Are Crickets Dispersing Seeds of Parasitic Plants?

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Parasitic plants lead a rather unique lifestyle. Many have foregone photosynthesis entirely by living off fungi or their photosynthetic neighbors. Indeed, there are many anatomical and physiological adaptations that are associated with making a living parasitically. Whether they are full parasites or only partial, one thing that many parasitic plants have in common are tiny, dust-like seeds. Their reduced size and thin seed coats are generally associated with wind dispersal, however, there are always exceptions to the rule. Recent evidence has demonstrated that a handful of parasitic plants have evolved in response to a rather unique seed dispersal agent - camel crickets.

A research team based out of Japan recently published a paper describing a rather intriguing seed dispersal situation involving three species of parasitic plants (Yoania amagiensis - Orchidaceae, Monotropastrum humile - Ericaceae, and Phacellanthus tubiflorus - Orobanchaceae). These are all small, achlorophyllous herbs that either parasitize trees directly through their roots or they parasitize the mycorrhizal fungi associated with said trees. What's more, each of these species are largely inhabitants of the dense, shaded understory of rich forests.

These sorts of habitats don't lend well to wind dispersal. The closed forest canopy and dense understory really limits wind flow. It would appear that these three plant species have found away around this issue. Each of these plants invest in surprisingly fleshy fruits for their parasitic lifestyle. Also, their seeds aren't as dusk-like as many of their relatives. They are actually quite fleshy. This is odd considering the thin margins many parasitic plants live on. Any sort of investment in costly tissues must have considerable benefits for the plants if they are to successfully get their genes into the next generation.

Fleshy fruits like this are usually associated with a form of animal dispersal called endozoochory. Anyone that has ever found seed-laden bird poop understands how this process works. Still, simply getting an animal to eat your seeds isn't necesarly enough for successful dispersal. Seeds must survive their trip through the gut and come out the other end relatively in tact for the process to work. That is where a bit of close observation came into play.

After hours of observation, the team found that the usual frugivorous suspects such as birds and small mammals showed little to no interest in the fruits of these parasites. Beetles were observed munching on the fruits a bit but the real attention was given by a group of stumpy-looking nocturnal insects collectively referred to as camel crickets. Again, eating the fruits is but one step in the process of successful seed dispersal. The real question was whether or not the seeds of these parasites survived their time inside either of these insect groups. To answer this question, the team employed feeding trials.

They compared seed viability by offering up fruits to beetles and crickets both in the field and back in the lab. Whereas both groups of insects readily consumed the fruits and seeds, only the crickets appeared to offer the greatest chances of a seed surviving the process. Beetles never pooped out viable seeds. The strong mandibles of the beetles fatally damaged the seeds. This was not the case for the camel crickets. Instead, these nocturnal insects frequently pooped out tens to hundreds of healthy, viable seeds. Considering the distances the crickets can travel as well as their propensity for enjoying similar habitats as the plants, this stacks up to potentially be quite a beneficial interaction. 

The authors are sure to note that these results do not suggest that camel crickets are the sole seed dispersal agents for these plants. Still, the fact that they are effective at moving large amounts of seeds is tantalizing to say the least. Taken together with other evidence such as the fact that the fruits of these plants often give off a fermented odor, which is known to attract camel crickets, the fleshy nature of their fruits and seeds, and the fact that these plants present ripe seed capsules at or near the soil surface suggests that crickets (and potentially other insects) may very well be important factors in the reproductive ecology of these plants.

Coupled with previous evidence of cricket seed dispersal, it would appear that this sort of relationship between plants and crickets is more widespread than we ever imagined. It is interesting to note that relatives of both the plants in this study and the camel crickets occur in both temperate and tropical habitats around the globe. We very well could be overlooking a considerable component of seed dispersal ecology via crickets. Certainly more work is needed.

Photo Credits: [1]

Further Reading: [1] [2]

Broomrape: What's in a Name?

One can turn a lot of heads by speaking publicly of the plants in the family Orobanchaceae. This interesting and often beautiful parasitic plant family is collectively referred to as the broomrape family. Species with common names like “naked broomrape” and “spiked broomrape” can really make a casual plant conversation turn sour in no time.

Despite how heinous the name sounds, its origin is a bit more innocent. I have really grown to appreciate etymology. Learning the hidden meaning behind the words we utilize for taxonomy can be a lot of fun. It can also teach you a little bit more about the species itself. 

In this context, rape stems from the Latin word “rapum,” which roughly translates to “tuber” or “turnip.” Broom is an English word that, in this context, refers to a shrubby plant related to vetch, which is often parasitized by broomrapes. So, the literal meaning of broomrape is something akin to “broom tuber.” In other words, they are plants growing on the roots of vetch. So, yea, the more you know…

Further Reading: [1]

Nicholas Turland

Nicholas Turland

A Peculiar Parasite at Berkeley

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Parasitic plants are fascinating. I never pass up an opportunity to meet them. On a recent trip to California, my host for the day mentioned that something funny was growing in a patch of ivy on the Berkeley Campus. I had to know what it was. We took a detour from our intended rout and there, growing underneath a pine tree in a dense patch of ivy were these odd purple and brown stalks. This was definitely a parasitic plant.

The plant in question was the ivy broomrape (Orobanche hederae). As both its common and scientific name suggests, it is a parasite on ivy (Hedera spp.). As you can probably guess based on the identity of its host, ivy broomrape is not native to North America. In fact, the population we were looking at is the only known population of this plant you will find in the Americas. How it came to be in that specific location is a bit of a mystery but the proximity to the life sciences building suggests that this introduction might have been intentional. Personally I am quite alright with this introduction as it is parasitizing one of the nastier invasive species on this continent.

The ivy broomrape starts its life as a tiny seed. Upon germination, the tiny embryo sends out a thin thread-like filament that spirals out away from the embryo into the surrounding soils. The filament is looking for the roots of its host. Upon contact with ivy roots, the filament penetrates xylem tissues. The ivy broomrape is now plugged in, receiving all of its water, nutrient, and carbohydrate needs from the ivy. At this point the embryo begins to grow larger, throwing out more and more parasitic roots in the process. These locate more and more ivy roots until the needs of the ivy broomrape are met. Of course, all of this is going on underground.

Only when the ivy broomrape has garnered enough energy to flower will you see this plant. A stalk full of purple tinged, tubular flowers emerges from the ground. At this point its membership in the family Orobanchaceae is readily apparent. Like all members of this family, its parasitic lifestyle is so complete that it is beginning to lose genes for the production of chlorophyll and Rubisco, all things we generally associate with plants. This is why I love parasites so much. Not only are their ecological impacts bewilderingly complex, their evolutionary histories are such a departure from the norm. I will never tire of appreciating such species and I am happy to have met yet another awesome member of this group.

Further Reading:
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1925.tb06671.x/pdf

http://cat.inist.fr/?aModele=afficheN&cpsidt=4107447

Pretty Parasites

Three owl's clovers from western North America - Triphysaria eriantha ssp. rosea, Triphysaria eriantha, and Triphysaria versicolor ssp. versicolor. These lovely plants are hemiparasites belonging to the family Orobanchaceae. This group is thought to be closely related to the genera Castilleja and Orthocarpus.

Photo Credits: Tom Hilton (http://bit.ly/1Ikg9Vy) and Ken-ichi Ueda (http://bit.ly/1CRPxaT)