High outcrossing rates and short-distance pollination in a species restricted to granitic inselbergs

Abstract

Plant populations that are spatially isolated may experience genetic isolation if gene flow via pollination or seed dispersal is limited. The high genetic differentiation among populations of Encholirium horridum L.B.Sm. indicates low gene flow for this species that occurs exclusively on granitic inselbergs in South-east Brazil. Here we describe reproductive and pollination attributes of this bromeliad assessing how they influence the current degree of genetic isolation among populations. The mating system, estimated using five microsatellites markers, indicated allogamy and low pollen pool structure. The breeding system assessed by hand-pollination treatments supported partial self-incompatibility combined with inbreeding depression. Flowers of E. horridum are bell-shaped with crepuscular/nocturnal anthesis, and high nectar production, suggesting chiropterophily. Its pollination system was actually generalist but bats and hummingbirds were more frequent than hawkmoths and crepuscular bees. The high local abundance and large floral display per individual plant contributed to short distance pattern of pollen movement by pollinators. At the intra-population level, this pollinator foraging pattern, with inbreeding depression, promotes low seed viability and individual variations on outcrossing rate. At the inter-population level, those features led to low gene flow. Overall, heterozygosity, maintained by self-incompatibility and inbreeding depression, and pollination by vertebrates described here for E. horridum are consistent with other attributes that are predicted for inselberg plants by the OCBIL theory, such as reduced dispersability and genetic isolation among populations. Those characteristics contribute to the isolation of E. horridum populations and heighten the importance of population-based conservation strategies for taxa that occur exclusively in those naturally fragmented inselbergs.