Genetic diversity analysis of cassava germplasm resources based on phenotypic traits and SSR molecular markers

Objective This study aimed to analyze the genetic diversity of 114 cassava germplasm resources， so as to provide a theoretical basis for germplasm resource conservation， evaluation， and utilization.Method Twelve phenotypic traits （including plant height， main stem height， internode length， plant shape， root number per plant， main stem dia-meter， cortex color of main stem， epidermis color of main stem， and root skin） were measured for genetic diversity analysis across 114 Chinese and foreign cassava germplasms， and SSR fluorescent molecular markers and capillary electrophoresis were used to evaluate the genetic diversity of cassava.Result The coefficients of variation （CV） of the 12 phenotypic traits ranged from 15.73% to 81.74%， with an average of 39.86%. The highest CV was observed in fresh root weight per plant， while the lowest was observed in stem diameter. The Shannon-Wiener diversity index values ranged from 0.5671 to 2.0059， with an average of 1.4202， indicating a relatively high overall genetic diversity of the tested germplasms. Four phenotypic traits （main stem height， fresh root weight per plant， main stem diameter， and cortex color of main stem） were preliminarily screened from the twelve phenotypic traits with high CVs and Shannon-Wiener diversity indexes. Cluster analysis based on the 12 phenotypic traits showed that the 114 germplasms were classified into six categories at a Euclidean distance of 20： categories I–III contained 62， 38， and 5 germplasms， respectively， while each of categories IV， Ⅴ and VI contained 3 germplasms. Using 28 pairs of SSR core primers to conduct PCR amplification for the 114 cassava germplasms， a total of 168 alleles （Na） were detected， with 3–14 alleles detected per primer pair （average=6）； the effective number of alleles （Ne） ranged from 1.5777 to 7.1136， with an average of 3.1217； Shannon’s index （I） ranged from 0.6418 to 2.1958 （average=1.2712）； observed heterozygosity （Ho） ranged from 0.2280 to 0.8070 （average=0.5877）； expected heterozygosity （He） ranged from 0.3661 to 0.8594 （average=0.6444）； the polymorphic information content （PIC） ranged from 0.3205 to 0.8472， with an average of 0.5978， demonstrating a high genetic diversity among the 114 cassava germplasms. Cluster analysis based on SSR molecular markers divided the cassava germplasms into nine groups： groups I–IX contained 1， 1， 13， 4， 11， 7， 9， 15， and 53 germplasms， respectively. The cluster results for phenotypic traits and SSR molecular markers were inconsistent， and neither pointed to a clear geographic distribution pattern.Conclusion Main stem height， fresh root weight per plant， stem diameter， and cortex color of main stem are important phenotypic traits for germplasm identification and core germplasm screening， which can be applied to germplasm resource evaluation， variety innovation， and elite germplasm screening. The 114 cassava germplasms exhibit a high genetic diversity， and the 28 pairs of SSR core primers could effectively characterize genetic diversity and can be used to analyze relationships among germplasms.