From Herbivore to Pollinator Thanks to a Parasitoid

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In the Atlantic forests of Brazil resides a small orchid known scientifically as Dichaea cogniauxiana. Like most plant species, this orchid experiences plenty of pressure from herbivores. It faces rather intense pressures from two species of weevil in the genus Montella. These weevils are new to science and have yet been given full species status. What's more, they don't appear to eat the leaves of D. cogniauxiana. Instead, female weevils lay eggs in the developing fruits and the larvae hatch out and consume the seeds within. In other words, they treat the fruits like a nursery chamber.

This is where this relationship gets interesting. You see, the weevils themselves appear to take matters into their own hands. Instead of waiting to find already pollinated orchids, an event that can be exceedingly rare in the dense Amazonian forests, these weevils go about pollinating the orchids themselves. Females have been observed picking up orchid pollinia and depositing the pollen onto the stigmas. In this way, they ensure that there will be developing fruits in which they can raise their young.

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Left unchecked, the weevil larvae readily consume all of the developing seeds within the pod, an unfortunate blow to the reproductive efforts of this tiny orchid. However, the situation changes when parasitoid wasps enter the mix. The wasps are also looking for a place to rear their young but the wasp larvae do not eat orchid seeds. Instead, the wasps must find juicy weevil larvae in which to lay their eggs. When the wasp larvae hatch out, they eat the weevil larvae from the inside out and this is where things get really interesting.

The wasp larvae develop at a much faster rate than do the weevil larvae. As such, they kill the weevil long before it has a chance to eat all of the orchid seeds. By doing so, the wasp has effectively rescued the orchids reproductive effort. Over a five year period, researchers based out of the University of Campinas found that orchid fruits in which wasp larvae have killed off the weevil larvae produced nearly as many seeds as uninfected fruits. As such, the parasitoid wasps have made effective pollinators out of otherwise destructive herbivorous weevils.

The fact that a third party (in this case a parasitic wasp) can change a herbivore into an effective pollinator is quite remarkable to say the least. It reminds us just how little we know about the intricate ways in which species interact and form communities. The authors note that even though pollination in this case represents selfing and thus reduced genetic diversity, it nonetheless increases the reproductive success of an orchid that naturally experiences low pollination rates to begin with. In the hyper diverse and competitive world of Brazilian rainforests, even self-pollination cab be a boost for this orchid.

Photo Credits: [1] [2]

Further Reading: [1]

Parasitic Protection

Strangler figs are remarkable organisms. Germinating in the canopy of another tree, their roots gradually wrap around the host, growing down towards to forest floor. Once in the soil, the interwoven structure of the fig begins to grow and swell. Over time, the strangler fig does what its name suggests, it strangles the host tree. Strangling is bad news for the host, however, new research suggests that strangler figs may actually provide some benefit to larger host trees, at least for part of its life. 

Cyclones are a force to be reckoned with. Their punishing winds can quickly topple even the sturdiest of trees. This is exactly what happened in 2013 when Cyclone Oswald struck Lamington National Park in Australia. Many trees fell victim to this storm but not all. Survival was not random and an interesting pattern started to emerge when researchers began surveying the damage. 

The hollow center of an ancient strangler fig where its host tree once grew and has long since rotted away.  

The hollow center of an ancient strangler fig where its host tree once grew and has long since rotted away.  

They found that large trees hosting strangler figs survived the storm whereas those without were more likely to be uprooted. It appears that hosting these parasitic figs just might have some benefits after all. There are a handful of mechanisms with which strangler figs could be helping their hosts. First is that figs spanning multiple trees may provide stability for the host and its neighbors. Another could come in the form of additional leaf area. The canopy of both the fig and its host tree may help reduce the impact of the cyclone winds. Additionally, once they make it to the soil, the roots of the strangler fig may act as guy-wires, keeping the host tree from uprooting. Finally, The interwoven roots of the strangler fig may act as scaffolding, providing additional structural integrity to the host tree. 

More work will be needed to see which of these are the most likely mechanisms. The mere fact that this parasitic relationship might not be so one-sided after all is quite interesting. What's more, by keeping large tree species alive through devastating cyclone events, the figs are essentially keeping legacy trees alive that can then reseed the surrounding forest. This could explain why host trees have not evolved any obvious mechanism to avoid strangler fig infestation. 

