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 Fern With Flower Genes - An Odd Case of Horizontal Gene Transfer

When researchers at Harvard decided to take a look at the genome of the rattlesnake fern (Botrypus virginianum) they found something completely unexpected. Whereas one set of genes they looked at placed this species firmly in the family to which it belongs, Ophioglossaceae, two other genes placed it in the Loranthaceae, a completely unrelated family of flowering plants. What are flowering plant genes doing in a fern?

The rattlesnake fern is a ubiquitous species found throughout the northern hemisphere. It is believed to have evolved in Asia and then radiated outward using ancient land bridges that once connected the continents. At some point before this radiation occurred, the rattlesnake fern picked up some genes that were entirely foreign.

Horizontal gene transfer, the transfer of genes from one organism to another without reproduction, is nothing new in nature. Bacteria do it all the time. Even plants dabble in it every now and then. The surprising thing about this recently documented case is that it is the first discovery of horizontal gene transfer between an angiosperm and a fern. Up until this point, examples within the plant realm have been seen between ferns and hornworts as well as some parasitic plants and their hosts.

This is why the rattlesnake fern genome is so interesting. How did this occur? Though there is no way of telling for sure, researchers believe that one of two things could have happened. The first involves root parasitism. The family Loranthaceae is home to the mistletoes, a group of plants most famous for their parasitic nature. Although the majority of mistletoe species are stem parasites, at least three genera utilize root parasitism. It could be that an ancient species of mistletoe transferred some genes while parasitizing a rattlesnake fern.

This scenario seems to be the least likely of the two as no representatives of the root parasitic mistletoes currently exist in Asia, though it is entirely possible that some did at one time. The other possibility doesn't involve parasitism at all but rather fungi. Rattlesnake ferns are obligate mycotrophs and thus cannot live without certain species of mycorrhizal fungi. Perhaps the transfer of genes was achieved indirectly via a shared mycorrhizal network. This hypothesis is especially tantalizing because if it is found to be true, it would help explain many other examples of horizontal gene transfer that currently lack a mechanism. Only time and more research will tell.

Photo Credit: Aaron Carlson (http://bit.ly/1OAVhNZ)

Further Reading:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1560187/