Population genetic diversity and structure of Pilosocereus tillianus (Cactaceae, Cereeae), a columnar cactus endemic to the Venezuelan Andes

Genetic diversity has been examined in only few species of the Cactaceae, however these studies show high genetic variability even when they have a restricted distribution. We examined the factors that can contribute to maintain high genetic diversity under this condition. We examined levels of allozyme variation and fine-scale genetic structure of Pilosocereus tillianus, a columnar cactus endemic to the Venezuelan Andes. Genetic diversity was considerably high compared to other species previously examined (species level: Ps = 100%, APs = 2.783, Hs = 0.352; population level: Pp = 97.8%, APp = 2.645, Hp = 0.350) levels. We propose three explanations for the levels of genetic variability detected: (1) poliploidy, (2) gene dispersal predominantly driven by bat and bird mediated pollination and seed dispersal, and (3) a vicariant speciation scenario, in which the current reduced geographic distribution of P. tillianus represents a relict of a former widespread species. High genetic variability in spatially restricted columnar cacti is emerging as a common condition between members of this family. Under the current scenario of habitat fragmentation in dry ecosystems in the Neotropics, cactus species with this trait can be more resilient to genetic erosion than expected according to their limited geographic distribution.