When I was in the second year of my undergrad, I took a class on comparative plant physiology. For some reason, I found this to be one of the more boring classes ever (until I had to sit through comparative animal physiology....zzzzzzzzzzz). While studying for a final, one of my roommates told me that I needed to think of it like a soap opera. Not 30 seconds later, I'm in my room reading about how in angiosperms (flowering plants), the pollen tube (the male reproductive organ) swells and bursts during reproduction.
And that is when I realized that plant sex can be quite raunchy.
Like I mentioned in my last plant post, many people do not actually think of plants as having these crazy, complex lives. But they do, and they compete for mates just like animals do. A new study published a few days ago in the New Phytologist (which I believe is open access), found that the pollen of milkweed actually use weapons to get an advantage over competitors.
It was previously believed that the only type of sexual selection plants like milkweed could participate in was just sending out massive amounts of pollen, in the hopes that one of those grains would be the first to reach the stigma - a part of the female reproductive organ, it looks like this:
Pollen is generally captured by the stigma either through the air or by pollinators. Milkweed pollen develops in a pollen sac (technical term pollinium), which attach to the feet or mouthparts of pollinators, like bees, wasps, and butterflies, who then unknowingly contribute to a no-holds-barred sexual competition between plants. Two pollinia are attached to form a complex (which is confusingly called a pollinarium). There are five pollinaria per milkweed plant. The pollinators pick up one of these complexes, then go off to another flower where they pick up another complex, and so on, and these pollinaria attach to each other like a chain, which is called concatenation, and it looks like this:
Basically what this diagram is showing you is that once the pollinaria attach, it leaves some pollen sacs more likely to leave their pollen on a stigma than the other pollen sacs. This study looked at sexual selection in milkweed mediated through physical male interference, like where one pollen sac is located in the chain, and through the effect of physical traits, like the horn-like extensions, that contribute to one pollen sac maintaining its coveted position.
Alright, so the authors, led by Andrea Cocucci, studied four species of milkweed. They analyzed pollen donation efficiency, and they found that there may be a competition for a "coveted spot" in the chain, but the chance of pollination is pretty similar no matter where the pollen sac is. This position may only give a slight advantage, but not enough for it to be a sexually selected competitive trait. Then, the authors looked at these horn-like extensions, called caudicle horns, that prevented concatenation in ancestral species.
(horns) (no horns)
The horns on the pollinarium make it hard for the pollinaria of other species to attach in a chain. That pretty much gives that plant a selective advantage over another plant, whose pollinaria allow for concatenation.
Plants are not typically known to use these types of "weapons" and interference strategies in reproduction the way animals do. This shows that there is a confrontation process in milkweed plants during reproduction that involves a direct physical interaction, and traits like these horns are an adaptive trait likely evolved from male-male competition in plant reproduction! This is the first-ever report of an active competitive interaction among pollen before landing on the stigma, and the authors go as far as comparing these horns to those of bucks, who violently compete for mates:
This study also shows that sexual selection is possible even without the ability to move around, see, and smell. The only thing that's needed for sexual selection is proximity and physical contact.
Note: I grabbed most of these figures from the research paper. If you're going to use them, please cite the original paper
If you're interested in learning about animal sexual selection, I'd suggest checking out Dr. Tatiana's Sex Advice to All Creation. This was one of my "textbooks" for my biology of sex class in second year, super funny and informative!