.
Heterosis or hybrid vigor is the difference between the hybrid and the mean of the two parents and is characterized by increased vigor and superior qualitative or quantitative traits Chen, ; Dhawan and Lavania, ; Lamkey and Edwards, Over the last several decades, breeders have increased the world food production by utilizing the concept of heterosis in hybrid cultivars Kempe and Gils, S Stuber, However, unlike diploids which may lose heterosis with each consecutive generation due to segregation, alloploidy and autoploidy imposes pairing of homologous chromosomes, thus preventing intergenomic recombination Comai, This concept is called preferential or selective pairing and is the tendency for a doubled set of chromosomes to pair independently of the doubled set of chromosomes of the other species Acquaah, In this way, heterozygosity is maintained throughout generations Acquaah, ; Comai, Generally, the parents used in hybrid formation should be within subspecies or between subspecies.
An example of a man-made interspecies allopolyploid hybrid is triticale. It is derived from crossing two cereals, hexaploid bread wheat T.
Triticale was developed to combine good qualities of wheat including high yield and grain quality with the hardiness disease and stress tolerance of rye Acquaah, ; Chen, ; Haesaert and De Baets, ; Wolski and Pojmaj, The process of hybrid formation for polyploids is not without setbacks.
Many interspecific hybrids have low fertility and viability due to hybrid incompatibilities Chen, ; Orr, Hybrid incompatibility results from genes that are functionally diverged in the respective hybrid forming species. This may lead to silencing of protein encoding genes and has been reported in interspecific hybrids of Arabidopsis Chen, To increase the heterosis, fertility and viability of interspecific hybrids, several factors should be considered. The parents used should be of diverse genetic background and preferably heterozygous Acquaah, ; Chen, Self pollination is an important method for attaining homozygosity in breeding.
Through this process, it is possible to fix desired alleles in the background of a crop. In general, it takes approximately 3. Fixing a trait controlled by a single gene in an autotetraploid, would require four identical alleles to achieve homozygosity. For example, in a segregating, tetraploid F2 population the proportion of the homozygous loci would be Fig 5.
An immediate consequence of polyploidy is the change in gametic and filial frequencies Comai, This is because polyploids have multiple alleles associated with a single locus. For example, a hexaploid has six alleles per locus while a tetraploid has four.
The genetics of polyploids is often complicated by multi-allelism at loci thus altering segregation ratios and inheritance patterns expected in diploids. Provided a polyploid species behaves like a diploid at meiosis through normal bivalent pairing disomic inheritance , such as in wheat or tobacco, normal biometric analysis of inheritance apply Kearsey and Pooni, However, several autotetraploid crop plants including potatoes, coffee and lucerne and some forage grasses have tetrasomic inheritance Killick, With this knowledge, it is necessary to make accommodations in population structure and breeding strategy to account for differences in gamete structure Katepa-Mupondwa et al.
For example, breeding schemes that maximize heterozygosity are frequently used for the autotetraploid alfalfa in an attempt to utilize multi-allelic interactions Katepa-Mupondwa et al. Altered genotypic ratios are apparent in polyploids when compared with diploids. For example an arbitrary locus with B dominant and b recessive alleles, following selfing, an autotetraploid BBbb would produce 5 possible genotypes while a diploid Bb would generate 3 possible genotypes Fig 5.
Distinguishing a quadruplex BBBB from a triplex BBBb in the segregating population using a progeny test presents difficulty in breeding because both would breed true to the dominant allele. An extra generation would be required to identify the triplex by observing the formation of duplex plants Acquaah, Since autoploids contain more than two homologous chromosomes, meiosis results in the formation of univalents and multivalent, unlike in diploids where bivalents are usually formed Acquaah, For instance during meiosis, autotetraploids may form bivalents, quadrivalents and univalents Fig 5.
The ratio of these gametes following meiosis determines the fertility of a polyploid individual. Univalents and trivalents result in non-functional sterile gametes and are the most common in triploids, making them sterile. Rigorous and effective selection strategies for fertile autoploids are practiced in the development of inbred lines. Breeders rogue out autoploids with low seed set as well as those with morphological abnormalities Andrus et al.
Sterile alloploids arise from the pairing of homeologous chromosomes from separate genomes during meiosis instead of homologous chromosome Chen et al. This results in non-functional gametes. A viable allopolyploid requires a diploid-like meiosis behavior to establish disomic inheritance and full fertility.
