I recently had the pleasure of talking to an extrodinarily bright high school student who, among other things, taught me about yellow dodder. It's one of
the coolest pieces of little-known science I've heard about all year, so I thought I'd do a bit more research on my own.
What is Dodder?
Dodder, or Cuscuta, is a genus of flowering plant, but if you saw one growing in the wild you might not realize that. Dodders have no roots or leaves, and they're
not even green! They look more like a bowl of alien spaghetti than any kind of flower you're probably used to. Still, even without the ability to photosynthesize,
these plants have no problem feeding themselves. The reason? They're parasites.
Dodders wind their stems around unexpecting host plants and tap into their vascular systems using special organs called haustoria. Haustoria are similar to roots,
but instead of taking water and nutrients from the soil, they siphon sugars, vitamins, and minerals directly from the xylem and phloem
of other plants (Kaiser et al, 2015).
Among angiosperms, we think that
haustoria evolved twelve independent times. Over 4,700 species use these strange organs
(Nickrent, 2020).
That's about 1% of all flowering plants (Sun et al, 2018)! So what makes dodder so special?
Well, in 2018 yellow dodder became the first species of parasitic plant to have its full genome sequenced
(Vogel et al, 2018), and the findings were... weird.
One of the many strange things about dodder's genome is that it lacks over 1,000 of the most highly conserved genes in plants. I.e., genes that are considered to
be the most fundamental for life. Specifically, the missing genes seem related to root and leaf development, nutrient uptake, photosynthesis, and immunity.
While losing these types of genes would be catastrophic for free-living plants, parasitic dodder does just fine without the ability to produce its own food or
grow the organs necessary to do so. Additionally, because most plants become infected with pathogens through their roots and leaves, dodder is not especially at
risk for infection. This means its immune system can be a little weaker without causing any massive negative side-effects
(Sun et al, 2018).
Even though actively losing genes may sound counter-productive, it's actually a pretty
common process. We call it "regressive evolution," and among other things, it's how whales lost their legs, and humans lost our tails. It can happen when natural selection
is no longer acting on a trait, and allows organisms to save their energy for more useful purposes.
Because parasites often rely on their hosts for a lot of the fundamental functions of life, regressive evolution is pretty common amongst biotic freeloaders- plants and
animals alike (Sun et al, 2018).
What's far more interesting about this plucky little parasite is that we think it can steal genes from its hosts, and incorporate them into its own DNA. The reasoning
behind this is twofold:
Firstly, Yellow dodder contains 64 genes that seem like they come from dozens of other species. The genes have strikingly similar sequences
to ones that we find in plants from entirely different taxonomic families and orders. Even if the dodders and these other species all inherited the genes from a
common ancestor, we would expect the sequences to be a lot more dissimilar due to the accumulation of random mutations over time
Secondly, the plants that the genes come from are the ones that dodder parasitizes the most.
 |
| Electron microscopy image of conjugation pilus |
This is an example of horizontal gene transfer (HGT): a transmission of genetic material between organisms that doesn't involve a parent passing genes to their offspring.
HGT is fairly common in bacteria and archaea. These single-celled organisms can directly exchange DNA by forming an anatomical structure called a conjugation pilus (picture).
Viruses that infect bacteria, called bacteriophages, can also pass genetic material from one host to the next. However, the finding that HGT was occurring rampantly
between dodder and its hosts was a bit of a shock, since scientists used to think that horizontal gene transfer between two large, multicellular organisms was extremely
rare. But that idea is being challenged as more and more plant genomes get sequenced with better and better technology. In fact, newer studies are starting to suggest
that HGT between plants may be a lot more ordinary than we'd ever imagined (El Baidouri et al, 2014) and
(Kado and Innan, 2018).
At this point, very little is understood about exactly why or how dodder is stealing genes from its hosts. Though nothing has been definitively proven, researchers
hypothesize that horizontal gene transfer may
aid evolution in two ways. The first is that taking in a host's genes may help an invader camoflauge itself from the immune system. If the parasite
is making the host's own genes, they won't be detectable as a foreign elements. The second idea is that if a parasite steals its host's defense genes,
it could give it an edge while fighting off the host's immune system: essentially, fighting fire with fire.
Still, our understanding of how this is occurring is even more barren. One possibility is that simply being in close proximity makes it easier for dodders to
become infected with the same pathogens as their hosts, as we know viruses and bacteria can move DNA around. Another idea is that DNA-containing mitochondria in a plant's
cells may have the ability to move through haustoria. Experiments have shown that when two separate plant species are grafted together,
mitochondria can move from one plant to the other through cell-to-cell connections (Gurdon et al, 2016).
All in all, we're a long way from being able to prove any of these ideas, and a lot more research needs to be done before we really understand these weird little plants
and their even weirder genomes.
