Construction of the Core Collection of Catalpa fargesii f. duclouxii (Huangxinzimu) Based on Molecular Markers and Phenotypic Traits

To promote the conservation and utilization of Catalpa fargesii f. duclouxii (Huangxinzimu) germplasm resources, a total of 252 accessions were used to construct a preliminary core collection according to phenotypic traits and single nucleotide polymorphism (SNP) markers. In this study, 24 phenotypic traits, namely, 9 quantitative traits and 15 qualitative traits, were investigated. The core collection of C. fargesii f. duclouxii (Huangxinzimu) was constructed to remove redundant samples from the collected materials. First, the phenotypic core collection, with a sample proportion of 30, consisting of 24 clones, was constructed according to two genetic distances (Euclidean distance and Mahalanobis), four system clustering methods (the unweighted pair-group average method, Ward’s method, the complete linkage method, and the single linkage method), and three sampling methods (random sampling, deviation sampling, and preferred sampling). The best construction strategies were selected for further comparison. Three core collections (D2C3S3-30, D2C3S3-50, and D2C3S3-70) were constructed according to the optimal construction strategy at three sampling proportions. The core collection D2C3S3-30 with the best parameters was evaluated by using six parameters: the mean difference percentage (MD), variance difference percentage (VD), periodic rate of range (CR), changeable rate of the coefficient of variation (VR), minimum rate of change (CRMIN), and maximum rate of change (CRMAX). Three core collections (M-30, M-50, and M-70) were constructed by molecular markers, and the optimal core collection M-30 was selected by using five parameters, namely, Ho, He, PIC, MAF, and loci. The combination of D2C3S3-30 and M-30 was used to construct the final core collection DM-45, 45 samples representing the complete range of phenotypic and genetic variability. In this study, phenotypic traits combined with molecular markers were used to construct core collections to effectively capture the entire range of trait variation, effectively representing the original germplasm and providing a basis for the conservation and utilization of C. fargesii f. duclouxii (Huangxinzimu).