More examples of breakdown the 1:1 partner specificity between figs and fig wasps

Abstract

Background: The obligate mutualism between fig trees (Ficus, Moraceae) and pollinating fig wasps (Agaonidae) is a model system for studying co-evolution due to its perceived extreme specificity, but recent studies have reported a number of examples of trees pollinated by more than one fig wasp or sharing pollinators with other trees. This will make the potential of pollen flow between species and hybridization more likely though only few fig hybrids in nature have been found. We reared pollinator fig wasps from figs of 13 Chinese fig tree species and established their identity using genetic methods in order to investigate the extent to which they were supporting more than one species of pollinator (co-pollinator).

Results: Our results showed (1) pollinator sharing was frequent among closely-related dioecious species (where pollinator offspring and seeds develop on different trees); (2) that where two pollinator species were developing in figs of one host species there was usually one fig wasp with prominent rate than the other. An exception was F. triloba, where its two pollinators were equally abundant; (3) the extent of co-pollinator within one fig species is related to the dispersal ability of them which is stronger in dioecious figs, especially in small species.

Conclusions: Our results gave more examples to the breakdown of extreme specificity, which suggest that host expansion events where pollinators reproduce in figs other than those of their usual hosts are not uncommon among fig wasps associated with dioecious hosts. Because closely related trees typically have closely related pollinators that have a very similar appearance, the extent of pollinator-sharing has probably been underestimated. Any pollinators that enter female figs carrying heterospecific pollen could potentially generate hybrid seed, and the extent of hybridization and its significance may also have been underestimated.

Keywords: Agaonidae; Co-speciation; Ficus; Fig wasps; Host specificity; Hybrids.

Fig. 1

COI ML phylogenetic tree of the pollinators of the sympatric figs, with sequences of two non pollinators (Pteromalidae) as outgroups. Node support rates are shown. Haplotypes and sampling sites are also listed together with their host figs

Fig. 2

Cumulative distribution of Kimura pairwise genetic distances (K2P) for COI of pollinators associated with Ficus species. Intraspecific distance ranged between 0–1.3 % and interspecific distances ranged from 3.3–29.2 %. A marked barcoding gap separated the within- and between-putative species pairwise distances

Fig. 3

The proportion of different pollinator species in each Ficus species. The columns with red marks show the same major pollinator species in F. hirta and F. triloba; while those with blue marks show the same major pollinator species of F. erecta var. beecheyana, F. pyriformis and F. variolosa. The number on each column represent the sample size of each fig species within the site with more than pollinator species, the exception is F. hirta and F. hispida which are the total number of two sites each with only one same pollinator species