Genetic Diversity of Landraces in Gossypium arboreum L. Race sinense Assessed with Simple Sequence Repeat Markers

Asiatic cotton (Gossypium arboreum L.) is an "Old World" cultivated cotton species, the sinense race of which is planted extensively in China. This species is still used in the current tetraploid cotton breeding program as an elite germplasm line, and is also used as a model for genomic research in Gossypium. In the present study, 60 cotton microsatellite markers, averaging 4.6 markers for each A-genome chromosome,were chosen to assess the genetic diversity of 109 accessions. These included 106 G. arboreum landraces,collected from 18 provinces throughout four Asiatic cotton-growing regions in China. A total of 128 alleles were detected, with an average of 2.13 alleles per locus. The largest number of alleles, as well as the maximum number of polymorphic loci, was detected in the A03 linkage group. No polymorphic alleles were detected on chromosome 10. The polymorphism information content for the 22 polymorphic microsatellite loci varied from 0.52 to 0.98, with an average of 0.89. Genetic diversity analysis revealed that the landraces in the Southern region had more genetic variability than those from the other two regions, and no significant difference was detected between landraces in the Yangtze and the Yellow River Valley regions. These findings are consistent with the history of sinense introduction, with the Southern region being the presumed center of origin for Chinese Asiatic cotton, and with subsequent northeastward extension to the Yangtze and Yellow River Valleys. Cluster analysis, based on simple sequence repeat data for 60 microsatellite loci, clearly differentiated Vietnamese and G. herbaceum landraces from the sinense landrace. No relationship between inter-variety similarity and geographical ecological region was observed. The present findings indicate that the Southern region landraces may have been directly introduced into the provinces in the middle and lower Yangtze River Valley, where Asiatic cotton was most extensively grown, and further race sinense crops were subsequently produced.