Fertility problems in allopolyploids also occur when crossing crops of different ploidy levels as a result of formation of multivalents. To improve fertility, breeders use the parent with the lowest chromosome number as the female parent so as to increase seed set Olmo, High frequencies of chromosome mutations are desirable in modern breeding techniques, such as tilling, as they provide new sources of variation.
The multiallelic nature of loci in polyploids has many advantages that are useful in breeding. The masking of deleterious alleles, that may arise from induced mutation, by their dominant forms cushions polyploids from lethal conditions often associated with inbred diploid crops Gaul, This concept has been instrumental in the evolution of polyploids during bottlenecks where there is enforced inbreeding Comai, Mutation breeding exploits the concept of gene redundancy and mutation tolerance in polyploid crop improvement in two ways.
First, polyploids are able to tolerate deleterious allele modifications post-mutation, and secondly, they have increased mutation frequency because of their large genomes resulting from duplicated condition of their genes Gaul, The high mutation frequencies observed with polyploids may be exploited when trying to induce mutations in diploid cultivars that do not produce enough genetic variation after a mutagenic treatment.
This approach has been used in mutation breeding of Achimenes sp. In this study, the autotetraploids were found to have times higher mutation frequency than the corresponding diploid cultivar due to the large genome Broertjes, The seedless trait of triploids has been desirable especially in fruits.
Commercial use of triploid fruits can be found in crops such as watermelons and are produced artificially by first developing tetraploids which are then crossed with diploid watermelon. In order to set fruit, the triploid watermelon is crossed with a desirable diploid pollen donor. Another breeding strategy that utilizes the reproductive superiority of polyploids is bridge crossing.
When sexual incompatibilities between two species are due to ploidy levels, transitional crosses can be carried out followed by chromosome doubling to produce fertile bridge hybrids.
This method has been used to breed for superior tall fescue grass F. The same principle has been applied in fixing heterozygosity in hybrids by doubling the chromosomes in the superior progeny Comai, One of the immediate and obvious consequences of polyploidy in plants is an increase in cell size which in turn leads to enlarged plant organs, a phenomenon termed gigas effect Fig 5.
For example, the volume of tetraploid cells usually is about twice that of their diploid progenitors Acquaah, ; Emsweller and Ruttle, ; Levin, ; Schepper et al. The increase in cell volume however is mainly attributed to increased water and not biomass.
Therefore, its application is limited for breeding agronomically important crops such as cereals. Although chromosome doubling may result in significantly larger seeds and increased seed-protein content in cereal crops, this advantage is offset by low seed set Dhawan and Lavania, In contrast, the gigas effect has been explored in tree, ornamental, forage crop and fruit breeding Emsweller and Ruttle, ; Schepper et al.
For example, through induced polyploidy, breeders have developed Bouschet tetraploid grapes with more yield and juice content than the diploid progenitor Alicante Olmo, Ornamental crops such as snapdragons and marigolds have been bred through chromosome doubling to improve the quality and size of their blossoms Emsweller and Ruttle, A strong inverse correlation between DNA content and development rates in plants has been reported by several authors Levin, ; Smith and Bennett, It has been attributed to lower auxin levels, reduced surface to volume ratio and altered nuclear surface to cell volume ratio Acquaah, ; Levin, The mitosis that follows the treatment of colchicine is referred to as C — mitosis and it results in forming bivalents.
Many cultivated plants are autopolyploids. Examples include tetraploid Potato and alfalfa. Allopolyploidy is the phenomenon in which a hybrid variety is formed as a result of receiving three or more sets of chromosomes from genetically nonidentical varieties. Therefore, they do not have similar genomes and they belong to different types of species.
Allopolyploids can have either an even or an odd number of chromosomes. Multivalents are formed instead of bivalents in allopolyploidy. Examples of allopolyploids are cotton — 13 pairs and 53 chromosomes, wheat — 7 pairs and 42 chromosomes. Polyploids are formed as a result of non-disjunction taking place in the mitosis phase which will either result in bivalents or multivalents. Autopolyploidy is the phenomenon in which an organism receives three or more sets of chromosomes from organisms which have similar genomes, whereas allopolyploidy is the phenomenon in which the hybrid organism receives three or more sets of chromosomes from organisms which do not have similar genomes.
Producing these two types of polyploids have shown to be beneficial in plant breeding and crop cultivation. This is the difference between autopolyploidy and allopolyploidy. You can download PDF version of this article and use it for offline purposes as per citation note. Baron, Adrianne. Available here 2. Sengbusch, Peter v. Available here 3. Anand, Sandhya.
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