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第一段
1 .Listen to part of the lecture in a biology class.
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第二段
1 .Ok, today I'd like to spend some time going into more detail about symbiosis.
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2 .Symbiosis, what is it? Anyone?
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第三段
1 .Urn, I thought it's when two organisms are in a relationship that they both benefit from.
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2 .Well, at least that's what I thought it was until I did the reading last night.
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3 .Now, I am kind of confused about it, because the book used that definition to describe mutualism.
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4 .Could you explain the difference?
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第四段
1 .Good. I was hoping that someone would bring that up.
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2 .Sometimes scientists working in different fields use the term symbiosis to mean slightly different things.
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3 .And it can get confusing.
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4 .Uh, for example, when symbiosis is used as a synonym for mutualism.
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5 .But there are quite a few of us out there who think there should be a clearer distinction made between the two.
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第五段
1 .Ok, where to begin...
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2 .Um, the original definition of symbiosis is pretty simple.
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3 .It simply means living together.
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4 .So, any close relationship between two organisms of different species would be considered a symbiotic relationship,
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5 .including positive and negative relationships.
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第六段
1 .Mutualism then is a kind of symbiosis,
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2 .a specific type of symbiotic relationship where both organisms benefit somehow.
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3 .So, your book is correct.
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第七段
1 .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.
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2 .It's not a one-to-one ratio here.
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第八段
1 .Is everyone with me so far?
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2 .Symbiosis, general term;
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3 .mutualism, a narrower or more specific kind of symbiosis. Okay.
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第九段
1 .Now, let's take a closer look at mutualistic relationships.
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2 .Um, I'll start off by describing a case of mutualism that involves a certain butterfly species found in South Africa and Australia.
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3 .It's a good example of how dependence on a mutualistic relationship can vary.
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第十段
1 .Ok, there's this butterfly family and I'll spare you the fancy Latin name because it is not important for our purposes here.
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2 .Uh, I'll call them Coppers and Blues, well, because most members of this family have blue or copper colored wings.
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3 .I think this is one of the most interesting cases of mutualism.
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4 .These butterflies require the presence of ants to complete their life cycle.
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5 .Their interaction with ants is obligatory.
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第十一段
1 .So, this is what happens.
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2 .A female butterfly of these Coppers and Blues will lay eggs only on vegetation where there are ants of a particular species.
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3 .The butterflies can smell,
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4 .well, ants leave behind pheromones, a special chemical signal.
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5 .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.
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6 .As the caterpillar gets a little older and find shelter under nearby rocks or stones to protect itself from predators.
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7 .It's always attended or escorted by ants.
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8 .And it always makes its way back to the host plant to feed,
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9 .guided by the ants,
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10 .the ant escort service, so to speak.
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第十二段
1 .Now, why would the ants go through all this trouble?
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2 .What's their benefit? Mary?
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第十三段
1 .It's probably related to food?
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第十四段
1 .Uh-huh? You are onto something.
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第十五段
1 .Ok, ants feed on sweet stuff, right?
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2 .So the caterpillar must have some kind of special access to honey or sugars, or something like that.
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3 .Maybe caterpillars produce honey somehow.
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4 .On second thought, um, I'm probably way off.
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第十六段
1 .You are pretty close actually.
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2 .The caterpillars have a honey gland, an organ that secretes an amino acid and carbohydrate liquid.
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3 .The caterpillar secretes the liquid from the honey gland, rather large quantities, enough to feed several ants.
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第十七段
1 .But what makes this relationship obligatory for the caterpillar?
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第十八段
1 .Well, if the ants don't feed regularly on the liquid from the caterpillar's honey gland, the gland overloads and gets infected.
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2 .The infection will kill the caterpillar and it will never reach its final stage of development, becoming a butterfly.
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第一十九段
1 .John?
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第二十段
1 .Ok, I just want to make sure I'm following here.
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2 .The caterpillar needs the ants or it won't make it to the stage where it can become a butterfly.
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3 .And the ants do this because they get an easy meal out of it, right?
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4 .But the ants don't absolutely need the caterpillar for survival, 'cause they can get food from other places right?
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5 .So it's still called mutualism even though it seems like the caterpillar's getting way more out of it.
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6 .Oh, wait, you said they don't have to equally benefit. Never mind, sorry.
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第二十一段
1 .Yes. But there is a type of mutualism where the relationship is necessary for both organisms to survive.
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2 .It's called obligatory mutualism.
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3 .And we'll talk about that in the next class.
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