Further Reading: [1]

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

The Explosive Dwarf Mistletoes

I used to think mistletoes were largely a southern phenomenon, preferring regions with mild or even no winters. Then I was introduced to the dwarf mistletoes in the genus Arceuthobium. These odd parasites can be found growing throughout the northern hemisphere. Their affinity for conifers has landed them on the watch list of many a forester yet, despite their economic implications, the dwarf mistletoes are fascinating parasitic plants. 

First and foremost, these are aggressive little plants. They vary in their host specificity. Some species can grow on a wide variety of conifer species from Abies balsamea (balsam fir), Larix laricina (American larch), to Pinus strobus (eastern white pine), whereas others are more specialized, preferring only spruces (Picea spp.). Regardless, infestations of these parasites can do some interesting things to conifer stands. 

Similar to other mistletoes, the dwarfs are stem parasites. They penetrate into their hosts vascular tissues and set up shop, sucking up water and photosynthates and giving nothing in return. Because of this, large infestations can seriously drain their host trees as they themselves have reduced or even no photosynthetic capacity. Additionally, they interfere with nutrient and hormone flows throughout the branches of their host. Such disruptions can result in the formation of dense clusters of branches called "witches brooms." Some dwarf mistletoe infestations can become so intense that they effectively girdle their host tree.

In natural settings, this serves an ecological function. By weakening their hosts, dwarf mistletoes can leave room for other plant species to take root. They also keep one species from becoming too dominant. As such, mistletoe infestations can actually increase plant diversity in the long run. Dwarf mistletoe infestations only become an issue once humans get involved. They can cause serious financial issues for foresters as well as damage important or valued specimen trees. In our highly fragmented forests, their natural behavior can get in the way of human ideals. 

All of this talk of damage can distract us from just how amazing some of these species really are from an organismal standpoint. For instance, the lodgepole pine dwarf mistletoe, Arceuthobium americanum, is capable of thermogenesis. Unlike the other examples of thermogenesis in the plant world, this has nothing to do with flowers. Instead, thermogenesis in A. americanum is used as a seed dispersal agent. 

The dwarf mistletoes don't rely on fleshy fruits to get their seeds from one tree to another. Instead, they utilize ballistic means. As their seed pods mature, they gradually swell. Once pressure is great enough, the seed pods erupt, sending their sticky seeds flying through the canopy at speeds of up to 62 mph (100 km/h)! If lucky, the seeds will stick to the branches of a viable host or be transported there in the fur or feathers of an animal. For A. americanum, the eruption of its seed pods is triggered by heat. Using specialized metabolic pathways at the cellular level, A. americanum is able to heat its seed pods up to ~2 °C warmer than its surroundings, thus triggering its pods to explode. 

Pretty incredible for a species so often labelled as a pest. 

Photo Credit: [1]

Further Reading: [1] [2] [3] [4]

Host Coercion

Moving from one host to another can be difficult for parasites, especially for those specializing on plants. Because they rely on other organisms for their survival, they have evolved some amazing strategies at getting what they need. A recent study published in PLOS Biology has shed some light on one interesting strategy.

Phytoplasma are bacterial parasites of a variety of plant species. In order to get from one host to another, these bacteria utilize insect hosts. How they do this is quite incredible. These bacteria produce specialized proteins that have some strange effects on plant tissues.

The proteins actually sterilize the host plant. They do this by interfering with the proteins responsible for flower development. Instead of producing normal flowers, the plants produce mutated leaf-like structures. You can see an example of a healthy plant on the left and an infected one on the right. So, why does the bacteria do cause such mutations?

This is where the insects enter the picture. Researchers found that infected plants that produced these mutated leaf-like structures were more attractive to leaf hoppers. The leaf hoppers readily feed and reproduce on these infected plants at a higher rate than they do healthy plants. In feeding, the leaf hoppers inevitably suck up bacteria in the sap.

When the leaf hoppers go on to feed on healthy plants, some of the bacteria get transferred in their saliva, thus completing the parasitic lifecycle. This is what parasitologists call "host coercion." The parasite, in this case phytoplasma bacteria, alter their host in some manner that increases the fitness of the parasite. This is one of the first examples in which researchers have been able to identify the exact mechanism by which a parasite makes this happen.

Photo Credit: John Innes Centre (https://www.jic.ac.uk/)

Further Reading:
http://journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.1001835