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When you can’t find something you know should be there, it makes you a little crazy, right? Well, that’s exactly how two groups of researchers must feel. After looking for plastid genomes in different organisms, they can’t find the DNA—it’s gone missing.
Plastids are organelles found in the cells of plants and algae. The Scientist describes chloroplasts—a type of plastid—as “small structures that were once free-living bacteria and retain their own DNA.” Plastids are instrumental in photosynthesis, but also are found in plants that don’t photosynthesize, as in the various parasitic species that are found scattered throughout the plant kingdom. According to Wikipedia, plastids also “possess a double-stranded DNA molecule.” But in the cases below, maybe “should possess” would be a better description.
Filipino researchers looked at plastids in the very stinky Rafflesia lagascae, a parasitic plant that doesn’t photosynthesize, but rather mooches its food off of another plant, a vine in the grape family. Through sequencing Rafflesia, the team discovered that while parts of plastid genes existed, the genome was non-existent. And the scientists believe that many of the fragment plastid genes came from the host plant.
At the same time, a team of Canadian researchers examined four alga species in the Polytomella genus, sequencing their DNA. These algae don’t photosynthesize either, instead living off of food in their freshwater habitat. Again, no plastid genome; in fact, no plastid genes were found at all.
What’s the big deal? According to Rappler.com, “These two studies are challenging some of the basic ideas of plant life.” ScienceShot asks, “When is a plant no longer a plant?”
We asked the Academy’s Peter Fritsch about the non-findings. “Much of the plastid DNA’s genome in normal plants is made up of genes for photosynthesis, but parasitic plants no longer need these genes. Also, the close association of the parasite and host can lead to the loss of key genes in the parasite because they can rely on the host for these functions. For both of these reasons, the plastid DNA’s of parasitic plants are typically much smaller than those in photosynthetic plants and the genes that remain are often highly altered and nonfunctional. This is an example of evolutionary reduction—examples in animals are the degeneration of eyes in many cave-dwelling organisms, and the vestigial hind limbs in whales. In Rafflesia and Polymella, it appears that extreme evolutionary reduction has resulted in the complete elimination of DNA from their plastids.
“As to whether these plants are no longer plants—it might seem as though, because they don’t photosynthesize like all the other plants, they shouldn’t be considered plants anymore. This isn’t really the case though, because they still have many other key features that place them solidly in the plant kingdom. For example, Rafflesia still has flowers with organs homologous to those of other flowers (although highly modified in this case), and regions of their DNA outside of the plastid clearly show their strong phylogenetic relations with other flowering plants, namely those in the spurge family (Euphorbiaceae).”
Image: Steve Cornish