Abstract:

There are 51 species in Gossypium including 5 tetraploid species (AD genome, 2n=52) and 46 diploid species which belong to the genome A, B, C, D, E, F, G, and K, respectively. Among them, there are 4 cultivated species including 2 tetraploid species, G. hirsutum and G. barbadense, and 2 diploid species belonging to genome A, G. arboreum and G. herbaceum. All the 5 tetraploid cotton species were testified to be AD allopolyploids, which was developed from the hybridization between two diploid species related to the genome A and genome D. Since their parental genome groups exist in diploid form, the question “how allopolyploid cotton formed” has stimulated discussion for more than 50 years. Furthermore, the species in Gossypium genus represent a vast resource of genetic multiplicity for the improvement of cultivated cotton. Research on genetic diversity and phylogenetic relationships among cultivated and wild cotton species is necessary for better understanding of cotton evolution. In order to determine the genetic diversity and relationships within a diverse collection in genus of Gossypium, 24 cotton species in gossypium including 20 diploid species representing 7 basic genome groups and 4 AD allotetraploid cotton accessions were assessed by RAPD, using their relative plant, T. populnea, as contrast. 26 of 40 RAPD primers were polymorphic, with the percentage of polymorphic bands of 21.0%，and the dendrograms were constructed by the Unweighted Pair Group Method of Arithmetic Average （UPGMA） based on Jaccard，s genetic similarity coefficients using the NTSYS-pc（Version 2.00）. The results showed that there was an obvious genetic diversity among 25 species. Among 20 diploid cotton species, the genetic similarity coefficient between G. aridum and G. capitis-viridis was the lowest, so did between G. aridum and G. australe. Therefore, the genetic relationship between G. aridum and G. capitis-viridis / G. australe was the farthest. Genetic similarity coefficients between allotetraploid and the diploid species belonging to genome A and genome D showed that the A-genome donor was much similar to the present-day G. herbaceum and G.arboretum，and they played an equal role on the course of formation of allotetraploid cotton species. The highest genetic similarity coefficient between allotetraploid species and G. raimondii indicated that G. raimondii was the possible D genome donor of allotetraploid cotton species. According to the results of this experiment and others, the ancient allotetraploid cotton species might be formed by hybridization and chromosome doubling between the species related to G. arboreum or G. herbaceum and G. raimondii, then different allotetraploid cotton species were appeared by further geographical and genetical isolation and separating differentiation, which supported the evolution theory of sole origin of allotetraploid cotton species. In addition, this result illuminated that the RAPD is an useful method in study of genetic diversity and pedigree classification of cotton resources at the genomic level, and the clustering analysis based on RAPD data was coincident with the results obtained from the traditional classification.

Key words: Gossypium, RAPD, Genetic diversity