Primary Studies On Chromosome Doubling In Three Diploid Cotton Species

Polyploidization is a fundamental process in plant evolution,leading to the merger of two or more genomes into the same nucleus.All flowering plant lineages are known to have experienced at least one round of polyploidization in their evolutionary histories,as indicated by genome analysis in diverse lineages of plants.Colchicine have been used to induce polyploidy in plant species.Diploid Gossypium species have inheritable and desirable genes that are resistant to biotic and a biotic stresses and possesses useful fiber quality traits,such as longer fiber length and enhanced fiber strength.To transfer these desirable traits to the most cultivated tetraploid cotton species through hybridization is not possible due to hybrid sterility,in order to achieve polyploidy in diploid species,chromosome doubling could only be the possible mechanism to initiate polyploidy in Gossypium herbaceum,Gossypium arboreum and Gossypium raimondii and use their tetraploids forms to improve the narrow genetic diversity of the cultivated cotton.In this study,we explored the use of different concentrations of colchicines with varying applications techniques and time of exposure.Three different applications techniques were adopted;seeds soaked in different colchicine concentration,seeds germinated in Hoagland nutrient solution with colchicine and pre-germinated seeds in MS media with colchicine.In the first two applications techniques,seeds soaked in different colchicine concentration and seeds germinated in Hoagland nutrient solution with colchicine,The highest percentage of polyploidy was achieved in 0.5%concentration of colchicine;87.3% in G.herbaceum,82% in G.raimondii and 69.3% in G.arboreum in seeds soaked in colchicine and 84.9% was obtained in G.herbaceum,64.3% in G.raimondii and 66.9 in G.arboreum in seeds pre germinated in Hoagland nutrient solutions.In MS media,the highest polyploidy was achieved at 2000 ppm of colchicine concentration;34.8% in G.herbaceum,32.8% in G.raimondii and 34.8% in G.arboreum.Twenty-five cells from each sample were screened and number of chromosome was determined by the microscope.We observed chromosome abnormalities in all species as the concentration of colchicine increases even with less time of exposure.Among the treatments techniques employed to induce polyploidy in the diploid cotton species,seed soaking was the most effective and easy as compared to other two methods.Though sterilized tissue culture demands less experimental space and amount of colchicine,the high risk of contamination and high sensitivity of embryos to colchicine makes it less preferable.Colchicine treated plants are highly sensitive to environmental stimuli,caution is necessary,such as exposure high temperature and direct sunlight.The optimum dose of colchicine and the incubation period is species specific,under various environmental conditions.Chromosome doubling technique will be a valuable molecular tool in cotton study,to solve the sterility of interspecific hybrids,important agronomic traits bulking and will help breeders and molecular scientists in solving the perennial problem of narrow genetic diversity in Gossypium species.The doubled chromosome plants can be used for various genetic studies.