The Largest Mistletoe

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When we think of mistletoes, we generally think about those epiphytic parasites living on branches way up in the canopy. The mistletoe we are discussing in this post, however, is a decent sized tree. Nuytsia floribunda is a native of western Australia where it is known locally as moojar or the Christmas tree. To the best of our knowledge, it is the largest mistletoe known to science.

Nuytsia floribunda is a member of the so-called showy mistletoe family (Loranthaceae). It along with all of its mistletoe cousins reside in the order Santalales but from a phylogenetic standpoint, the family Loranthaceae is considered sister to all other mistletoes. This has excited my botanists as it allows us a chance to better understand how parasitism may have evolved in this group as a whole.

Speaking of parasitism, there are some incredible things going on with N. floribunda that are worth talking about. For starters, it is not fully parasitic but rather hemiparasitic. As you can tell by looking at the tree decked out in a full canopy of leaves, N. floribunda is entirely capable of photosynthesizing on its own. In fact, experts feel that it is fully capable of meeting all of its own carbohydrate needs. Instead, it parasitizes other plants in order to acquire water and minerals. How it manages this is remarkable to say the least.

Nuytsia floribunda is a root parasite. Its own roots fan out into the surrounding soil looking for other roots to parasitize. Amazingly, exploratory roots of individual N. floribunda have been found upwards of 110 meters (360 ft.) or more away from the tree. When N. floribunda do find a suitable host root, something incredible happens. It begins to form specialized roots called “haustoria”, which to form a collar-like structure around the host’s roots.

Whole haustoria of Nuytsia (white [ha]) and host root (dark brown). * indicates `gland' and developing `cutting device.

Whole haustoria of Nuytsia (white [ha]) and host root (dark brown). * indicates `gland' and developing `cutting device.

The collar gradually swells and a small horn forms on the inside of the haustoria. Swelling of the haustoria is the result of an influx of water and as the pressure around the host root builds, the haustorial horn of N. floribunda physically cuts into its victim. Once this cut is formed, the haustoria form balloon-like outgrowths which intrude into the xylem tissues of the host root, thus forming the connection that allows N. floribunda to start stealing the water and minerals it needs.

Even more amazing is the fact that roots aren’t the only thing that N. floribunda will attempt to exploit. Many inanimate objects have been found wrapped up in a haustorial embrace including dead twigs, rocks, fertilizer granuals, and even electric cables! Its non-selective parasitic nature appears to have left it open to exploring other, albeit dead end options. I don’t want to paint the picture that this tree as the enemy of surrounding vegetation. It is worth noting that N. floribunda extracts very little from any given host so its impact is spread out among the surrounding vegetation, making its overall impact on host plants minimal most of the time.

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Provided its needs have been met, N. floribunda puts on one heck of a show around December. In fact, the timing of its blooms is the reason it earned the common name of Christmas tree. Flowering for this species is not a modest affair. Each tree is capable of producing multiple meter-long inflorescences decked out in sprays of bright orange to yellow flowers. The flowers themselves produce copious amounts of pollen and nectar, making it an important food source for resident pollinators. Though many different species have been documented visiting the flowers, it is thought that beetles and wasps are the most effective at pollination.

Seed dispersal for N. floribunda is mainly via wind. Each fruit is adorned with three prominent wings. After they detach from the tree, the fruits usually break apart into three samaras, each with its own wing. The key for success of any propagule is ending up in a site suitable for germination. According to some, this can be a bit tricky and attempts at cultivating this plant in captivity have not been terribly successful. It would seem that nature knows best when it comes to reproductive success in N. floribunda. It may be worth trying to figure it out though because recent evidence suggests that this species is not faring well with human development. As the surrounding landscapes of western Australia become more and more urbanized, plants like N. floribunda seem to be on the decline. Perhaps renewed interest in growing this species could change the tide for it as well as others.

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Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

Is Love Vine Parasitizing Wasps?

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No, that's not dodder (Cuscuta sp.), its love vine (Cassytha filiformis), a member of the same family as the avacados in your kitchen (Lauraceae). It is a pantropical parasite that makes its living stealing water and nutrients from other plants. To do so, it punctures their vascular tissues with specialized structures called "haustoria." Amazingly, a recent observation made in Florida suggests that this botanical parasite may also be taking advantage of other parasites, specifically gall wasps.

