Genetic diversity and structure of the introduced germplasm of the rubber tree (Hevea brasiliensis Willd. ex A. Juss) in Costa Rica

Hevea brasiliensis, the rubber tree, is a species of major economic and ecological importance. Although it was introduced to Costa Rica in the early twentieth century, its genetic diversity and structure remain poorly characterized. This study assessed the genetic variability of introduced H. brasiliensis from old germplasm in Costa Rica using nine microsatellite markers from six representative sites. Among 122 individuals, 113 unique multilocus genotypes were identified, with 9 identical genotypes shared within and among distant sites, supporting centralized germplasm distribution. The genetic diversity was moderate and relatively uniform (He = 0.56–0.61), with the highest allelic richness in La Tirimbina (Na = 5.67) and the lowest in Potrero Grande (Na = 4.11). Nei’s distances indicated close relationships among HCR, Los Gavilanes, and Potrero Grande and strong differences between Potrero Grande and La Tirimbina. The unweighted pair group method with arithmetic mean (UPGMA) separated the samples into two major groups, whereas discriminant analysis of principal components (DAPC) resolved six genetic clusters with limited geographic structure and evidence of admixture, except for Guápiles, which formed a homogeneous cluster. Genetic diversity across clusters was moderate, and Cluster 3 (with members from La Tirimbina) showed the highest allelic richness and heterozygosity. Cluster 4, dominated by individuals from Potrero Grande, presented the lowest effective number of alleles and heterozygosity, indicating a restricted genetic base. These results show that the genetic structure of Costa Rican rubber plantations reflects historical germplasm movement rather than spatial proximity and identifies distinct genetic clusters. The results demonstrate moderate diversity and genetic structuring among H. brasiliensis populations, providing essential insights for germplasm conservation, germplasm management, and the development of breeding strategies aimed at enhancing the genetic basis of rubber tree cultivation in Costa Rica.