Jump to content Jump to navigation

Resource allocation in fig wasps

Fig trees belong to a large and ecologically diverse genus of at least 750 species occurring globally in tropical forests.

Supervisors
Aafke Oldenbeuving, J.C. Biesmeijer, J.J.M. van Alphen

Contact
aafke.oldenbeuving@naturalis.nl

Period & duration
Starting dates are flexible, this project involves 1 month preparation (learning techniques, reading relevant literature and making a project description), 2 months of field work (STRI/Panama), 3 months of lab work (STRI/Panama) and 1 month to write a report.
(Note that funding applications will have to precede this project.)

Study & level
MSc-students with an interest in (tropical) ecology and evolution.

Background & context
Fig trees (genus Ficus) belong to a large and ecologically diverse genus of at least 750 species occurring globally in tropical forests (Berg 1989) . Every fig species is associated with one or two host-specific pollinators: fig wasps (figure 2). Fig wasps are instrumental in pollinating figs and depend on figs for their own reproduction.

A fig is not a fruit in the classical sense, but an inflorescence with the flowers inside a sphere. Adult female fig wasps enter receptive figs through an opening at the apex of the inflorescence, bringing pollen with them from the fig they have emerged. They then pollinate a number of flowers within the fig and oviposit in other flowers, inducing gall formation in these flowers, the galls serving as food for their offspring. Most fig species have synchronized development of  inflorescences and synchronized maturation of figs. Emerging fig wasps, therefore, cannot reproduce on the same tree, but have to find a different tree of the same species with receptive infloresences. Such receptive trees are often at a great distance from the tree that produces adult wasps. Evidence exists that fig wasps disperse over distances from 15 to 160 km.

station on Barro Colorado Island

Figure 1: The field station on Barro Colorado Island, Panama

Background & context
Fig trees (genus Ficus) belong to a large and ecologically diverse genus of at least 750 species occurring globally in tropical forests (Berg 1989). Every fig species is associated with one or two host-specific pollinators: fig wasps (figure 2). Fig wasps are instrumental in pollinating figs and depend on figs for their own reproduction.
A fig is not a fruit in the classical sense, but an inflorescence with the flowers inside a sphere. Adult female fig wasps enter receptive figs through an opening at the apex of the inflorescence, bringing pollen with them from the fig they have emerged. They then pollinate a number of flowers within the fig and oviposit in other flowers, inducing gall formation in these flowers, the galls serving as food for their offspring. Most fig species have synchronized development of  inflorescences and synchronized maturation of figs. Emerging fig wasps, therefore, cannot reproduce on the same tree, but have to find a different tree of the same species with receptive infloresences. Such receptive trees are often at a great distance from the tree that produces adult wasps. Evidence exists that fig wasps disperse over distances from 15 to 160 km.

 

Pollinating fig wasp

Figure 2: Pollinating fig wasp entering a fig through the ostiole (photo Christian Ziegler).

Collecting fig wasps

Figure 3: Collecting fig wasps with sticky traps in the field

Fig wasps develop on a fixed amount of food, provided by the gall in which they are born. Thus, young adult females have a limited amount of lipid resources, that could either be allocated to egg production or to body maintenance and fuel for flying. Wasp that allocate all the lipids into egg production would have no time and energy to find a tree with receptive trees. Wasps that would allocate all the lipids into the maintenance of their body and into flight fuel would maximize the probability of arriving at a tree with receptive fruits, but will have no eggs to lay. The optimum allocation of lipids will be between these extremes and depends on the expected flight distance to a receptive tree (figure 2). In a poor habitat, where these distances are long, optimal lifespan should be longer, and hence, lipid reserves should be larger than in a richer habitat.

Objectives & goals
We can study our hypothesis with fig wasps, by ranking the different species of fig in a habitat according their abundance and measure egg loads and lipid reserves in freshly emerged females. The prediction is then that fig wasps from the commoner species would have lower lipid reserves than females from the rarer species of fig. Moreover, the hypothesis predicts that wasps arriving on a receptive host tree have used a large proportion of their lipid reserves. 

Methods, tasks & approach
Field site - This project will be carried out in Panama at the Smithsonian Tropical Research Institute. On and around their field station on Barro Colorado Island, some 20 species of figs can be found, which allows us to compare fecundities and lipid reserves at eclosure and amount of lipids burnt when arriving on a host tree of a similar number of species of fig wasps and see how these relate to ecological traits (e.g. single or multiple foundresses, fig size, host tree abundance etc.).

Field work - By collecting figs from which adults are about to emerge, we can obtain females that have their full lipid reserves. By placing yellow sticky traps on receptive host trees, we can obtain females wasps arriving on a receptive host tree (figure 3).

We can measure the remaining lipid reserves and compare the values with those of freshly emerged females to obtain information on how much fat wasps, on average, need to use to arrive on a host tree and on the variance between wasps.

Laboratory studies - Fecundities of wasps can be measured by removing the ovaries from adult wasps and separate the eggs in a droplet of Ringers’ solution. Eggs can be counted from a photograph of the complete egg load. For measuring lipid contents, we will either use the method described by Ellers (Ellers 1996) or the one described by Giron et al. (Giron, Rivero et al. 2002).

Requirements
MSc-student applying for this project will need to provide their own funding for their stay at STRI (room and board / lab fees / registration costs) and travel costs to and from Panama. If you are studying in the Netherlands, you will be eligible for several Dutch funds and for a STRI short-term fellowship (which is competitive).

Besides, there will be other research projects on the same study system (e.g. behavioral and chemical studies) that can be carried out during the same time period. Students therefore can jointly apply for these projects so that they can easily collaborate during their research project.

Last but not least, doing an introductory course of Spanish is recommendable.

References
BERG, C.C., 1989. Classification and Distribution of Ficus. Experientia, 45(7), pp. 605-611.

ELLERS, J., 1996. Fat and eggs: An alternative method to measure the trade-off between survival and reproduction in insect parasitoids. Netherlands Journal of Zoology, 46(3-4), pp. 227-235.

GIRON, D., RIVERO, A., MANDON, N., DARROUZET, E. and CASAS, J., 2002. The physiology of host feeding in parasitic wasps: implications for survival. Functional Ecology, 16(6), pp. 750-757.

 

The optimum number of eggs

Figure 4: The optimum number of eggs and the optimum life span varies between habitats