Kjellberg, Finn. et al. Pollination Mode in Fig Wasps: The Predictive Power of Correlated Traits. The Royal Society 10.1098, February 14,2001.

In this paper the authors elaborate on the concept of pollinating and non-pollinating wasps based on the fact that previous research has determined that some of the nearly 700 species of wasp are active pollinators while others are passive pollinators. While the Ficus-Agaonidae obligate mutualism is understood to be one the best studied species-specific, plant-animal mutualistic symbioses, these authors look to determine how the mutualism arrived and is maintained. This is especially important in the association of fig and fig wasps given the belief that the species have coevolved together. They based their study off of the fact that “How these selective conflicts translate into longer-term evolution can be visualized by determining the evolutionary trajectories of coevolved traits” (Kjellberg 1113). In addition, systems that have many different species participating in the same specific mutualism (over 7000 Ficus species and over 200 wasp species) because the evolution of different lineages happen independently of one another allowing for many different trajectories to study. “The Ficus-Agaonidae pollination mutualism is an appropriate model for investigating such trajectories. It is obligate and species specific, and there are over 700 Ficus species which are thought to have co-speciated with the associated agaonid wasps” (Kjellberg 1113). In order to determine what causes active pollination verses passive pollination it was necessary to look at both the lineage of the figs and the wasps.

The foundation for the question comes from the fact that once female agaonid’s enter some wasps demonstrate signs of active deposit of pollen while others do not. This distinction is the primary basis of the difference between active and passive pollination. “Either the wasps have pollen loaded into thoracic structures called pollen pockets and each time they deposit an egg they also discharge some pollen with their forelegs, or they show no such behavior” (Kjellberg 1113). In passive pollination, the pollen is released only as the wasp’s body swells in the water-saturated fig. After the female larvae emerge from the gall there is another distinction between active and passive pollination. “Either they go to the anthers and pick up pollen with their forelegs, deposit it on the ventral part of the thorax and shovel it into the mesothoracic pollen pockets using their fore coxae (active pollination) or they totally lack this behavior (passive pollination) “ (Kjellberg 1113). It is easy to tell which female agaonid is active or passive upon emergence from the fig because active pollinators do not have any physical evidence of pollen with the exception of what is stored in the pockets, while the passive pollinators are covered in pollen. So far, research has shown that from one species of Ficus, only active or passive pollinators will emerge from the fig, not both. The question then shifts to the differences between the fig tree species and how that affects the mode of pollination in being passive or active. “It is widely accepted, though not proven, that actively pollinated Ficus species produce less pollen because wasps born within a fig will only load a limited amount of pollen into their pockets. If this is true then passively pollinated Ficus should produce much more pollen so that emerging insects become covered with it” (Kjellberg 1114). Based on these different characteristics between active and passive pollination both in regards to the wasps and the figs the authors set out to do a survey both by direct observation and review of previous literature to create a set of traits that indicate active verses passive for both the fig and wasp.

Kjellberg et al. relied on direct observation, taxonomic analysis and classification data of both the figs and the wasps. The determinates for the wasp species were based on the appearance of the female wasp when the exited the fig; if she was covered in pollen it was assumed she had participated in passive pollination. The timing was critical for this observation because passive pollinating wasps clean themselves immediately upon exit of the fig. On the other hand, direct evidence for active pollination came from “observation of pollen loading behavior, observation of pockets fileld with pollen, or observation of pollen-deposition behavior” (Kjellberg 1115). In addition, the authors also performed direct observation to look at the traits of the wasps to find correlations between them and the mode of pollination. For instance, when they were able to determine the mode based on the state of the species when they came out of the fig, they also looked at the fore coxae which function in loading pollen into pockets (active pollination). “This allowed us to establish whether the fore coxa bore a comb (a line of setae), whether pollen pockets were present, either fully developed or reduced, and, when no pockets were observed, whether traces of pockets were present” (Kjellberg 1115). In this way they were able to determine if they found there to be a correlation between physiological traits and mode of pollination for the agaonid wasps.

In terms of the fig, the authors looked at the count of flowers as well as the abundance of pollen and anthers. They did this “this in order to test the hypothesis that actively pollinated fig species produce less pollen than passively pollinated species and hence have lower anther-to-ovule ratio” (Kjellberg 1115). To ensure good results they took multiple figs from one tree and then also counted the whole figs, because often one half to the other showed very different ratios. In addition, they took into consideration maturity of the fig and tried to perform counts late in fig development but before wasp emergence. Finally, for “three different subgenera counts on several hundred figs were used to establish the within-species distribution of the anther-to-ovule ratio” (Kjellberg 1115). These counts were used to help determine the active verses passive fig species and create a data set that could be used in comparison to the data collected on the wasps to determine if there were any correlations that contribute to the species-specific symbiotic association between agaonid wasps and Ficus.

CONCLUSIONS!! “Based on direct observation of mode of pollination in 88 species, we show that mode of pollination can confidently be predicted from fig traits only (anther-to-ovule ratio) or from wasp traits only (presence of coxal combs)” (Kjellberg 1113). Furthermore they found that the mode of pollination is not determined by the presence of pollen pockets.

Back to Fig-Wasp Symbiosis.