نطق سوم

Lecture 3 - Symbiotic relationship

Narrator: Listen to part of the lecture in a biology class.

Professor: Ok, today I’d like to spend some time going into more detail about symbiosis. Symbiosis, what is it? Anyone?

Student: Um, I thought it’s when two organisms are in a relationship that they both benefit from. Well, at least that’s what I thought it was until I did the reading last night. Now, I am kind of confused about it, because the book used that definition to describe mutualism. Could you explain the difference?

Professor: Good. I was hoping that someone would bring that up. Sometimes scientists working in different fields use the term symbiosis to mean slightly different things. And it can get confusing. Uh, for example, when symbiosis is used as a synonym for mutualism. But there are quite a few of us out there who think there should be a clearer distinction made between the two.

Ok, where to begin… Um, the original definition of symbiosis is pretty simple. It simply means living together. So, any close relationship between two organisms of different species would be considered a symbiotic relationship, including positive and negative relationships.

Mutualism then is a kind of symbiosis, a specific type of symbiotic relationship where both organisms benefit somehow. So, your book is correct.

Now, I want to make it clear that, um, the positive result from being in a mutualistic relationship doesn’t have to be equal for both organisms. It’s not a one-to-one ratio here.

Is everyone with me so far? Symbiosis, general term; mutualism, a narrower or more specific kind of symbiosis. Okay.

Now, let’s take a closer look at mutualistic relationships. Um, I’ll start off by describing a case of mutualism that involves a certain butterfly species found in South Africa and Australia. It’s a good example of how dependence on a mutualistic relationship can vary.

Ok, there’s this butterfly family and I’ll spare you the fancy Latin name because it is not important for our purposes here. Uh, I’ll call them Coppers and Blues, well, because most members of this family have blue or copper colored wings. I think this is one of the most interesting cases of mutualism. These butterflies require the presence of ants to complete their life cycle. Their interaction with ants is obligatory.

So, this is what happens. A female butterfly of these Coppers and Blues will lay eggs only on vegetation where there are ants of a particular species. The butterflies can smell, well, ants leave behind pheromones, a special chemical signal. The butterfly recognizes the ants’ pheromones on the plant and then the newly hatched butterflies, the caterpillars will feed on this plant after they hatch from the eggs. As the caterpillar gets a little older and find shelter under nearby rocks or stones to protect itself from predators. It’s always attended or escorted by ants. And it always makes its way back to the host plant to feed, guided by the ants, the ant escort service, so to speak.

Now, why would the ants go through all this trouble? What’s their benefit? Mary?

Student: It’s probably related to food?

Professor: Uh-huh? You are onto something.

Student: Ok, ants feed on sweet stuff, right? So the caterpillar must have some kind of special access to honey or sugars, or something like that. Maybe caterpillars produce honey somehow. On second thought, um, I’m probably way off.

Professor: You are pretty close actually. The caterpillars have a honey gland, an organ that secretes an amino acid and carbohydrate liquid. The caterpillar secretes the liquid from the honey gland, rather large quantities, enough to feed several ants. But what makes this relationship obligatory for the caterpillar?Well, if the ants don’t feed regularly on the liquid from the caterpillar’s honey gland, the gland overloads and gets infected. The infection will kill the caterpillar and it will never reach its final stage of development, becoming a butterfly. John?

Student: Ok, I just want to make sure I’m following here. The caterpillar needs the ants or it won’t make it to the stage where it can become a butterfly. And the ants do this because they get an easy meal out of it, right? But the ants don’t absolutely need the caterpillar for survival, ‘cause they can get food from other places right?

So it’s still called mutualism even though it seems like the caterpillar’s getting way more out of it. Oh, wait, you said they don’t have to equally benefit. Never mind, sorry.

Professor: Yes. But there is a type of mutualism where the relationship is necessary for both organisms to survive. It’s called obligatory mutualism. And we’ll talk about that in the next class.