Sex ratio strategies in species subject to local mate competition (LMC), and in particular their fit to quantitative theoretical predictions, provide insight into constraints upon adaptation. Pollinating fig wasps are widely used in such studies because their ecology resembles theory assumptions, but the cues used by foundresses to assess potential LMC have not previously been determined. We show that Liporrhopalum tentacularis females (foundresses) use their clutch size as a cue. First, we make use of species ecology (foundresses lay multiple clutches, with second clutches smaller than first) to show that increases in sex ratio in multi-foundress figs occur only when foundresses are oviposition site limited, i.e. that there is no direct response to foundress density. Second, we introduce a novel technique to quantify foundress oviposition sequences and show, consistent with the theoretical predictions concerning clutch size-only strategies, that they produce mainly male offspring at the start of bouts, followed by mostly females interspersed by a few males. We then discuss the implications of our findings for our understanding of the limits of the ability of natural selection to produce 'perfect' organisms, and for our understanding of when different cue use patterns evolve.
Fig trees (Ficus) and their species‐specific pollinators (Agaonidae) represent a remarkable example of a coevolved mutualism. A number of non‐pollinating fig wasps (gallers and parasitoids, NPFW) are also an integral part of the mutualism, but have a negative impact on the reproductive success of the mutualists. Most NPFW belong to subfamilies only associated with figs and clearly have a long association with the plants and their pollinators.
In the present study, the costs imposed by an undescribed parasitoid Sycoscapter sp. on its host pollinator Kradibia (= Liporrhopalum) tentacularis of a dioecious fig tree Ficus montana maintained under glasshouse conditions are described.
It was asked whether pollinator numbers and sex ratios are changed by the presence or absence of parasitoids within individual figs. The effect of fig densities on parasitism rates at two spatial scales and within the general glasshouse population was also recorded. Parasitoid aggregation in relation to pollinator densities inside figs was also examined.
Sycoscapter sp. significantly reduced the numbers of pollinators emerging from the figs, but host sex ratios were not distorted. The parasitoid showed host density independence at both spatial scales of fig densities, but targeted individual figs that contained higher initial densities of pollinators.
1. Figs on male dioecious fig trees (Ficus, Moraceae) are breeding sites for pollinator fig wasps (Hymenoptera, Agaonidae), but figs on female plants are traps that produce only seeds. As the short-lived fig wasps cannot reproduce in female figs, natural selection should favour individuals that avoid them. Several studies have failed to detect such discrimination, a result attributed to inter-sexual mimicry and 'selection to rush' in the wasps, but their experiments failed to explicitly take into account fig age (how long they had been waiting to be pollinated). 2. We compared the relative attraction of male and female figs of known ages of the South East Asian Ficus montana Burm. f. to its pollinator Liporrhopalum tentacularis Grandi and examined how the reproductive success of the plant and its pollinator change with the age of the figs. 3. Mean retention time for un-pollinated figs on female plants was 16 days whereas in male figs it was 12 days. Female figs remained attractive for up to 2 weeks, although the wasps were less willing to enter older figs. After pollinator entry, receptivity continued for several days, lasting longer in figs entered by a single wasp. Consistent with abortion rates, attractiveness persisted longer in female figs. Older figs produced fewer fig wasp offspring, but similar numbers of seeds. 4. The sexual differences in floral longevity in F. montana may represent part of a previously un-recognised reproductive strategy in some fig trees that allows male plants to 'export' pollinators while also maintaining a resident fig wasp population.
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