Population expansion and genetic structure in Cephalocereus nizandensis (Cactaceae), a microendemic cactus of rocky outcrops of the Tehuantepec basin, Mexico

Abstract

Background and aims – Cephalocereus nizandensis is a microendemic columnar cactus that grows isolated in xerophytic enclaves associated with rocky outcrops in the Isthmus of Tehuantepec, in the south of Mexico. Its demographic history and genetic structure were assessed to determine the main events that shaped its current restricted distribution.Material and methods – Chloroplast intergenic sequences of 40 individuals and inter simple sequence repeats (ISSRs) of 45 individuals from four isolated populations were used to estimate haplotypic and nucleotide diversity, using expected heterozygosity and the Shannon index. AMOVA, population pair-wise FST, and Bayesian clustering analyses were performed to explore the genetic structure. Demographic history was estimated with neutrality tests, mismatch distribution analysis, and Bayesian skyline plots. Phylogenetic relationships and divergence times were determined using a median joining network and a Bayesian molecular clock.Key results – C. nizandensis has a high diversity and moderate genetic differentiation. The lowest elevation locality was found to be the most genetically distinct. The species has undergone a process of population expansion that began 150,000 years ago and has remained without evidence of a population contraction in the transition from the Pleistocene to the Holocene (11,700 years ago).Conclusions – C. nizandensis presents moderate but significant genetic differentiation, which may be due to an early divergence of its populations. Currently observed levels of genetic diversity are the result of historical maintenance of high population sizes and a population expansion approximately in the last 150,000 years, which was sustained independently of the climatic fluctuations of the Early Quaternary, due in part to the stability of the rocky habitat.

Evolutionary Genetics of Cacti: Research Biases, Advances and Prospects

“…27%) as the most relevant hypotheses to explain range shifts, distributions, and diversification of cactus species in the Neotropics. For instance, the climatic oscillation involving the glacial cycles of the Pleistocene is thought to have promoted the expansion and contraction (fragmentation) range of populations from distinct genera, such as Pilosocereus , Cephalocereus , Rhipsalis , and Echinopsis [ 21 , 29 , 31 , 50 , 80 ]. Moreover, paleoclimates drove marine transgressions/regressions, which have been inferred to have impacted the population dynamics of coastal and island populations [ 3 , 60 , 81 ].…”

“…Moreover, as many species may hybridize in nature (e.g., [ 13 , 27 , 28 ]), cacti may provide many insights into hybrid zones, species cohesion, and species delimitation. Finally, due to its intrinsic association with drought stress, cacti are useful models for biogeographic approaches aiming to investigate the effects of Quaternary climate oscillations on xeric biomes (e.g., [ 21 , 29 , 30 , 31 , 32 ]). As a result, many studies with an evolutionary focus have been conducted on cacti in recent decades.…”