Shazia Raja

The mechanism of sex ratio adjustment in a pollinating fig wasp

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.

Only pollinator fig wasps have males that collaborate to release their females from figs of an Asian fig tree

Male insects rarely collaborate with each other, but pollinator fig wasps (Hymenoptera: Agaonidae) are said to be an exception. Immature fig wasps feed on galled ovules located inside figs, the inflorescences of Ficus species (Moraceae). After mating, adult pollinator males chew communal exit-holes that allow mated females (which are often also their siblings) to escape. Figs also support non-pollinating fig wasps (NPFWs), some of which produce exit-holes independently. We determined whether collaboration between pollinator males (Kradibia tentacularis from Ficus montana) was necessary for the release of their females, and used the relationship between male numbers and likelihood of success to measure the extent of cooperation during exit-hole production. These attributes were then compared with those of an NPFW (Sycoscapter sp.) from the same host plant. Pollinators were more abundant than NPFW, but their more female-biased sex ratio meant male pollinator densities were only slightly higher. Individual males of both species could produce an exit-hole. Single males of the NPFW were just as successful as single male pollinators, but only male pollinators cooperated effectively, becoming more successful as their numbers increased. The lack of cooperation among NPFW may be linked to their earlier period of intense inter-male aggression.

A comparison of growth and reproduction, under laboratory conditions, of males and females of a dioecious fig tree

The interaction between Ficus spp. (Moraceae) and their pollinating wasps (Chalcidoidae: Agaonidae) is a highly co-evolved mutualism. Approximately half of all fig species are monoecious and produce a mixture of wasps and seeds within the same fig. In functionally dioecious fig trees male and female functions are separate. Figs on male trees produce wasps and pollen, whereas figs borne on female plants produce only seeds. Dioecious fig phenology provides an excellent opportunity to investigate the effect of sexual specialization on the obligate fig–fig wasp interaction and the non-pollinators associated with the system. Here we describe laboratory studies of phenological variation between two sexes in terms of vegetative growth and fig production in a dioecious fig tree Ficus montana. We also describe reproductive output in terms of wasp production in males and seeds in females. Intrasexual asynchrony was observed for the plants, with synchrony between the sexes with year-round production of figs. Male plants grew more rapidly, but leaf phenology was very similar. Crop sizes and development times were the same for males and females. Seasonal effects were strong for leaf phenology and fig initiation, but had a very limited effect on fig composition. The results show that the phenological differences described for other dioecious figs do not apply to all species.

Parasitism of a pollinator fig wasp: mortalities are higher in figs with more pollinators, but are not related to local densities of figs

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.

Sexual differences in the attractiveness of figs to pollinators: females stay attractive for longer

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).

Do fig wasps produce mixed paternity clutches

Pollinating fig wasps (Hymenoptera, Agaonidae) have been the focus of numerous studies examining sex ratio evolution. Recently, molecular genetic techniques have been introduced that assume single matings in fig wasps, yet their mating biology has not been investigated genetically. We used recently developed microsatellite markers to investigate whether a pollinating fig wasp (Liporrhopalum tentacularis Grandi) produces single or mixed paternity clutches. The clutches of 12 females which had had the opportunity to mate with males of different genotypes were investigated. The results suggest that, at least in this species of fig wasp, single paternity clutches are the norm. Based on our behavioural observations, this appears to be due to mating with a single male rather than sperm competition.

Putting your eggs in several baskets: oviposition in a wasp that walks between several figs

The interaction between figs (Ficus spp., Moraceae) and their pollinator fig wasps (Hymenoptera: Agaonidae) is an obligate mutualism, but females of dioecious fig trees exploit fig wasps without providing rewards. Figs are closed inflorescences that typically trap pollinator females after entry, but some fig wasp species can re‐emerge (although wingless) and subsequently oviposit in and pollinate further figs. Using glasshouse populations, we examined the sex ratios and clutches laid by single foundresses of Kradibia tentacularis (Grandi) in their first and subsequent male figs of Ficus montana Blume, and how the probability of emergence and entering a second fig varied between seasons. A maximum of four figs were entered by any one foundress. Wingless foundresses were able to locate and enter figs up to 60 cm from the first fig they entered, but the probability of entry declined sharply with distance from that fig. The foundresses that re‐emerged produced slightly higher adult offspring totals than those that failed to re‐emerge. Clutch sizes of a single foundress in its first fig equalled those in all the subsequent figs combined, with clutch size per fig decreasing when more figs were entered. Smaller clutches had less female‐biased sex ratios. Figs were more numerous in summer than in winter, but the proportion of figs entered by only wingless foundresses remained unchanged. Movement between figs increases pollinator reproductive success in male figs, thereby encouraging foundresses that encounter a female tree to also move between and pollinate several female figs.

