Sex in the Rain

No, parents, you don't need to cover your children's eyes, and yes, this post is safe for work. This is strictly science. I just couldn't resist having a little pun, er, fun with the title, not after I found out about some new research on pollination by raindrops.

It's old news that some plants depend on wind or water rather than animal pollinators like insects to make baby plants. Ferns, mosses, conifers, and other plants from more ancient lineages rely on these abiotic (non-living) vectors for reproduction. Among the angiosperms, or flowering plants—the evolutionary newcomers to the plant kingdom—only about 20% get down and dirty without any help from animals.

But even though water pollination can and often does occur, ombrophily, or pollination by raindrops*, has never been conclusively documented. Oh, there've been a few researchers who've claimed to have seen it with their very eyes in the field, but most of those studies have since been discounted.

*The authors of the study I'm focusing on today define ombrophily as "pollination by rain splashes," but it actually has a broader meaning: namely, the ability of some species to thrive in very rainy conditions. (Ombro = rain, phily = loving). For the purposes of this post, though, let's just roll with the pollination-specific definition.

Even among plants that relish a constant state of drizzle, rain can inhibit pollination by damaging fragile flowers, making pollen less viable (less able to germinate), or simply by deterring pollinators. Many plants have evolved to bloom in warm, dry periods to avoid those sorts of problems.

Some angiosperms, though, bloom exactly when you wouldn't expect them to: during the rainy season. Acampe rigida, a south China orchid that's the hero of today's study, puts out cute, fragrant blossoms like many orchids, but it does so during the late summer rains when animal pollinators (especially insects) tend to hunker down instead of bustling from one flower to the next. WTF? It's also a "deceptive orchid," meaning its pretty little flowers offer no nectar, no reward for insects that take the trouble to stop by for a visit. Deceitful species like this tend to have rather low rates of fruit-set (fruit-set is an indicator of successful pollination).

A. rigida inflorescence (basically, a stem with flowers). From Figure 1 in Fan et al.

Yet despite its apparently dismal sense of timing, A. rigida boasts a relatively high rate of fruit-set. A research team led by Jiang-Yun Gao set out of figure out why.

These guys ran a good number of different observations and field and lab experiments—it ended up being a pretty well controlled study, in my opinion—to test if the flowers were 1) self-compatible (more on that in a minute) and 2) rain-pollinated. The dreariest part, without a doubt, was the 132 hours they collectively spent spying on select flowers to see if they were visited by insects. In all that time, just one single piddly little wasp stopped by one of the orchids. The next time you have a particularly boring day at work, just think: you could be sitting in the rain in China, staring at a flower and hoping against hope that some bug will chance to pay it a visit and break the awful monotony of your life.

Other aspects of the study were more involved. The scientists hand-pollinated dozens of flowers, both by self-pollination (fertilizing a flower with its own pollen) and cross-pollination (taking pollen from one flower and using it to fertilize another). The purpose of this was to see if A. rigida is self-compatible, or able to produce normal (viable) seeds with its own pollen. Many flowers are self-incompatible, meaning they can't produce viable seeds if a pollen grain from the anther (male part of the flower) lands on the flower's own stigma (female part).

Self-incompatibility and cross-pollination lead to genetic diversity among a plant's offspring, making it more likely for a few to survive environmental changes or disease. But self-compatible plants take the risks associated with breeding with oneself because hey, they can fulfill their biological imperative to go forth and multiply even if they never encounter another flower of their species.

Anyway, these rain-drenched botanists also found ways to protect the orchids from the rain. In some experiments, they bagged flowers on multiple plants to shield them from both rain and insect pollinators. They also hung clear plastic sheeting a foot or so above groups of flowers. The plastic stopped raindrops from falling on the plants but allowed insects to freely come and go; runoff was sufficient to keep the soil moist. In all experiments, they kept tabs on control patches where orchids weren't interfered with in any way.

Back at the lab, the team ran tests on seeds collected from the hand-pollinated plants to determine their viability (whether the seeds were normal and could grow into normal orchids). They also—this is my favorite part—used a showerhead to create synthetic rain. I'm not entirely sure what was the point of that particular experiment… why mimic rain when you've got real showers and drizzles and downpours and storms happening right outside? Maybe one of the scientists wanted a new showerhead at home and needed a good excuse to order one of the lab's dime.

The results of all this grunt work provided pretty conclusive support for the hypothesis that "rain-assisted self-pollination contributes significantly to maintaining relatively high levels of fruit-set in natural populations of A. rigida."

The orchid flowers are self-compatible, it turns out; the scientists saw no difference in viability between their hand-pollinated self- vs. cross-pollinated test plants.

More evidence: Neither pollen nor floral structures were damaged by falling rain. Only 3% of rain-sheltered plants set fruit, compared to 23% of exposed plants. And most tellingly, the flowers they enclosed in plastic bags didn't set any fruit. In other words, even though this species theoretically can produce perfectly normal seeds all on its lonesome, it doesn't self-pollinate. It needs some form of outside intervention. And those 132 miserable hours of rain-sodden observation showed that bugs very, very rarely stop by.

The final piece of evidence? It rains on these flowers. No, seriously. "We video recorded pollination by rain in the field and this is available at the XTBG website." I've embedded the video below.

I don't speak Chinese, but they seem to be exclaiming every time a raindrop makes a direct hit on a flower. Yes, we scientists get excited about strange things sometimes.

What's actually going on (i.e. what they could've filmed if they'd spent their research money on a nice high-speed camera instead of a showerhead) is actually kinda cool. Here's a close-up of an A. rigida blossom:

Close-up and extreme close-up of A. rigida. From Fig. 1 in Fan et al.

Do you see a face? Because I see a face. One with googly eyes and a bifurcated nose.

When a raindrop splashes down on the flower, it knocks the anther cap out of the way (the anther cap serves to protect the underlying pollen… it doesn't seem to be shown in the figure above). The yellow googly eyes—which are actually pollinia, or sticky clumps of pollen grains—bounce up but are immediately yanked backward by the stipe (indicated by "St." in the figure). The pollinia ("P") plop down into the stigmatic cavity ("S"), landing on the stigma, or female part of the flower. And that, kids, is how a baby A. rigida is born.

What's really curious here is the fact that this flower goes through the trouble of ombrophily (raindrop pollination) at all. Why would a self-compatible flower depend on falling rain to pollinate itself? Wouldn't completely independent self-pollination make more sense?

What Gao's team conjectures is that A. rigida saves ombrophily as a last resort. It's genetically preferable for the plant to get cross-pollinated by an insect. Increased genetic diversity, greater chance of offspring survival, all that jazz.

But if an animal pollinator doesn't chance to stop by—and per their observations, those visits are pretty rare—it rains almost every day in this region. Insect visitors may choose to stay chaste and dry, but the flowers can always fall back on a quick fling in the rain for some precipitation-induced pollination.


Source: Fan, Xu-Li et al. Rain pollination provides reproductive assurance in a deceptive orchid. Annals of Botany. First published online July 31, 2012. doi:10.1093/aob/mcs165