| Cotton(Gossypium spp.)is the most important economic crop due to its contribution of providing human beings with natural textile fiber.It is also widely used as oil and protein crop with a high level of oil content of 35?38%and protein content of 35~39%,cottonseed is an excellent source of oil and protein for food from the view of nutritional point.Upland cotton as the main cultivar in the world,because of its cottonseed containing the toxin gossypol makes cottonseed processing and using to be difficult,human beings and non-ruminant animals can't take a long-term of consumption of cottonseeds products containing gossypol.Interestingly,some of the Australian wild cotton species possess a unique character,known as the "glanded-plant and glandless-seed" trait,the trait also be called as "delayed gland morphogenesis".The cottonseeds of these species have no glands and gossypol inside until the seeds germinated.These species with the trait of"glanded-plant and glandless-seed" are the ideal germplasm resources to transfer the trait into the cultivar of upland cotton.To breed the low-gossypol content cotton cultivar is the most effective way to overcome the limitation of gossypol for the massive usage of cottonseeds directly.G.australe is an Australian wild diploid cotton species possessing many available advantage characters in cotton breeding include the resistance of verticillium wilt,mites and drought,high level of fiber maturity and lint rate,trait of "glanded-plant and glandless-seed",etc.,for improving the quality of some characters in upland cotton cultivars.G.herbaceum is a diploid cotton cultivar with tolerance to drought and angular leaf spot.G.herbaceum is the closely related species of G.hirsutum in the phylogenetic relationship of A subgenome in Gossypium.Creating synthetic allotetraploid cotton from these two species would lay the foundation for simultaneously transferring favorable genes into cultivated tetraploid cotton.The new synthetic allotetraploid(A1A1G2G2)between G.herbaceum(maternal parent for cross)and G.australe is based on the doubling-treatment of burgeons on hybrid Fi with colchicine.The chromosomes are doubled complement in one branch on the treated plant grafted onto the G.barbadense and several S1 seeds are harvested on this branch.When the seeding of geminated from S1 seeds grown up to the two true leaves after transplanted on soil in small plastic pots,the seedlings gradually became flavescent beginning at the top leaves,followed by the lower leaves after the fifth true leaf emerged,the seedlings become too weak to grow.The stem apex from one S1 seedling is graft onto a G.barbadense plant to rescue the allotetraploid.This seedling is recovered to further grow after twice of grafting and set S2 seeds after propagating through the winter by grafting.We invest the morphological traits of synthetic allotetraploid(S1)among the grafted plants and their parents grafted plants during flowering.The shape and size of the leaves on S1 showed typical intermediate morphological traits between the parents.The flowers of S1 showed us a typical polymerization of several kinds of morphological traits from the flowers of its parents,petal color is similar with the paternal parent,G.au,strale and anther color is similar with the maternal parent in S1,at the same times the sepals,bracts and nectaries showed the typical intermediate shape and color.The most parts of traits invested in the study trend to resemble those from maternal or paternal parent or polymerize both parents' to exhibit a typical intermediate phenotype in both F1 and S1,but the traits of F1 are weaker than the chromosome-doubled hybrid S1.To confirm the authenticity of the synthetic allotetraploid and to examine its genome components,we performed GISH targeting to somatic S2 mitotic cells by using the G2 gDNA from G.australe labeled with digoxigenin as a labeled probe and A1 gDNA from G.herbaceum race kuljianum cv Hongxingcaomian as unlabeled DNA.The results showed 26 chromosomes detected by the probe were from G2 genome and the other 26 chromosomes without signal were from A1 genome.Chromosome associations at meiosis of pollen mother cells(PMC)from the synthetic allotetraploid were performed by cytological observation with a result that all of the cells have 52 chromosomes.The chromosomes observed in mitotic and meiosis metaphase are 52 respectively which confirmed the authenticity of the new synthetic allotetraploid.The high frequency of univalents in PMCs at Metaphase I in meiosis explained why the synthetic allotetraploid plants were partially fertile.This study confirmed the authenticity of the new synthetic allotetraploid and investigated the variation of genome DNA methylation in interspecific hybrids and their parents which under the stress from the genome interaction and extreme conditions by MSAP analysis.The results of this study provide the possibility to transfer the characters like "glanded-plant and glandless-seed" into the cultivar of upland cotton and something useful to understand the methylation variation in the early stage of the allotetraploid synthesized. |
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