Interactions between pollinator and non‐pollinator fig wasps: correlations between their numbers can be misleading

Ficus and their species–specific pollinator fig wasps represent an obligate plant–insect mutualism, but figs also support a community of non‐pollinating fig wasps (NPFWs) that consist of phytophages and parasitoids or inquilines. We studied interactions between Kradibia tentacularis, the pollinator of a dioecious fig tree species Ficus montana, and an undescribed NPFW Sycoscapter sp. Members of Sycoscapter sp. oviposited 2–4 weeks after pollinator oviposition, when host larvae were present in the figs. No negative correlation was found between the numbers of the two wasp species emerging from figs in a semi‐natural population. However, in experiments where the numbers of pollinator foundresses entering a fig were controlled, Sycoscapter sp. significantly reduced the numbers of pollinator offspring. Consequently, it can be concluded that Sycoscapter sp. is a parasitoid of K. tentacularis (which may also feed on plant tissue). Sycoscapter females concentrate their oviposition in figs that contain more potential hosts, rendering invalid conclusions based on simple correlations of host and natural enemy numbers.

Floral constraint resulting from intersexual mimicry in a gynodioecious fig tree

Fig trees (Ficus: Moraceae) are pollinated by female fig wasps (Agaonidae) whose larvae develop inside galled flowers of unusual inflorescences (figs). Most fig trees also support communities of non‐pollinating fig wasps. Figs of different species display great size variation and contain tens to tens of thousands of flowers. Around one‐half the species of fig trees have the gynodioecious breeding system, where female trees have figs that produce seeds and male trees have figs that support development of pollinators. Mutual mimicry between receptive male and female figs ensures that pollinators enter female figs, even though the insects will die without reproducing, but the need to give no sex‐specific cues to the pollinators may constrain differences in size between receptive male and female figs. We compared relationships between inflorescence size and some measures of reproductive success in male and female figs of Ficus montana grown under controlled conditions in the presence of the pollinator Kradibia tentacularis and its main parasitoid Sycoscapter sp. indesc. Female figs that contained more flowers produced more seeds, but male figs did not increase the production of female pollinator K. tentacularis fig wasps in proportion of the flower number. Although more flowers were galled by the pollinators in male figs containing more female flowers, the high larval mortality caused by parasitism and nutritional limitation prevented the increase in the production of adult female offspring. Selection may favor the increase in flower numbers within figs in female plants of F. montana, but contrarily constrain this attribute in male plants.

Foundress Fig Wasps are More Likely to Re-emerge From Older Figs

The mutualistic interaction between Ficus spp. and their pollinating fig wasps (Agaonidae) centres on the plants’ unique inflorescences—their figs. Each Ficus species is pollinated by foundresses of host-specific fig wasps which enter figs to lay eggs in the female flowers. Most foundresses are trapped in the first figs they enter, but in some species wingless foundresses can re-emerge and subsequently enter and oviposit into further figs. We investigated whether number of potential oviposition sites, age of the fig and age of the wasp influence the likelihood of re-emergence of lone foundresses of the Asian fig wasp Kradibia (=Liporrhopalum) tentacularis from previously un-entered figs of Ficus montana. Likelihood of re-emergence was not influenced by wasp age or flower numbers (resource abundance), but was more frequent from older figs that had waited longer to be pollinated. Laying eggs in several figs offers clear advantages, but foundresses often failed to re-emerge despite being unable to lay all their eggs. Resource quality not quantity appears to be the main influence on the fig wasp’s oviposition decisions. The physical difficulty that the wasps experience when trying to re-emerge may prevent it, even when re-emergence would be advantageous for both the insect and its host plant, but older fig wasps were not detectably ‘weaker’ than younger individuals.