In 1862, when Charles Darwin was finishing up an exhaustive study of orchids, he noted an odd problem. Almost all the orchids he saw were pollinated by insects. Each orchid produced a little nectar to lure those insects. And in return, those insects unwittingly performed a service for the orchid—exchanging its pollen with other orchids—thus completing the plant’s reproductive cycle. But Darwin noticed that one orchid species in Madagascar, Angraecum sesquipedale, kept its nectar at the bottom of an extremely long spur, 30 centimeters in length. What insect could possibly reach it? Certainly none that Darwin had ever seen.
Darwin couldn’t see any reason for the plant to produce nectar other than to lure pollinators, so he predicted that a moth with an extremely long proboscis must exist. In 1903, long after Darwin’s death, such a moth was found. Indeed, its proboscis is so long that the moth must first approach the flower to determine by smell if it’s the correct species, then back up and unroll the proboscis, carefully threading it down the spur to reach the tiny amount of nectar at the orchid’s base. The flower’s stamens are positioned such that while the moth is feeding, the pollen rubs off on its head for distribution to the next flower.
Symbiotic relationships such as this are common in nature: Each organism gives a little and gets a little in return. The orchid benefits because it need not waste energy producing much nectar; only one insect can reach it. The moth benefits because it doesn’t have to compete with other species for its food, a reasonable tradeoff for its unwieldy proboscis.
Two weeks ago at the Science Online 2010 conference, I met Jeremy Yoder, one of two graduate student bloggers who were awarded a NESCent fellowship to attend the event. Yoder normally studies the relationship between Joshua trees and their pollinator moths, but on his blog last week, he discussed new research about a different symbiosis, one between plants and nitrogen-fixing bacteria in their roots.
Life as we know it requires nitrogen to produce amino acids and proteins, but the abundant nitrogen in the earth’s atmosphere is non-reactive and inaccessible to most organisms. Just as you can’t get the iron your body needs by eating nails, you can’t get the nitrogen you need from the air you breathe. Humans get nitrogen by eating plants and animals, but even plants can’t process atmospheric nitrogen by themselves.
Fortunately, some plants have developed a symbiotic relationship with bacteria that grow in nodules in their roots. The bacteria have the ability to produce ammonia from nitrogen gas, using energy provided by their plant-hosts, in a process called nitrogen fixation. The plants get nitrogen in a usable form, and the bacteria get food. But how did this relationship start? A French team led by Marta Marchetti has found a clever way to replicate one part of the process, published this month in PLoS Biology. Marchetti’s group transferred a gene for nitrogen fixation into a pathogenic bacterial species. The pathogen could now fix nitrogen, but when it infected a plant, it didn’t produce the nodules like benign nitrogen-fixers, and a true symbiotic relationship wasn’t established.
So the researchers took one more step, modifying the pathogen to be more prone to mutation, and thus to variation. As the bacteria reproduced and mutated, eventually three strains developed that were capable of producing nodules, but were genetically distinct from the original source of the nitrogen-fixation gene. This result shows that many different bacteria can become partners with plants—even pathogens that previously were unable to fix nitrogen—to produce the biologically useful nitrogen that all forms of life require.
An even more bizarre symbiotic relationship is described by the anonymous biochemistry student and blogger “Lab Rat”: A 2006 study published in Protist found that while the bacterium Hatena arenicola doesn’t have the ability to photosynthesize, it occasionally enters into a symbiotic relationship with another organism, nephroselmis, which can. The two organisms merge, with nephroselmis providing food for both. But when Hatena replicates by dividing in half, nephroselmis only stays with one of the two new cells. The Hatena deprived of its photosynthetic partner must fend for itself, scavenging for food unless it happens upon another nephroselmis, in which case, the cycle begins anew.
Not all stages of this cycle have been observed, but Lab Rat says this may be evidence of how plants evolved. Initially, chloroplasts—the photosynthesizing components of plant cells—were probably independent organisms. At some point in the distant past, other cells may have subsumed them like Hatena, and eventually, at some later stage, the chloroplasts themselves were able to replicate along with the parent organism, with no need to live independently from their host.
Symbiosis is nearly ubiquitous in nature. Most organisms can’t exist in isolation; they need other species to survive. New research continues to uncover novel relationships between species, and using a customized search, you can follow along on ResearchBlogging.org. Try searching for symbiosis and symbiont to learn more.
Originally published January 27, 2010