Wind-borne insects mediate directional pollen transfer between desert fig trees 160 kilometers apart

Abstract

The question of how far pollen can move between plants has implications for topics as diverse as habitat fragmentation, conservation management, and the containment of genetically modified crops. The monoecious African fig tree Ficus sycomorus L. relies on the small, short-lived, night-flying, host-specific fig wasp Ceratosolen arabicus Mayr for pollination. We used microsatellite markers to characterize a geographically isolated riparian population of F. sycomorus growing along the Ugab River in the Namib Desert, Namibia, together with paternity analysis of seedlings from known mothers, to map pollen movement within this population. In this way we tracked insect movements between individually recognizable trees by means of their pollen cargo and documented the movement of C. arabicus between known trees separated by more than 160 km, with a mean distance for confirmed successful pollination events of 88.6 km. The predominant observed movement of pollinators was in a westerly direction, toward the sea, reflecting seasonal nighttime wind direction and the wind-borne dispersal of fig wasps. Our results suggest the existence of an extensive panmictic population of trees that are well suited to overcome the effects of geographical isolation.

Fig. 1.

Distribution of trees within the study site and pollen movements between them. (A) Position of the field site in Namibia, Southern Africa. (B) Distribution of adult trees mapped along the Ugab River. Individuals or clusters of trees are represented by white dots (for map coordinates see Table S1). Paternity assignments relating to individual female parents (red) and assigned pollen donors (blue) as follows: (C) Tree-1 mother, (D) Tree-3 mother, (E) Tree-17 mother, (F) Tree-41 mother. The base map was obtained from www.maps-for-free.com.

Fig. 2.

Distribution of paternity assignments. The observed numbers of progeny assigned (solid bars) and simulated numbers of assignments (± SD) (open bars) under the assumption of panmixia, with mothers and numbers of offspring defined by the observed data and fathers chosen at random from sampled trees up- or down-stream of the mother, with no limitation on distance, in 5-km distance increments for (A) year 2000 data (maternal Tree-1 only) and (B) year 2001 data.