Natural Hybridization between Bursera bicolor × B. glabrifolia (Burseraceae) Complex: Molecular and Chemical Evidence

The hybridization phenomenon is recognized as an important evolutionary force that influences the diversification and evolution of different vascular plant groups. Hence, it is important to identify hybrid individuals. In Mexico, the dry tropical forest (DTF) is considered as the main center of diversification and endemism of the Bursera genus, containing 85% of the approximately 106 described species worldwide. In the Mexican DTF, a complex of Bursera species was identified, which is formed by two putative parental species Bursera bicolor (Will. ex Schltdl.) Engl. and B. glabrifolia (H.B.K) Engl. Their putative hybrids were analyzed using molecular (cpSSR) and chemical markers (monoterpenes, sesquiterpenes, and triterpenes) in two pure sites (one site for each parental species) and two hybrid zones. We tested whether individuals with atypical morphology between B. bicolor and B. glabrifolia from sympatric sites were the result of genetic flow between these two species. A total of 80 individuals were analyzed with 4 microsatellite diagnostic primers and 37 secondary metabolites (SMs). The genetic and chemical markers were highly coincident according to the statistical analyses performed, and they supported the hybridization hypothesis in this Bursera complex, with both species remaining distinct even in sympatric zones. α-Amyrin (b), β-amyrin (a), and β-amyrin (b) (triterpenes) were the SMs that most contributed to differentiating putative parental species according to SIMPER analysis. The putative hybrids registered the highest genetic diversity values along with the highest number of SMs, out of which 11 were novel and distributed as follows: triterpenes > monoterpenes > sesquiterpenes. Finally, the chemical markers of the three analyzed families give a framework for future studies to explore hybridization events between Bursera species.