Are woody seeder plants more prone than resprouter to population genetic differentiation in Mediterranean-type ecosystems?

Abstract

High diversification of woody seeder lineages is characteristic of the south-western cape floristic region (CFR), South Africa, which has been explained as a consequence of its mild Mediterranean climate and reliable winter rainfall. Such climatic regime reduces the risk of post-fire recruitment failure, acting as an ecological filter that favours seeder populations, thus promoting genetic differentiation and diversification in seeder populations, as previously seen in the South African heath Erica coccinea. To explore this hypothesis further, genetic population structure was investigated in two Mediterranean Erica species, one seeder (Erica umbellata) and the other resprouter (Erica australis), using nuclear microsatellites. These two species are endemic to the western Mediterranean Basin and co-occur in heathlands of the Strait of Gibraltar region. Mean annual rainfall in this region is similar to that from the south-western CFR, but summer stress is more marked and winter rainfall is much less reliable. Contrary to what was found in E. coccinea, average genetic diversity levels were considerably lower in seeder populations (E. umbellata), regardless of an apparently higher gene flow among them. No differences in genetic differentiation among populations were found between the two species. The occurrence of less favourable climatic conditions for post-fire recruitment in the western Mediterranean compared to the south-western CFR may affect seeder populations more strongly than resprouter and may thus account for lower levels of within-population genetic diversity in the seeder E. umbellata. In addition, putatively higher migration rates in the seeder E. umbellata, may contribute to reduce its potential for genetic differentiation. This study provides evidence that high divergence of seeder populations is not a general rule in fire-prone, Mediterranean-type ecosystems.