Breeding of Fruit and Plantation Crops 2 + 1

Types of polyploidy

Autopolyploidy

• 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.

Allopolyploidy

• 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.