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TOP: Ficus tremula with figs receptive for pollination(left). cross-section of fig with afig wasp pollinatorfemale inside the central cavity (centre), and amagnified view of the pollinator 'ovipositing' down thestyle into the ovary, simultaneously depositing pollenon the stigmas with her forelegs.Illustation: Simon van NoorLABOVE: Underside of the head of the fig wasp pollinator of Ficus bubu, Gourtella michaloudi showingmandibles and mandibular appendages bearingmany rows of backward pointing teeth that she usesto facilitate her entry into the fig through the ostiole(a tiny bract-lined opening at the apex of the fig).Photo: Simon van NoorL
The ultimate challenge
How fig treesare pollinatedby Simon van Noort, Iziko South African Museum
Pollinators of plants can often be predicted, based on flower colour, fragrance andshape. White, fragrant flowers with a long corolla tube, for example, are usuallypollinated by hawk moths, which are nocturnal and have a long tongue. Fig trees,
however, are unique. Their flowers are completely concealed within the fig withhundreds of tiny florets lining the inside of a central cavity. Attempting to guesswhat pollinates figs and how the act is carried out would no doubt lead to the conclusion that the pollinator, as for many plant species, must be highly specialized.
In fact, fig trees are completely dependent on tiny wasps, a couple of millimeterslong, for their propagation and survival. These fig wasps are the sole pollinators offig trees and in turn, fig wasps can breed nowhere else but inside figs, a relationship that is a classic example of an 'obligate mutualism' where neither party can
survive without the other, and which has evolved over ninety million years.Each species of fig tree is usually pollinated by one fig wasp species that is only
associated with that fig species, a host-specific relationship that plays a major rolein the prevention of hybridization between different species of fig trees. However,only 300 of a potential 750 fig wasp pollinator species are currently known (there
are 750 species of Ficus in the world), and already we know that the one-to-onerule between fig tree and wasp does not always hold, so the interaction is not aswell defined as initially appears. What the exceptions do tell us, however, is thatwe are dealing with a continually evolving dynamic system, the intricacies of which
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we are only just beginning to understand.
How then do these tiny wasps thatonly live for a few days manage to perform their amazing task of finding andpollinating flowers that are hiddeninside the fig? Female fig wasps, leaving the fig they have bred in, need to flyoff in search of another fig tree to continue the reproductive cycle. This isoften a long and arduous journey thatonly a few out of thousands managesuccessfully. This remarkable feat isachieved by homing in on host treespecific 'volaWes' (a chemical signalreleased by the fig when it is receptivefor pollination).
Completion of this journey is the firsttest of endurance, as once the fig wasphas located a receptive fig, she needs tocircumvent the next barrier. The onlylink the fig cavity has to the outsideworld is through a tiny bract-linedopening at the apex of the fig, called theostiole, and it is by means of this passage that the pollinating fig wasp gainsaccess to the florets. Negotiating theostiole is no easy task, with the femalewasp having to squeeze and labour herway between the tightly closed bracts.She is, however, remarkably adapted todo so. Her body, in particular her headand thorax, is extremely flattened andelongate. She also has row upon row ofbackward pointing teeth on hermandibular appendage, situated on theunderside of her head, as well as a few
RIGHT: The fig wasp Courtellawardi, pollinator of Ficus tremu/a,the female at top and male below.Illustration: Simon van Noor\.FAR RIGHT: Afig wasp,Ceratoso/en capensis, femaleemerging from her natal gall withinthe cavity of Ficus sur.Photo: Simon van Noor\.
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strong teeth on her legs. These teethassist her progress through the ostioleand also prevent her slipping backwards. Nevertheless, the process ofgaining access to the fig cavity is so difficult that her wings and antennaeusually break off in the ostiole, but thisfortunately does not influence her pollinating or egg-laying ability.
