Cheating fig trees


People don’t think plants are sexy. Even biologists don’t think plants are sexy. In my first year as an undergraduate, an enterprising algal biologist asked us at the start of his lecture series how many of us thought plants were interesting, and was met with a deafening silence. By the end of his teaching block, when he repeated the question every hand in the lecture theatre went up.

But plants are sexy: and their sexual reproductive systems are far more variable than you might think. In school we’re taught that flowers have petals and sepals and carpels and stamens. All very nice and dull.

Typical anatomical drawing from EnchantedLearning.com

But in reality there are lots of other cool versions of flowers. There are plants like Anthurium that use a brightly coloured leaf in place of true petals. And plants that have separate male and female flowers, like Euphorbia – in which the flowers are often so reduced that they consist of barely more than the sexual organs.

Left: Anthurium andraeanum
Right: Euphorbia c.f. serrata
Wikipedia

And species of plant where individual plants are male or female. This latter syndrome is called dioecy and I happen to think it’s incredibly cool. When I first graduated I worked as a lab slave on a project about a plant called Mercurialis annua, which is androdioecious – a sort of half way house where there are males and hermaphrodites, but sometimes the hermaphrodites function as females (in that they never use their pollen).

Figs and fig wasps

Ecology Letters is carrying a paper this week about a functionally dioecious species of fig: similarly to Mercurialis above, the ‘male’ trees have female flowers, but when female trees are in flower they do not produce fruit. Figs are quite unusual in that they are only pollinated by a single type of aganoid wasp, which climbs inside the fig synconium (the part we think of as the fruit, though functionally it is not) to lay its eggs. In order to get to the flowers, the lady wasp must squeeze through a tiny hole, often losing her wings in the process. This is a suicide mission. Once she’s dropped her pollen bomb, she lays her eggs in the ovaries of some of the flowers, and then dies. She’s still there when the fig is picked. Every time you eat a fig, you’re eating fig wasp too. Eww. 

A complicated sexual system

Ficus carica has two flowering seasons, in the spring and in the summer. In the summer male and female fig trees open their flowers at the same time, and the wasps that pollinate the female flowers die without producing offspring, because the long style of the flower prevents them from laying their eggs. In the spring only the ‘male’ plants flower, and their short-styled female flowers allow the wasps to reproduce. Because the fig wasps cannot lay eggs in the inflorescences of the female tree, from an evolutionary point of view it makes no sense for them to visit the female plants. The plant has to trick them, by making both male and female flowers smell exactly the same. 

A fig tree in any other season would smell as sweet.

The authors of the study sampled 9 or 10 trees of each type (spring male, summer male, female) and used gas chromatography to figure out what the proportions of around two dozen chemicals were in the scent of the flowers. The expectation is that female figs would smell like male figs, such that pollinators are attracted to them too. The actual results were rather surprising: in the summer, male and female figs emit similar chemical bouquets. But in the early season, the male plants produce a different volatile blend. They also managed to show that this still held true if you removed the chemicals that the fig wasps did not have receptors for. 

One suggestion for why the males change their scent in the spring is that their bouquets are less complicated and variable between individuals in spring. In the summer, the plant ‘wants’ the fig wasp to fly to a different tree, and attempt to lay her eggs. In the spring, the plant needs the wasp to self-pollinate it, and so an uncomplicated odour with a single strong element (linalool) may reduce the chances of the wasp flying to a different tree.

_____________________________________________________

Soler, Catherine CL et al (2012) Evidence for intersexual chemical mimicry in a dioecious plant. Ecology Letters doi: 10.1111/j.1461-0248.2012.01818.x

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