Flying genes, barriers and vicariance: Bat pollination and gene flow in a widespread columnar cactus
Stenocereus thurberi is a xenogamous columnar cactus endemic of the Sonoran Desert with a disjunct distribution on both sides of the Gulf of California. Bats are their main pollinators and seed dispersers. The comparison among populations along its distribution allowed us to assess the effect of spatial and temporal variation in the reproductive success and population genetic structure. We examined the effects of pollinator guild and of variation in time and space on the reproductive success of S. thurberi in three populations along a transect spanning about 800 km and five degrees of latitude in Sonora, Mexico.We study 12 populations of S. thurberithroughout its range to explore the effect of the current geographical distribution in the genetic structure. Also, we examine the effect of the separation of the peninsula and its complicated evolution between insular and peninsular systems. They were tested phylogeographic methods for modeling processes based on chloroplast DNA sequences: trnL-trnF, rpl32-trnL intron and petB D4.
In the Northern and Central populations reproductive success depends on the nectar-feeding bat Leptonycteris yerbabuenae (specialized pollinator). In the southern population, a combination of pollinators was more effective. However, significant differences were detected in the southern population, which indicates temporal differences in pollinator abundance or arrival time. The high mobility of pollinators (and also with high dispersal by bats) has a strong effect on the structure and genetic diversity. The genetic variance was higher in continental populations than peninsular populations (Hd = 0.81336; Hd = 0.71335), and the structure was more marked in the peninsular (Gst = 0.4677). A Bayesian analysis shows a partial separation of Baja California haplotypes, suggesting and ancient vicariance event with a subsequent dispersion across the Gulf of California. Two continental ancestral haplotypes were reconstructed and one peninsular. The latter includes one coastal population of Sonora, probably with contact from the last interglacial. In conclusion, the large genetic variation and diversity found in S. thurberiagrees with its open reproductive system and the high mobility of bats. However, directionality in the migratory routes of bats, small changes in number or kind of pollinators and seed dispersal agents, and/or lags in the timing of flowering and fruiting among populations, can critically affect gene flow dynamics of this emblematic species of the Sonoran Desert.