Gall wasps are also plant parasites. They lay their eggs in developing plant tissues and the larvae release compounds that coax the plant to form nutrient-rich galls packed full of starchy goodness. Essentially you can think of galls as edible nursery chambers for the wasp larvae. While looking for galls on sand live oak (Quercus geminata) growing in southern Florida, Dr. Scott Egan and his colleagues noticed something odd. A small vine seemed to be attaching itself to the galls.

Love vine draping a host plant. 

Love vine draping a host plant. 

The vine in question was none other than love vine and they were attached to galls growing on the underside of the oak leaves. What is strange is that, of all of the places that love vine likes to attach itself to its host (new stems, buds, petioles, and on the top and edge of leaves), the only time this vine showed any "interest" in the underside of oak leaves was when galls were present. Obviously this required further investigation.

The team discovered that at least two different species of gall wasps were being parasitized by love vine - one that produces small, spherical galls on the underside of oak leaves and one that forms large, multi-chambered galls on oak stems. Upon measuring the infected and uninfected galls, the team discovered that there were significant differences that could have real ecological significance.

(A)  Cassytha filiformis  vine attaching haustoria to a leaf gall induced by the wasp  Belonocnema treatae  on the underside of their host plant,  Quercus geminata . (B) Labeled graphic of insect gall, parasitic vine, and vine haustoria. (C) Box plots of leaf gall diameter for unparasitized galls (control) and galls that have been parasitized by  C. filiformis . (D) Proportion of  B. treatae  leaf galls that contained a dead ‘mummified’ adult for unparasitized galls (control) and galls that have been parasitized by the vine  C. filiformis .  [SOURCE]

(A) Cassytha filiformis vine attaching haustoria to a leaf gall induced by the wasp Belonocnema treatae on the underside of their host plant, Quercus geminata. (B) Labeled graphic of insect gall, parasitic vine, and vine haustoria. (C) Box plots of leaf gall diameter for unparasitized galls (control) and galls that have been parasitized by C. filiformis. (D) Proportion of B. treatae leaf galls that contained a dead ‘mummified’ adult for unparasitized galls (control) and galls that have been parasitized by the vine C. filiformis. [SOURCE]

For the spherical gall wasp, infected galls tended to be much larger, however, the team feels that this may actually be due to the fact that the vine "prefers" larger galls. Astonishingly, larvae living in the infected galls were 45% less likely to survive. For the multi-chambered gall wasp, infection by love vine was associated with a 13.5% decrease in overall gall size. They suggest this is evidence that love vine is having net negative impacts on these parasitic wasps.

Subsequent investigation revealed that these wasps were not alone. In total, the team found love vine attacking the galls of at least two other wasps and one species of gall-making fly (though no data were reported for these cases). To be sure that love vine was in fact parasitizing these galls, they needed to have a closer look at what the vine was actually doing.

Figure S2. (A)  Cassytha filiformis  vine attaching haustoria to a leaf gall induced by the wasp  Callirhytis quercusbatatoides  on the stem of their host plant,  Quercus geminata . (B) Labeled graphic of insect gall, parasitic vine, and vine haustoria on  C. quercusbatatoides . (C) Exemplar of parasitic vine wrapping tightly around the stem directly proximate to a gall induced by the wasp  Disholcaspis quercusvirens  on  Q. geminata . (D) Field site where love vine,  C. filiformis , is attacking the sand live oak,  Q. geminata , and many of the gall forming wasps on the same host plant.  [SOURCE]   

Figure S2. (A) Cassytha filiformis vine attaching haustoria to a leaf gall induced by the wasp Callirhytis quercusbatatoides on the stem of their host plant, Quercus geminata. (B) Labeled graphic of insect gall, parasitic vine, and vine haustoria on C. quercusbatatoides. (C) Exemplar of parasitic vine wrapping tightly around the stem directly proximate to a gall induced by the wasp Disholcaspis quercusvirens on Q. geminata. (D) Field site where love vine, C. filiformis, is attacking the sand live oak, Q. geminata, and many of the gall forming wasps on the same host plant. [SOURCE]
 

Dissection of the galls revealed that the haustoria were plugged into the gall itself, not the wasp larvae. However, because the larvae simply cannot develop without the nutrients and protection provided by the gall, Eagan and his colleagues conclude that these do indeed represent a case of a parasite being parasitized by another parasite.

At this point, the next question that must be asked is how common is this in love vine or, for that matter, across all other parasitic plants that utilize haustoria. Considering that parasites of parasites are nothing new in the biosphere, it is a safe bet that this will not be the last time this phenomenon is discovered.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]