BREEDING OF FRUIT CROPS - Lecture.3
An organism having more than two sets of homologous chromosomes is known as a polyploid. Polyploidy is of general occurrence in plants while it is rare amongst animals. If the somatic chromosome sets in a diploid be represented by AA BB CC then the genome, i.e., the number in the genomes will be A B C. If this is represented by ‘n’ then the simple polyploid series would be:
2n – diploid
3n – triploid
4n – tetraploid
5n – pentaploid
6n – hexaploid
7n – heptaploid
8n – octaploid
9n – Nonaploid
10n – decaploid and so on
Polyploidy is pervasive in plants and some estimates suggest that 30-80% of living plant species are polyploids, and many lineage show evidence of ancient polyploidy (paleopolyploidy) in their genomes. Polyploid plants can arise spontaneously in nature by several mechanisms, including meiotic or mitotic failures, and fusion of unreduced (2n) gametes. Both autopolyploids (e.g. Potato) and allopolyploids (e.g. canola, wheat and cotton) can be found among both wild and domesticated plant species.
Most polyploids display heterosis relative to their parental species. The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, and epigenetic remodeling, all of which affect gene content and or expression levels. Many of these rapid changes contribute to reproductive isolation and speciation.
Behaviour of polyploid crops
Polyploid plants tend to be larger and better at thriving in early succession habitats such as farm fields. In the breeding of crops, the tallest and best thriving plants are selected for. Thus, many crops (and agricultural weeds) may have unintentionally been bred to a higher level of ploidy. The induction of polyploidy is a common technique to overcome sterility of a hybrid species in plant breeding. In some situations, polyploid crops are preferred because they are sterile. For example, many seedless fruit varieties are seedless as a result of polyploidy. Such crops are propagated using asexual techniques such as grafting. Polyploidy in crop plants is most commonly induced by treating seeds with the chemical colchicine.
Examples of polyploid crops
- Triploid crops : banana, apple, ginger, watermelon, citrus
- Tetraploid crops : potato, cabbage, leek, tobacco, peanut, kinnow, pelargonium
- Hexaploid crops : chrysanthemum, bread wheat, triticale, oat, kiwifruit
- Octaploid crops : strawberry, dahlia, pansies, sugar cane
Some crops are found in a variety of ploidy. Apples, tulips and lilies are commonly found as both diploid and triploid. Bananas are available as diploid, triploid, tetraploid, and pentaploid. Daylilies (Hemerocallis spp) cultivars are available as either diploid or tetraploid. Kinnows can be tetraploid, diploid, or triploid.
A survey of the chromosome numbers of the species in a genus or a family shows that these species generally fall into a polyploid series. The species are grouped together under a taxonomic head because of certain morphological resemblances and relationships. They may be crossable or may not hybridize at all with one another. However, the chromosome numbers of the species show a general relationship, i.e., they form multiples of a common basic number. The chromosome numbers of the family Solanaceae may be considered as an example.
|Capsicum annum nigrum||12|
|S. nigrum var. gigas||144|
Autopolyploids are polyploids with multiple chromosome sets derived from a single species. They can result from a spontaneous, naturally occurring genome doubling, like the potato. Others might form following fusion of 2n gametes (unreduced gametes). Bananas and apples can be found as autopolyploids. Autopolyploid plants typically display polysomic inheritance, and are therefore often infertile and propagated clonally.
Allopolyploids are polyploids with chromosomes derived from different species. Precisely, it is the result of doubling of chromosome number in an F1 hybrid. Triticale is an example of an allopolyploid, having six chromosome sets, allohexaploid, four from wheat (Triticum turgidum) and two from rye (Secale cereale). Amphidiploid is another word for an allopolyploid. Mango and banana are also allopolyploids. Doubled diploids are known as amphidiploids. Some of the best examples of allopolyploids come from the Brassicas, the three diploid Brassicas (B. oleracea, B. rapa, and B. nigra) and three allotetraploids (B. napus and B. juncea).
Problems due to polyploidy and heterozygosity nature of fruit crops
Fruit crops such as mango, banana and citrus pose the problem of polyploidy, and crops such as mango, papaya and citrus are highly heterozygous. Choosing of polyploid varieties with desirable qualities may have the hindrance in developing hybrids as sometimes they exhibit sterility and obtaining a good hybrid may be questionable. In banana, when tetraploid is crossed with a diploid or triploid the genome of the segregating population will be unpredictable because of the restitution or unreduced chromosomes arising from the female parent. Heterozgosity on the other hand, create more complexity in breeding of mango, papaya and citrus because of wide segregations in the progenies. Hence, the breeding cycle is extended when compared to self pollinated crops because in every generation careful selection of progenies is required and high level of purity has to be maintained in each generation.