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]

How Plants Influence Honeybee Caste System

Is has long been known that food fed to larval honeybees influences their development and therefore their place in the hive. Larvae fed a mixture of pollen and honey, often referred to as "bee bread," develop into sterile workers whereas larvae fed special secretions termed "royal jelly" from nurses within the colony will develop into queens. Despite this knowledge, the mechanisms underpinning such drastic developmental differences have remained a mystery... until now.

A team of researchers from Nanjing University in China have uncovered the secret to honeybee caste systems and it all comes down to the plants themselves. It all has to do with tiny molecules within plants called microRNA. In eukaryotic organsisms, microRNA plays a fundamental role in the regulation of gene expression. In plants, they have considerable effects on flower size and color. In doing so, they can make floral displays more attractive to busy honeybees.

Photo Credit: [1]

Photo Credit: [1]

As bees collect pollen and nectar, they pick up large quantities of these microRNA molecules. Back in the hive, these products are not distributed equally, which influences the amount of microRNA molecules that are fed to developing larvae. The team found that microRNA molecules are much more concentrated in bee bread than they are in royal jelly. Its this difference in concentrations that appears to be at the root of the caste system.

Larvae that were fed bee bread full of microRNA molecules developed smaller bodies and reduced, sterile ovaries. In other words, they developed into the worker class. Alternatively, larvae fed royal jelly, which has much lower concentrations of microRNA, developed along a more "normal" pathway, complete with functioning ovaries and a fuller body size; they developed into queens.

All of this hints at a deep co-evolutionary relationship. The fact that these microRNA molecules not only make plants more attractive to pollinators but also influence the caste system of these insects is quite remarkable. Additionally, this opens up new doors into understanding co-evolutionary dynamics. If horizontal transfer of regulatory molecules between two vastly different kingdoms of life can manifest in such important ecological relationships, there is no telling what more is awaiting discovery. 

Further Reading: [1]