Hierarchical genetic and spatial structure among varieties and populations of Hymenaea stigonocarpa (Fabaceae) in Brazilian savannah

Abstract

Approaches in population genetics decompose species genetic variation at hierarchical level, understanding the action of microevolutionary processes at spatial scales. We evaluated the taxonomically complex species Hymenaea stigonocarpa, a tree species distributed in central Brazil, presenting three botanical varieties weakly distinguished by morphological traits. We aimed to investigate the spatial genetic structure of populations of H. stigonocarpa along Cerrado biome, assessing genetic differentiation and the existence of hybridization zones in this species. We genotyped 901 plants from 28 local populations using nine nuclear microsatellite loci. Analyses of genetic diversity, Bayesian inference estimated genetic differentiation and migration rate among populations and varieties. Mantel tests and sPCA evaluated spatial pattern of genetic variation. We observed that H. stigonocarpa var. brevipetiolata presented larger genetic differentiation compared to H. stigonocarpa var. stigonocarpa and H. stigonocarpa var. pubescens (FCT = 0.175). Bayesian approaches supported a genetic different cluster and hybridization process in sympatric populations for H. stigonocarpa var. brevipetiolata. Migration rate was high and not significantly higher among varieties, confirming the existence of hybridization. Spatial correlations were relatively low, but with exponential decrease of genetic similarity along of the geographic space. Spatial genetic structure was higher for H. stigonocarpa var. brevipetiolata and local structures were found with sPCA, indicating that neighboring populations are genetically different mainly in populations of eastern Cerrado. The genetic clusters are not congruent with the vegetative characters used to recognize the three botanical varieties, which should be viewed with skepticism. Our genetic analyses strongly support the need for a taxonomic review.