Photo by Martin LaBar licensed under CC BY-NC 2.0
The animal kingdom is rife with sexual conflict. We are all aware of what is going on when two stag deer lock antlers or when a group of male sage grouse flaunt themselves on leks as females look on. But what about plants? Is there sexual conflict among plant species? Whether pollen ends up on a stigma via wind or animal, is there any way for a plant to "choose" who gets to fertilize the ovule?
It turns out, yes, there is. Sexual competition is part of the pollination process. In fact, some of the most familiar floral morphologies may have evolved as a way of weeding out weak paternal lines. To understand this process better, though, we must first quickly review exactly what goes on during pollination.
Photo by Nick Fedele licensed under CC BY-NC-SA 2.0
Pollen is a male gamete. Each grain is haploid and contains only a single copy of a plant’s chromosomes. When a pollen grain lands on a stigma, the grain germinates like a tiny seed, sending down a root-like growth called a pollen tube. This tube grows down into the ovary until it finds an unfertilized ovule. At this point, sperm travels down the pollen tube where it can unite with the ovule, thus forming a seed.
By CNX OpenStax licensed under CC BY 4.0
It’s the formation of this pollen tube that introduces the idea of competition among pollen grains. Again, whether by wind or animal, the pollen arriving to a new plant generally doesn't come from a single individual. Pollen from many potential paternal lines can arrive all at once. As such, the race to fertilize the ovules can be quite intense, and this is where competition begins.
Remember, pollen only contains a single set of chromosomes from the parent plant, thus all alleles, both functioning and deleterious, are represented. During the growth of the pollen tube, upwards of 60% of the pollen genome is actively transcribed. Any pollen containing lots of deleterious alleles is going to have a much harder time competing with pollen grains that have fewer deleterious alleles. Their tubes have a harder time making it to the ovules in time to fertilize them.
Photo by Dartmouth Electron Microscope Facility, Dartmouth College
It is thought that the length of the style (the stem connecting the stigma to the ovaries) may also provide a sort of "proving ground" for pollen too. For instance, picture the flowers of a lily or a mallow. Those long, slender styles may actually be acting like a race track. Only the pollen with the best selection of genetic material will be able to grow their pollen tubes fast enough to reach the ovules, leaving the weaker competition in the dust. In this way, plants may actually be sorting out stronger paternal lines, which makes sense for sessile organisms that can't see.
As with everything in nature, there is far more nuance to this than what I have outlined above. Much work is being done to test some of the earlier assumptions and data surrounding this concept of pollen competition. It certainly happens but the degree to which any given species utilizes such methods is up for debate. Still, it paints a much more interesting picture of mate selection in plants. Static, plants are not!
Photo Credits: [1] [2] [3] [4]
Further Reading: [1] [2] [3] [4] [5] [6] [7]