The female wasp then proceeds to layeggs in the ovules of some of the florets,and to pollinate the stigmas at thesame time. This she does by insertingher long ovipositor down the inside ofthe style. The florets that have styleslonger than the wasp's ovipositor arepollinated, but no eggs reach the ovuleand hence these florets set seed. Thewasp larvae feed on the endosperm tissue in the galled ovary (where the eggswere laid) and the larvae take anythingfrom three to twenty weeks to develop,depending on the season.
Once the wasps have reached maturity they chew their way out of the gallsand emerge into the fig cavity at muchthe same time. The wingless malesmate with the females before chewing ahole through the fig wall to the exteriorto allow the females to escape - themale's only two functions in life, as hedies soon afterwards! The femaleseither actively load up pollen from ripeanthers into special pollen pockets, orin some species, they passively becomecovered with pollen, before emergingfrom the fig in search of young recep-
tive figs to complete the cycle.Once the female fig wasps have left
the fig, it ripens, changing colour andsmell, and becomes attractive to seedor fruit eating birds, bats, monkeysand even lizards. Fig trees are considered to be keystone species in manytropical and subtropical ecosystems,because of their all year round production of figs, providing food in seasonswhen other fruiting trees do not. Fruiteating animals help to disperse theseeds.
Besides the pollinators there is asuite of non-pollinating fig wasps thatalso breed in figs, but which play norole in the pollination process. Fromthe fig's perspective they are unwantedinterlopers. In Africa, as many as thirty species of non-pollinating fig waspcan be associated with a particular figtree species, as in Ficus thonningii, butmore commonly three to flfteen speciesare associated with each fig species.One group of non-pollinators also enterthe fig to lay eggs, and exhibit parallelmorphological adaptations to those ofthe pollinators for getting through theostiole.
Most of the non-pollinators, however,lay eggs by inserting their ovipositorthrough the fig wall from the outside ofthe fig. These wasps often haveextremely long external ovipositor'tails', the length of which has beenevolutionarily determined by the thickness of the fruit wall of their host fig
TOP: Females of the non-pollinating fig wasp, Apocrypta guineensis, laying eggs through the fig wall of Ficus sur.ABOVE LEFT: Three non-pollinating fig wasps. Philocaenus (top) enters the fig to lay eggs, Sycoryctes (centre) has a long ovipositor for laying eggs through the fig wall,and Otitesella (below) also lays eggs through the fig wall, but the ovipositor is concealed within the body.ABOVE RIGHT: Philocaenus rotundus (a non-pollinating fig wasp that enters the fig) laying eggs inside a receptive Ficus abutilifolia fig that has been split open.Photos and illustrations: Simon van Noort.
species. Fig size varies tremendouslyacross species, from smaller than amarble to as large as a tennis ball.
Given the short life span of the adultpollinating fig wasps and the lack ofsynchrony of fig production, how thenis the relationship maintained? Usuallyon emergence the female wasps have toleave the tree they bred on, because thefigs in a particular crop are normally allat the same stage of development.Some species, however, have figs at different stages of development within thesame crop and in these cases the
female wasp has only to fly a short distance to locate a receptive fig.
The cycling of the mutualism can,therefore, continue on the same tree.More commonly the cycling phenomenon relies on the presence of a suitablysized population of trees of a particularspecies in a given area, so that somewhere within the population there will
be a tree with figs that are receptive forpollination. The lack of synchrony in figcrop production between trees is anessential trait to ensure this. If all thetrees in a population produced figs at
the same time, the fig wasp populationwould die out.
Local extinctions of wasps do occur,when they are not able to locate a treewith figs in the receptive phase, andthere will also be abortions of fig cropsthat pollinators have not managed tolocate.
Nevertheless, in the larger picturethis is a very successful interaction,which is borne out by the high diversity of both fig trees and fig wasps.
For more about figs and fig wasps,visit the website at www.figweb.org.
Further readingBurrows, J. & Burrows, S. (2003). The figs of southern & south-central Africa. Umdaus, Pretoria. Includes a chapter on the interaction of fig wasps and figs by the authorof this article. For more further reading material, contact the Editor at [email protected].
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