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The Bloodhounds of the Plant World (Cuscuta sp.)

(Via: Marine Science)

Common Names: Dodder, Goldthread, Witch’s Shoelaces

A.K.A.: Genus Cuscuta

Vital Stats:

  • Approximately 200 species
  • Part of the Convolvulaceae family, which includes morning glory and sweet potato
  • Only 15-20 species are considered to be problematic crop parasites

Found: Throughout temperate and tropical parts of the world

It Does What?!

We’ve discussed a few parasites on this blog already, and they’ve all been pretty typical of what comes to mind when we think of parasitic organisms- tiny, malignant little creatures that invade the host’s body, steal its resources, and, in some cases, eat its tongue. But when we think ‘parasite,’ we don’t usually think ‘plant.’ As it turns out, there are an estimated 4500 parasitic species just among the angiosperms, or flowering plants. Among them, dodders have to be one of the strangest.

Found nearly throughout the world, these vine-like plants begin as tiny seeds that germinate late in the spring or summer, after their potential host plants have established themselves. The young seedling has no functional roots and little or no ability to photosynthesize, so initially, it must make do with what little nutrition was stored in its seed. This isn’t much, so the plant has only a few days to a week to reach a host before it dies. To better its chances, the dodder stem swings around in a helicopter-like fashion as it grows, trying to hit something useful.

Much more impressive is the plant’s other method of finding suitable hosts- a sense of smell. Recent research has found that, uniquely among plants, the dodder can actually detect odours given off by surrounding plants and grow towards them. In experiments, the seedlings were found to grow toward the scent of a tomato, even if no actual plant was present. What’s more, they are capable of showing a preference among hosts. Presented with both tomato plants, which make excellent hosts, and wheat plants, which make poor hosts, seedlings were found to grow toward the aroma of tomatoes much more often. Like herbivores, they can use scent to forage amongst a variety of species for their preferred prey.

Smells like lunch… even to other plants.
(Via: Wikimedia Commons)

Once a host plant is found, the dodder begins to twine itself around the stem and to form haustoria (singular: haustorium). These are like tiny tap roots that pierce the host’s stem and actually push between the living cells inside until they reach the vascular system. Once there, the haustoria enter both the xylem (where water and minerals move upward from the roots) and the phloem (where sugars from photosynthesis move around the plant). From these two sources, the dodder receives all its nutrients and water, freeing it from any need for a root system, or even a connection to the soil. And since it doesn’t need to capture solar energy, all green pigment fades from the parasite, and it turns a distinctive yellow or red colour. Leaves aren’t necessary either, which is why the plant is essentially nothing but stem, explaining its common name of “witch’s shoelaces.”

Not what you want to see when you head out to weed the garden.
(Via: County of Los Angeles)

Once it gets comfortable on its new host, the dodder can grow at a rate of several centimetres a day (impressive for a plant) and produce stems of a kilometre or more in length, quickly overrunning an area. It can also attach itself to additional hosts – hundreds, in fact – which is problematic, because at this point it becomes the plant equivalent of a dirty shared needle. Since the vasculature of the hosts is connected, any virus present in one host can be freely transferred to any other. This ability, coupled with its affinity for potatoes, tomatoes, tobacco, and several other important crops, makes dodder a major nuisance for many farmers. And since it’s able to regenerate from just a single, tiny haustorium left in a host plant, it’s really hard to get rid of. There’s always a flip side, though; in some ecosystems, dodder can actually maintain biodiversity by preferentially parasitising the more competitive plants, allowing the weaker ones to survive. It seems dodder may also be the Robin Hood of the plant world.

[Extra Credit: Here’s a video showing how dodder can completely take over a group of nettle plants, complete with ominous soundtrack. Narrated by the fantastic Sir David Attenborough.]

Says Who?

  • Costea (2007-2012) Digital Atlas of Cuscuta (Convolvulaceae). Wilfred Laurier University Herbarium, Ontario, Canada
  • Furuhashi et al. (2011) Journal of Plant Interactions 6(4): 207-219
  • Hosford (1967) Botanical Review 33(4): 387-406
  • Pennisi (2006) Science 313: 1867
  • Runyon et al. (2006) Science 313:1964-1967

    Cuscuta: 1, Acacia: 0
    (Via: Wikimedia Commons)
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EVOLUTION TAG TEAM, Part 2: Sex & the Synconium

The second in an ongoing series of biology’s greatest duos. (Check out Parts One and Three)

(Via: Mastering Horticulture)

Common Name (Plants): Fig Trees

  • A.K.A.: Genus Ficus

Common Name (Wasps): Fig Wasps

  • A.K.A.: Family Agaonidae

Vital Stats:

  • Approximately 800 species of figs
  • Most are trees, but some are shrubs and vines
  • Approximately 640 species (20 genera) of fig wasps
  • All are obligate pollinators of figs

Found: Throughout the Tropics

It Does What?!

Snacked on any Fig Newtons lately? Tasty, right? Like the ad says, “A cookie is just a cookie, but a Newton is fruit and cake.”  …And wasps.

They must have run out of space on the package for that last part.

Before you toss out your favourite teatime treat, I should point out that without those wasps, the figs themselves wouldn’t exist. [Personally, I love Fig Newtons and will eat them regardless of any insects present.] This plant-insect pairing actually represents one of the most stable symbioses out there, with evidence suggesting it has existed for over 65 million years.

