Flat-headed Fig Invaders from Outer Space!

March 25, 2014

Photo credit: Kelsey Bailey

Photo credit: Kelsey Bailey Disclaimer: To our knowledge fig wasps are not really from outer space, they just look like miniature aliens. To understand the tiny wasps in the family Agaonidae, you must first understand their inverted-flower “spaceships of reproduction”: figs. A fig, although it masquerades as a simple fruit, is actually an inside-out inflorescence (cluster of flowers). This inflorescence, once pollinated, becomes an infructescence (cluster of fruits) that contains the fig tree’s seeds. Pollinating this “calzone of the flower world” is no easy task: enter the fig wasp. These flat-headed wonders of fig pollination measure out at a slender two millimeters in length, and have an obligate mutualism with fig trees —meaning the wasps and trees cannot live without the other. The ultimate example of a “pollination syndrome”, where a flower’s shape, size, coloration and other traits have evolved to facilitate pollination by a particular taxon, a fig can only be pollinated by this single family of miniature wasps. Despite their highly specialized co-evolution, the pollination of a fig tree by its wasp friends is not a cakewalk. Pollination is done exclusively by female wasps. These lovely ladies leave the figs of their youth to seek out new figs in which to lay their eggs. They hone in on the scent of a new tree, which is often species-specific (one species of wasp paired with one species of tree), and once they have it located, they must enter the fig to reach the flowers on the inside. This is a task of perseverance: the fig has only one small opening, called an ostiole, where the wasp can make her entrance. This small tunnel to the inner paradise of the inflorescence is lined with highly-modified leaves, called bracts. The narrow tunnel and its lining make entering the fig a tight, tortuous squeeze. Female fig wasps have a flattened shape to facilitate squeezing through this opening. They also have special teeth on the bottoms of their heads and on their legs to help them wiggle through. Often, these hard working women lose their wings and antennae in their struggle to gain entrance to the inner sanctuary of the fig. Once inside, however, their plush floral surroundings are the perfect site for egg deposition. As the wasp moves about laying eggs into the ovules of the flowers, she deposits pollen that she picked up in the fig of her birth. Any ovule that does not receive an egg, but does receive pollen, becomes a seed for the fig tree’s reproduction. The eggs mature inside the ovules of the fig’s inner flowers, creating galls. Once the larval wasps hatch, they feed off of the inner tissue of the flower until they are ready to emerge. Males and females emerge simultaneously, and mate inside the fig. Males are wingless and have two functions: mate and chew through the fig to create an escape for females. They die shortly thereafter. The females use the tunnels made by their mates to escape and go find a fig “spaceship” of their own. We know you are now wondering; am I eating fig wasps when I eat figs? The answer is: possibly, but they would be a different type of fig wasp than this alien-looking flat headed type. This particular genus of fig wasp, Pleistodontes, is a genus native to Australia that came over with ornamental fig trees. Because these wasps pollinate ornamental figs, they won't be found in commercial varieties in the store. Other fig wasps may be in commercial varieties, but different varieties are produced differently. There are fig trees that have been bred to produce figs without pollination, in which case wasps would not have an association with those breeds. There are also both monoecious and dioecious types of fig trees. In monoecious varieties, you will end up with figs that contain both seeds and wasps, whereas in dioecious varieties the male trees produce figs with mostly wasps (due to short styles that allow females to lay eggs in the ovules easily) and female trees produce mostly seeds (due to long styles that prevent egg implantation but allow pollination). If a fig comes from a male plant and is full of galls, you would certainly notice it. In female plants that have been pollinated (but not laid in) there may have been one, or several, female wasps that entered but never left. Luckily, figs produce enzymes that can break down any adult wasps that perish inside the developing infructescence. That “crunch” when you chew a fig? Rest assured those are seeds, which are considered a desirable trait in figs. When the fig was first being grown in California, we did not have the fig wasp here and our figs did not have those wonderfully crunchy seeds. It took a few years, but scientists finally figured out the problem — and we've been happily munching on crunchy figs ever since, all thanks to the introduced fig wasp.

(Posted by: Emily Hartop )

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