Types of allopolyploids
a) Monobasic allopolyploids: Triticum durum
b) Polybasic allopolyploids: Rapeseed
c) naturally occurring allopolyploids: Wheat, cotton, Sugarcane, oats, tobacco and mustard.
d) Artificially created allopolyploids: Triticale, Raphanobrassica.
Synthesis of allopolyploids
a) Hybridization of parental species followed by doubling of chromosome no. of F1.
b) Doubling chromosome number of parental species followed by hybridization
c) Through the use of FDR gametes of one or both of the diploid parental species or their F1 hybrid.
d) Fusion of Protoplast by somatic hybridization.
Application of allopolyploid
a) Alloployploidy has three major applications in crop improvement :
(1) as bridging species in the transfer of characters from one species into another, (2) in the production of new crop species, and
(3) for widening the genetic base of existing allopolyploid crop species.
b) The promising alloplyploids are Raphanobrassica, the triploid (AAC) obtained by crossing B. napus (AACC) with B. campestris (AA), allopolyploid clovers, Festuca-Lolium hybrids and some species hybrids in Rubus and Jute (Corchorus sp.,). In Raphanaobrassica, the breeding objectives are to combine the hardiness of B. oleracea with quick growth and disease resistance of fodder radish. The problems of Raphanobrassica are the same as those of triticale. i.e., low fertility, cytogenetic and genetic instability and leafy rape-like plants that do not produce bulbs. There is evidence that hybridization and selection at the polyploidy level would be effective in improving Raphanobrassica.
c) In cassava the triploids developed by crossing tetraploids and diploids, have been reported to be promising and it therefore seems possible to improve cassava by producing new chromosomal lines, in which the chromosome number does not go beyond an optimum level.The triploid plants derived from OP-4 (2x) X S-300 (4x) and OP-4(2x) X H-2304 (4x) consistently produced roots with high dry matter in the seedling and succeeding clonal generations, which ranged from 34 to 43%. Some of the triploids recorded high starch content from the eighth month onwards, being significantly higher than that of the control. Among these, the triploid 76-9 had a yield similar to that of H- 2304, the released cultivar, at CTCRI.
d) The amphidiploids B.napus (AACC) crosses very easily with B. campestris (AA) to produce the triploid (AAC), which has some desirable features. The triploid is produced so easily that it may be used as a hybrid variety, a special case of hybrid varieties produced by crossing two different species.
Limitations of allopolyploidy:
a) Low fertility and sterility
b) Cytogenetic and genetic stability
c) Time, labor and other resources consuming.
Effects of allopolyploidy
a) More vigorous than diploid
b) Greater adaptability than parents
c) Apomictic, particularly in grasses