DIPLOID AND POLYPLOID GAMETES IN CROP PLANTS: MECHANISMS OF FORMATION AND UTILIZATION IN PLANT BREEDING

 

 

 Haploid cells contain only one complete set of chromosomes which play important role in sexually reproducing crops at the time of fertilization. Diploid cells contain two complete sets of chromosomes and polyploids have multiple sets of chromosomes. Approximately 90 per cent of the genotypes of Ipomoea batatas, 4x produced 2n pollen with different frequencies. In contrast, none of the genotypes of I. triloba produced 2n pollen. The diameter of the 2n pollen was approximately 30 per cent larger than that of the n pollen. The abnormal spindle configurations are involved in the production of 2n pollen in I.batatas.¹          

           Large number of crosses were performed among the groups of lilies (Lilium), viz., Asiatic, Longiflorum and Oriental; and to some species, to produce F₁ inter-specific hybrids. Many of these hybrids were sterile; to overcome this sterility they used approaches like chromosome doubling by chemicals and the use of naturally occurring or induced unreduced (2n) gametes. The success in breeding lilies were achieved through the use of naturally occurring and induced 2n gametes.²

         To check the effect of genotype and temperature on male 2n gamete formation in Brassica allotetraploids and their interspecific hybrids based on estimates at the sporad stage, some interspecific hybrid genotypes produced unreduced gametes   at more frequency than the parents. These 2n gametes are required for the triploid bridge hypothesis of allopolyploid evolution, especially at cold temperatures.³

           Three cycles of phenotypic recurrent selection for 2n pollen production were conducted in adopted diploid red clover germplasm grown in the greenhouse. Average 2n pollen production per plant increased from 0.04per cent to 47.38per cent in third cycle and realized heritability for 2n pollen formation was about 0.50. It was estimated that two to six genes control the frequency of 2n pollen formation within a plant. Thus recurrent selection was utilized to enhance the 2n pollen formation frequency.⁴

           Allopolyploid hybridization serves as a major pathway for plant evolution, but in its early stages it is associated with phenotypic and genomic instabilities as observed in Arabidopsis thaliana. The variable phenotype of the allotetraploids could not be explained by cytological abnormalities. However, suppression of 20 of the 700 genes examined by AFLP of cDNA. Independent RT-PCR analyses of 10 of these 20 genes confirmed silencing in three of them, suggesting that~ 0.4 per cent of the genes in the allotetraploids are silenced.⁵ The utilization of polyploids in vegetatively propagated crops and diploids in sexually reproducing crops is more beneficial.

KEYWORDS: Diploidy, FDR, SDR, polyploidy and gene silencing.            

                                                                                                                                     

REFERENCES:

 1. Becerra, L. A. and   Orjeda, G., 2002, Occurrence and cytological mechanism of 2n pollen formation in a tetraploid accession of Ipomoea batatas (Sweet Potato).  The American Genetic Association., 93: 185–192.

2. Rodrigo, B.G., Byung , K. L.,  Shujun,  Z.,  Ramanna, M. S. and   Tuyl, M.V., 2008 , Interspecfic hybridization in Lily: The use of 2n gametes in interspecific Lily Hybrids. Global Science Books., UK.pp.138-145.

3. Mason, A. S., Nelson, M. N., Guijun, Y. and Wallace, A. C., 2011, Production of viable male unreduced gametes in Brassica interspecific hybrids is genotype specific and stimulated by cold temperatures. BMC Plant Biology., 11:103.

4. Parrott, W.A.  and  Smith, R . R., 1986 , Recurrent selection for 2n pollen formation in Red Clover. Crop Science., 26: 1132-1135.

5. Comai, L.,   Tyagi, A. P.,  Winter, K., Davis, R.H., Steven, H.R., Stevens, Y. and Breck, B., 2000,  Phenotypic instability and rapid gene silencing in newly formed Arabidopsis allotetraploids. The Plant Cell.,12: 1551–1567.