Reproductive coevolution in ficus

The Ficus genus is composed of 800 species of vines, shrubs, and trees, defined by their syconiums, the fruit-like vessels that either hold female flowers or pollen on the inside. In addition to being cultivated by humans for thousands of years, Ficus is also known for their reproductive mutualism with the fig wasp.

Fig trees either produce hermaphrodite fruit known as caprifigs, or female figs; only the female figs are palatable to humans. In exchange for a safe place for their eggs and larvae, fig wasps help pollinate the ficus by crawling inside the tiny hole in the apex of the fig, called the ostiole, without knowing whether they crawled into a caprifig or a fig. If the female wasp crawls into the caprifig, she can successfully lay her eggs and die. The males hatch first, mate with the females, dig tunnels out of the caprifig, and die. The females, now covered in fig pollen from the caprifig, fly out to begin the cycle again. If the female wasp crawls into a female fig, she will not be able to successfully lay her eggs despite pollinating the fig with pollen from the caprifig she hatched in. The fig will absorb her body and her eggs as the fruit develops.

Aristotle noted in Historia animalium that wild figs contain psenes that begin as grubs, but whose skin splits allowing the psen to fly out. The psen flies into a cultivated fig, and stops it from falling. He noted further that Greek farmers planted wild figs next to cultivated figs, and tied wild fig fruits on to the cultivated trees.

Overall, the coevolution of ficus and wasps can be explained through the specificity of ficus as a host and the tendency of the wasp species to frequent hosts different to that of their original Ficus plant, thus increasing the likelihood of hybrids and eventually new species.

Ficus typically control the reproductive mutualism with fig wasps by being highly selective in their choice of pollinator. Contributing to the evolution of Ficus, pollinator specificity in Ficus is a pre-zygotic mechanism of reproductive isolation among plant species in diverse angiosperm lineages with varying degrees of sympatry and of gene flow in hybrid zones. This is proven by the high host specificity and low pollinator sharing ratios found in fig pollinators, especially in the genera Ceratosolen and Kradibia. Morphological examples of such specificity includes the diameters of ostioles compared to the head widths of the respective wasp species. While the fig species F. wassa have a diameter of 1.0-1.5 mm, its pollinator K. wassae had an average head width of 0.58 mm.

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