Hybridization is a relatively common and important phenomenon in plant evolution. Although it is well documented in taxa of the Cactaceae family, most studies have used only species morphology to investigate the extent of hybridization. For example, the cactus species Pilosocereus arrabidae and P. brasiliensis occur in sympatry on the northern coast of Rio de Janeiro, where they are frequently misidentified and suspected to interbreeding due to confusing morphological variation in these populations. These species occur in restingas and inselbergs, the driest habitats of the Brazilian Atlantic Forest, ranging along Bahia, Espírito Santo, and Rio de Janeiro states. The present work aims to test the hypothesis of hybridization between these two Pilosocereus species using ten DNA microsatellite markers. The hybridization hypothesis will be tested using Bayesian clustering methods. If the hybridization hypothesis is true, it is expected the inference of two genetic clusters in the sympatry area, each one clustering individuals with admixed ancestrally. In case there is no hybridization, we expected two genetic clusters with little or no admixed ancestrality individuals. The knowledge about gene flow between these species is a piece of fundamental information for taxonomy, for the understanding of the relationships among species, and also for the conservation of a highly threatened group.
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