Ploidy and karyotypic variation in selected poplar ( <i>Populus</i> spp.) clones in Iran: Genetic insights for breeding strategies

Abstract The genus Populus (poplar) plays a major ecological role and is one of the fastest-growing woody species worldwide. Consequently, it serves as a model for genetic studies in woody plants. Identifying superior clones requires assessing genetic diversity, for which cytogenetic and karyotypic studies are essential. This study investigated 40 clones from 10 species. Significant differences ( p ≤ 0.01) were observed among clones for most chromosomal traits, indicating considerable karyotypic diversity. The base chromosome number was x = 19, with both diploid (2 n = 2 x = 38) and triploid (2 n = 3 x = 57) ploidy levels present. The clones P. euramericana var. vernirubensis (P. canadensis var. vernirubensis): 1(16) and P. nigra var. 63/135: 20(5) exhibited the most symmetric and most asymmetric karyotypes, respectively. The clones P. canadensis var. vernirubensis: 1(16) and P. caspica : (35) possessed the largest and smallest chromosomes, respectively. Quantitative analysis revealed that while individual chromosome volume was similar between ploidy levels (0.56–0.65 µm 3 in triploids vs. 0.52–0.81 µm 3 in diploids), the total chromosomal volume per cell was significantly greater in triploids (31.92–37.05 µm 3 ) than in diploids (19.76–30.80 µm 3 ). This ∼1.4-fold increase closely matched the theoretical 1.5-fold expectation based on chromosome number, indicating strict genomic additivity. Principal component analysis identified four main components: intra-chromosomal symmetry, chromosome length, inter-chromosomal symmetry and ploidy level. Cluster analysis grouped the 40 clones into 19 distinct classes, with different clones from the same species often classified separately, highlighting extensive intra-specific karyotypic diversity. The study demonstrates that triploid clones maintain diploid-scale chromosome dimensions, suggesting their possible phenotypic superiority stems from stable genome duplication rather than structural change, making them predictable candidates for breeding.