Pollination is the trademark of flowering plants, with animal pollinators like bees and birds maintaining the world’s food supplies, not to mention our cravings for coffee, honey and macadamia nuts. But new research raises the possibility that animal-assisted pollination originated in the sea, long before plants moved to land.
The study, conducted by research groups based in France and Chile, is the first to document a species of algae that depends on tiny marine crustaceans spotted in pollen-like spores to reproduce.
Because the red alga Gracilaria gracilis evolved long before land plants appeared, the researchers say their study shows that animal-assisted pollination may have emerged about 650 million years ago in the oceans once a suitable pollinator.
On land in flowering plants that bear seeds and gymnosperms, male reproductive cells or gametes, fly in the form of pollen grains, which are carried by wind, through water or uncovered insects, to land with a female counterpart somewhere far away. .
Scientists then discovered that mosses (a type of rootless, flowerless plant classified as bryophytes) and some fungi also use animals and insects to facilitate reproduction, changing what they knew about animal-mediated pollination.
Although often debated, researchers thought it had originated in concert with land plants about 140 million years ago, or at least during the Mesozoic, which dates back to about 252 million years ago.
Just a few years ago, scientists discovered marine invertebrates carrying seagrass sperm, blowing out the long-held theory that the oceans were devoid of pollinators.
Now, this new study by Emma Lavaut, a graduate student in evolutionary biology at the Sorbonne University in Paris, and her colleagues, describes how tiny crustaceans called isopods, Idotea balthica, help fertilize a species of algae red, G. gracilis, which evolved around 1,000 million. years ago, long before the 500 million years in which land plants appeared.
“The study by Lavaut et al. has expanded both the range and history of animal-mediated male gamete transfer, taking the concept of pollination from [land] plants to algae and potentially bring them back to the earliest evolution of marine invertebrates,” write Jeff Ollerton and Zong-Xin Ren, two ecologists at the Kunming Institute of Botany of the Chinese Academy of Sciences, in a perspective that accompanies the paper in Science.
A type of photosynthesizing algae, seaweeds are only distantly related to so-called true plants.
G. gracilis also differs from most other seaweeds in that its male gametes lack a flagellum to propel them through the water, left adrift in the ocean, unless they can hook a ridge in a passing creature, as this new work suggests they often do.
In a series of laboratory experiments, Lavaut and his colleagues showed how small marine isopods, which feed along threads of male G. gracilis, inadvertently pick up the alga’s male gametes (spermatia ) as they do so, transferring them to the female plants.
You can see in the image below, an idothea decorated with fluorescently stained spermatia, suggesting that the crustaceans may serve as pollinators.
An appendage of idothea covered with spermatia. (Sebastien Colin, Max Planck Institute of Biology/CNRS/SU).
“Our results demonstrate for the first time that biotic interactions dramatically increase the probability of fertilization in an alga,” write Lavaut and colleagues.
Fertilization success was about 20-fold greater in the presence of I. balthica that without creatures, the team found.
But they have not yet compared this crustacean pollination with the dispersal of pollen along water currents to find out which one plays a bigger role.
The origins of plants using animal pollinators also remain open, given that the researchers only inferred this from the evolutionary history of the animals involved.
Lavaut and his colleagues think the alga provides habitat, shelter and abundant food for grazing herons. In return, the tiny crustaceans not only help G. gracilis reproduce, but their appetite for the parasite-like plants that colonize G. gracilis fronds actually increases the algae’s growth rates, the researchers found. researchers
Idotea balthica, perched on a frond of red algae. (Wilfried Thomas, CNRS/SU).
However, in a world of rapid human-caused climate change, these delicate mutualistic relationships between plants or algae and animals are as much threatened as the ecosystems they support.
Seaweeds such as G. gracilis depend on calm coastal waters to reproduce, when coastlines are battered by storms and sea levels slowly rise inland. Meanwhile, ocean acidification may weaken the exoskeletons of crustaceans, although this remains to be studied in isopods.
Although the threat of global warming is abundantly clear, evolutionary-minded ecologists are still puzzled as to what G. gracilis was doing before I. balthica appeared on the scene, since isopods are not as old as algae, since they evolve a “simple” 300 million years ago.
Although they likely relied solely on ocean currents, “how these algae reproduced before that is a mystery,” Ollerton and Ren explain.
If there’s one thing science has taught us, it’s that we should always prepare for more surprises. Recent estimates by Ollerton suggest that only a tenth of the more than 300,000 known species of flowering plants pollinated by animals have had their pollinators documented.
So which spices work their magic? “Undoubtedly many more revelations await the careful observer of species interactions,” conclude Ollerton and Ren.
The study was published in Science.