The population genetic structure of Ficus craterostoma in South Africa

Abstract

In the eastern part of its distribution, Ficus craterostoma occurs in Afromontane forests whereas it also occurs in low-lying scarp and Indian Ocean coastal belt forests in South Africa. Ficus craterostoma must have dispersed to these low-lying forests from the Afromontane forests, even though forests became highly fragmented during the Pleistocene. To understand how these ancient changes have impacted the distribution and population structure of F. craterostoma we quantified the genetic variation in its slow-evolving chloroplast DNA with limited dispersal ability via seeds, and its highly variable nuclear microsatellites that reflect exceptional pollen flow. The chloroplast variation was highly structured and frequently monomorphic in nearby forests while the nuclear variation showed little structure and isolation by distance. From these data we reach several conclusions. Ficus craterostoma may have become extinct from South Africa's northern Afromontane forests during the Pleistocene. These forests were possibly subsequently recolonized from southern forests that may have been scarp or Afromontane in nature. Additionally, there was one scarp and one Indian Ocean coastal belt forest refugium, both of which were very isolated and small. Nuclear gene flow caused by pollen flow is very effective along the western part of the South African population, knitting together Afromontane and scarp forest fragments, dispersed over 1000 km, into one genetic population. Conversely, the Indian Ocean coastal belt forest refugium appears to have been isolated in terms of gene flow, but more recent gene flow with two nearby inland forests may have started to homogenize their genetic variation. Due to the unusual pollination system of fig trees, other forest tree species may display very different dynamics.

HIGHLIGHTS • In South Africa Ficus craterostoma inhabits Afromontane, scarp and coastal forests. • Genetic data suggest that F. craterostoma was affected profoundly by the ice ages. • Large F. craterostoma populations survived in southern scarp forests. • This tree may have recolonized Afromontane forests from the south. • Coastal forest populations of F. craterostoma are isolated and unique.