Fig Research

Figs (Ficus sp., Moraceae) and their pollinating wasps (Agaonidae, Chalcidoidea, Hymenoptera) constitute one of the most extraordinary pollination mutualisms known. The fig depends completely on its pollinating wasp for being pollinated and the wasps on the fig for completing its life cycle. In addition to the pollinating wasps, a diverse community of non-pollinating wasps also exploits figs. Individual host fig species may harbor many different species of non-pollinating wasps, and related host figs may possess very similar assemblages of these species. Further, species-specific parasitic nematodes (Parasitodiplogaster sp., Diplogasteridae) associated with the pollinating wasps are also found in the system.

We are interested in understanding the causes of fig species diversity

The nearly 750 described species of figs (Ficus sp.) occur worldwide in tropical and subtropical regions, and are considered “keystone” species in tropical forests due to their year-round production of fruit essential to a large number of frugivores. At the phenotypic level figs are an extremely diverse group of plants, and allozyme studies have shown that allele diversity and overall heterozygosity in Neotropical figs (sect. Urostigma) are among the highest reported in flowering plants. Such high genetic diversity could be simply the result of large historical effective population sizes. However, field observations on the figs and genetic data on the pollinators suggest a different but not mutually exclusive explanation: it is possible that Neotropical figs are not “good” biological species, and that gene flow between species is prevalent and plays a fundamental role in the generation of the tremendous diversity of what we recognize as different fig species. That interpretation arises from the observation of several cases of potential hybrid phenotypes in nature, the fact that pollinating wasps occasionally enter and successfully reproduce in the “wrong” host fig species (often resulting in viable hybrid seeds), and genetic data we have obtained that shows that there are several cases of more than one species of wasp pollinating the same species of fig.

To address this hypothesis we have developed a battery of sequencing markers based on sequences from cDNA clones obtained from cDNA libraries of two Neotropical fig species (F. citrifolia, F. popenoei) and one fig pollinator species (Pegocapus estherae) that were constructed in our lab. 2000 clones were sequenced and the sequences have been used to find variable markers that are being used in population genetic studies of species divergence in Neotropical fig species. These mini-genomic resources are being used to collect phylogenetic and population genetic data to test the long-held idea of strict-sense cospeciation between closely related species of figs and their pollinators, and to test our hypothesis that gene introgression in figs due to pollinator host switches or host sharing has been the main mechanism involved in the generation of fig species diversity.

Our initial phylogenetic and population genetic data has been published (Machado et al. 2005) and more data is being collected for a larger study. In the future we want to develop additional markers using our cDNA database, and to increase the geographic sampling of the study (currently confined to the Panama Canal area).

All contents copyright © 2005 Carlos Machado. All rights reserved.