Now with 10% more Wings
(Via: Wikipedia)

While it’s not entirely clear how this arrangement evolved in the first place, fig trees produce a unique structure called a synconium, in which the flowers are actually inside the part we think of as the fruit. This synconium, which can contain up to 7000 flowers, depending on the fig species, has a tiny hole at the tip called an ostiole. In order for the flowers to be pollinated and the fruit to grow, a female wasp must squeeze through that hole, often losing her wings and antennae in the process, and distribute pollen that she carries in a sac on her abdomen. As she does so, she also uses her ovipositor to reach down into some of the female flowers and lay her eggs in their ovaries, where a gall is formed and the larvae can develop. Then she dies and ends up in a cookie. The End.

But hold on, let’s remove humans from the equation for a moment. She dies, but her eggs hatch into little moth larvae which use the growing fig for nutrition. Once they’re old enough, the young wasps mate with one another inside the fig (another nice mental image for snacktime), and the females gather pollen from the male flowers and store it inside their abdominal pollen baskets (yes, that’s actually what they’re called). The wingless male wasps have a simple, three step life: 1) mate with females, 2) chew a hole through the fig so they can leave, 3) die. That’s pretty much it for them. They may escape the nursery with the females, but they’ll die shortly thereafter, regardless. In fact, even the females have a pretty rough deal; from the time they’re old enough to mate, they have about forty-eight hours to get their eggs fertilized, gather pollen, find a new synconium, distribute the pollen, and lay their eggs. Two days, and their life is over. No pursuit of happiness for the fig wasp, I’m afraid.

“What does it all mean?”
(Via: BugGuide.net)

As with any long-standing mutualism, there are, of course, parasites ready and waiting to take advantage of it. These parasites are wasps which are able to enter the synconium and lay their eggs, but which do not pollinate the fig. Although their eggs will crowd out those of the fig wasps, decreasing the number of fig wasp larvae born, they are kept in check by the fact that any unpollinated synconium will be aborted by the tree and drop to the ground, taking the parasite eggs with it. The nonpollinating wasps are therefore kept from being a serious threat to the tree’s pollinators.

So there you have it, another of evolution’s great matches. The wasps get an edible nursery, the trees get pollinated, and we get tasty fruits with suspicious crunchy bits that probably aren’t dead wasp bodies, so just try not to think about it too much…

Seeds, or wasp eggs? You be the judge!
(Via: This Site)

[Fun Fact: The symbiosis between fig species and their corresponding wasp partners is so specific (often 1:1), that the shape of the ostiole actually matches the shape of the head of the wasp species which will pollinate it.]

[For those who would like to read about figs and fig wasps in much greater detail (such as how this works when the male and female flowers are in different figs), check out this excellent site for all you could ever want to know.]

Says Who?

  • Compton et al. (2010) Biology Letters 6: 838-842
  • Cook et al. (2004) Journal of Evolutionary Biology 17: 238-246
  • Kjellberg et al. (2001)Proceedings of the Royal Society of London, Biology 268: 1113-1121
  • Proffit et al. (2009) Entomologia Experimentalis et Applicata 131: 46-57
  • Zhang et al. (2009) Naturwissenschaften 96: 543-549
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The Plight of the Spheroid Seaweed (Aegagropila linnaei)

Everyone’s Favourite Freshwater Pet
(via: http://commons.wikimedia.org/wiki)

Common Name: Lake Balls, Marimo

A.K.A.: Aegagropila linnaei

Found: Japan, Iceland, Scotland, Estonia, Germany

It Does What?!

Sure, you’ve had dogs and cats as pets, maybe even fish or lizards… but what about a big ball of algae? Probably not, but if you live in Japan, this idea won’t seem so odd.

Lake balls, or marimo, as the Japanese refer to them, are a rare and unique growth form of the filamentous green algae species Aegagropila linnaei. They occur in only a few isolated habitats worldwide because, unlike most algae, the species lacks a desiccation (dryness) resistant life stage which would allow it to be carried to distant bodies of water. The balls are formed from a densely-packed clump of algal strands which grow outward in all directions, and can reach up to 25cm (10 inches) in diameter. New balls can form from the free-floating form of the same species, or from the breakup and re-growth of an old ball. Found in shallow lakes with sandy bottoms, gentle wave action rolls the clump around, forming a near-perfect sphere and allowing all sides of the ball to receive light for photosynthesis. Seen rolling lazily around the lake bottom, and even rising and falling on columns of warm water, the marimo can almost seem sentient.

It is this bizarre movement and their strangely beautiful appearance which have made marimo so popular in Japan, where they are protected as a “natural monument” and even appear on postage stamps. Unfortunately, it has also been their downfall. Because the algae reside in fresh water and are adapted to low light conditions, they are easily cared for, leading many people to collect them and keep them in their homes. The Japanese believed that a healthy, well looked-after marimo would make the owner’s wishes come true. Lake balls eventually became so rare, due to both human collecting and pollution, that in the early 1950s, a campaign was launched asking Japanese citizens to return their beloved marimo to the lakes from which they had been taken. Impressively, people did so, and in large numbers. In honour of their selflessness, the first annual marimo festival was held, and has continued ever since. Today, the lake ball has become an important environmental symbol in Japan, and children even have their own stuffed marimo toy character, Marimokkori, to play with.

Japanese kids have the best toys, no?

Says Who?

  • Boedeker et al. (2010) BioScience 60(3): 187-198
  • Soejima et al. (2009) Aquatic Ecology 43: 359-370
  • www.marimoballs.com