Dodder: Parasite & Gene Thief Extraordinaire

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Apparently dodder (Cuscuta spp.) steals more than just water and nutrients from their hosts. They also steal genetic material. The movement of genetic material from the genome of one organism into the genome of another is called ‘horizontal gene transfer’ and it is surprisingly common in nature. Microbes like bacteria do it all the time and more and more we are finding examples in more complex organisms like plants (here and here). For plants, there is little evidence that the acquired genes serve many, if any, functions. This is not the case for dodder. It appears that many of the foreign genes within the dodder genome are being utilized.

Dodder are obligate parasites. They produce no chlorophyl nor any roots. Instead, they tap into their hosts vascular tissues via specialized structures on their stems called haustoria. It may be the intimacy of this parasitic connection that facilitates such high rates of gene transfer. Regardless of how they got there, the amount of genetic foreign material in the dodder genome is shocking. What’s more, much of it is functional.

Researchers have identified over 100 genes that have been added to the dodder genome via horizontal gene transfer. These genes comes from a wide variety of host lineages, including representatives from the orders Malpighiales, Caryophyllales, Fabales, Malvales, Rosales, and Brassicales. Interestingly, between 16 and 20 of these genes are thought to have been retained from the common ancestor of all living dodder species, which suggests that horizontal gene transfer occurred early on in the evolution of these parasites.

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Amazingly, the function of many of these genes appear to have been co-opted by dodder for use in their own biology. Not only were many of these genes complete copies, they were being actively transcribed by the dodder genome and are therefore functional. These include genes being used for the development of houstoria, genes being used for defense responses, and genes being used for amino acid metabolism. Researchers also found an instance of a gene that codes for micro RNAs. The micro RNAs are actually sent back into the host plant and may play a role in silencing host defense genes, allowing dodder to be a more successful parasite.

The plants themselves may not be able to select which genes get transferred. Indeed, some 42 regions of the stolen genome appear to have no function at all. Still, natural selection appears to be acting on newly acquired genes, incorporating those that serve a useful function and silencing the rest. We still don’t know exactly how this process unfolds over time, nor if gene transfer from host to parasite is largely a one-way street. Still, the evidence suggests that horizontal gene transfer is an important process in parasitic plant species and may contribute to their success through evolutionary time.

Photo Credits: [1] [2]

Further Reading: [1] [2]

A Dose of Dodder

Strangleweed, Devil's gut, witches shoelaces... all of these colorful nicknames have been given to a genus of plants that have evolved a very different way of survival. Dodder, genus Cuscuta, are a group of roughly 170 species of plants that make their living entirely off of other plants. We have talked about parasitic plants in the past but this group takes it to the extreme.

Dodder begin their lives just like any other plants. Seeds in the soil germinate under a certain set of conditions and begin their trek into the sunlight. However, unlike many other plants who spend a lot of their initial energy setting up root systems, dodder only sends sends out meager baby roots. It says "forget that" and starts searching for a victim. It whips about in a circular motion like a cowboy's lasso. It is looking for the nearest host. If dodder doesn't find a suitable host within 7 to 10 days it will wither and die. How does it find a host? Many theories have been put forth on the subject. From blind luck to changes in light levels, no one could seem to get a firm grasp on exactly how dodder knew where to go. Then, in 2006, a team of researchers discovered that dodder sniffs out its victims.

By honing in on green leaf volatiles, dodder sniffs out its potential prey. Even more interesting is that some species of dodder seem to show preferences. Tomatoes were a big hit with the species that were tested and indeed, many farmers will agree that dodder is a pretty serious agricultural pest. Once a host is located, dodder begins to wind around the stem. Its diminished root system completely dies off. It then uses specialized cells called haustoria to tap into the host's vascular tissue.

This is not so good for the host as it can severely weaken it, leaving it susceptible to viruses and other diseases. To ad insult to injury, these diseases can then be passed to other plants that the dodder has tapped in to. As far as we know, only one species of dodder undergoes any measurable level of photosynthesis. The rest are solely dependent on their host. That being said, there is evidence to support the idea that dodder actually increases plant diversity where it grows. By limiting the strength of dominant plants, dodder allows other, less competitive species to gain a root-hold in that habitat.

So, where does a plant like dodder fit into the evolutionary tree? It is quite a strange plant after all. Originally dodder was placed in its own family but recent genetic work has since changed all that. The genus Cuscuta is now considered to be a member of the morning glory family, Convolvulaceae! Regardless of how you feel about parasites, you really have to respect these plants.

Further Reading:
http://www.npr.org/templates/story/story.php?storyId=6160709

http://www.jstor.org/stable/4043367?seq=1#page_scan_tab_contents

http://www.cabdirect.org/abstracts/19500301574.html;jsessionid=94D7FF07A81A44560C60AFB44AC295B6;jsessionid=528A1AE7148BEA3A563B9F95419BC043;jsessionid=500949B7792FD68E5FA2E32A8E7AD291?freeview=true