At one point during my (extended) university years, I attended a campus screening of an insect documentary. Maybe it was Life in the Undergrowth? Probably, because, well, David Attenborough. Anyway, although the show delivered intriguing vignettes about ants, beetles, flies, and many other six-legged beasties, the audience received the overwhelming impression of a single take-home message. In the silence after the credits rolled, one commenter expressed it for all of us:
"F*ck wasps."
Now I'm not usually given to flights of colorful metaphor, but ever since that moment, every time I have learned something new about wasps, that viewer's succinct expression pops into my head.
Wasps are despicable.
Sure, some wasps sting humans, and that doesn't feel good. I know, because in first grade I got a wasp stuck behind one knee in the skirt of my uniform*, and thereby learned that these creatures sting repeatedly.
But stings, however painful, are a mere evolutionary afterthought to the real nightmare of wasps: the ovipositor. A female wasp uses her ovipositor to inject eggs into the living bodies of other animals, where her offspring incubate, hatch, and then assiduously consume their (still-living!) host from the inside out.
Say it with me: EWWWW.
Not all wasp species engage in this "parasitoid" lifestyle. In fact, any wasp that stings almost certainly does not also do the freaky egg-injection thing, because evolution has turned their ovipositors into stingers. In the grand scheme of wasp evolution, oviposition came first; stinging is a recent innovation.
Evolution came up with the sting because of the eggs, though. If Wasp Mom just dropped off her kids at the local caterpillar and flew away, the caterpillar's immune system would go ballistic on the baby wasp intruders. So evolution mixed up a cocktail of chemicals for Mom to inject along with her eggs, a sort of "baby care package" if you will.
This package can include a paralytic venom, which weakens the host so it's easier for the babies to eat. Over time, some of these venomous wasps evolved from parasitoids into simple predators, who paralyze food and carry it back to their babies instead of burying their babies directly in the food. These are the nest-building wasps, and they include the yellowjacket that employed its arsenal on my tender five-year-old popliteal fossa.
I encourage you to delve into the nearly inexhaustible and totally incredible literature about parasitoid wasps (including the "Russian doll warfare" so delightfully named by Carl Zimmer, and the news that parasitoid wasps may be the most speciose group on the planet). No matter how disturbing it gets, you've got to give them this: they know how to take care of their kids.
Despite the tender maternal care, however, these children are not helpless--ahem--parasites. Evolution has gifted Baby Wasp with some fantastically freaky survival techniques as well.
Rather like the embryos of animals that bear live young, wasp embryos develop entirely inside someone else's body. So they don't need a hard shell to protect them from the environment. Heck, they don't even need a soft shell. Nestled in the hemocoel (essentially the bloodstream) of a fellow insect, they're safe from wind and rain and surrounded by nutrition.
All they need is a placenta to absorb it.
Yup, wasps are the only non-mammals known to possess a placenta.** Of course, this doesn't indicate kinship with mammals! Evolution simply solved a similar problem in a similar way. Shortly after Mom Wasp does her business with the ovipositor, the embryo grows a membrane around itself. As Ahmed Sabri and colleagues explain in a 2011 PLoS paper, this membrane then "invades the host tissues . . . and form[s] a placenta like structure able to divert host resources and allowing nutrition and respiration of embryo."
For readers unfamiliar with the workings of the mammalian placenta, the authors draw an explicit parallel: "Such interspecific invasion, at the cellular level, recalls mammal's trophoblasts that anchors maternal uterine wall." I'm sure this could be construed as a contradiction to my "babies are not parasites" thesis (which I did not expect to be nearly as controversial as it apparently is).
As I noted four years ago, I love babies and I love parasites, so what could be more pleasing than these placenta-bearing parasitoid wasps? I'm sorry, Hymenoptera; I take it back. Wasps aren't despicable at all. They're delightful.
* I switched to public school for second grade, so this was my only year of mandatory school uniforms. The wasp incident didn't endear them to me.
** I first learned about these wasps from the unashamedly pro-insect book Planet of the Bugs by Scott Richard Shaw. I hope that I can do cephalopods half the justice that he did insects.
Showing posts sorted by date for query parasites. Sort by relevance Show all posts
Showing posts sorted by date for query parasites. Sort by relevance Show all posts
Friday, August 12, 2016
Thursday, January 1, 2015
How The Ocean Will Kill You, and Other Salty Truths
This article was originally posted on Dan Koboldt's website as part of Science in Sci-fi, Fact in Fantasy—a fun and educational series of blog posts by experts from various fields. Cross-posted here with permission.
The ocean covers 71% of our planet and probably leaks into at least that much of our collective psyche. You can’t dip your toe in a tidepool without getting bitten by symbolism. The depths of the sea are humanity’s unconscious; maritime weather is fickle fate; fish represent Jesus; and the white whale—well, we all know about him.
The ocean has shaped high fantasy like Ursula K. LeGuin’s Earthsea series and hard sci-fi like David Brin’s Uplift books, adventures like Verne’s 20,000 Leagues Under the Sea and thrillers like Benchley’s Jaws.
The immense popularity of these last two titles, however, has contributed to one of the most common misconceptions about the ocean among readers and writers alike.
When they hear that the ocean is dangerous, most people think of great white sharks, giant squid, maybe even sea serpents. But the most dangerous thing in the ocean is actually . . . water. Because you can’t breathe it.
In the US, about 3500 people die every year by drowning, and half of these occur in what the CDC calls “natural water” settings: the ocean, lakes and rivers. Compare 1750 annual deaths by natural water to less than one by shark (some years there are no fatal shark attacks) and zero by squid (there’s never been a confirmed fatal squid attack).
That’s why real people who work or play in the ocean, like divers and sailors and surfers, take safety seriously and focus primarily on the risks of drowning and exposure. Fictional characters should do the same. The Law of Conservation of Detail may prevent you from waxing lyrical about your kayaking protagonist’s life jacket, but at least you can make sure your divers follow the buddy system.
On the flip side, if you do want to hurt or kill a character in the ocean, it’s tragically easy to find inspiration in real headlines. Operating a boat under the influence. Night swimming alone. Ignoring the dive computer’s warnings.
But I know, I know. Sometimes you just have to threaten your characters with a deadly animal. In that case, may I suggest a cone snail or a blue-ringed octopus?
Or even (as much as it pains me to admit it) giant squid. In fact, the ocean is Earth’s premier showcase for the sheer diversity of animal life. It’s got dancing flatworms, sea cucumbers that breathe through their anuses, sailing jellyfish, octopuses that dress up like shrimp, shrimp that can break your thumb faster than you can blink . . . I could go on.
Most of these creatures are invertebrates, animals without a backbone. And some are truly bizarre. In college, my invertebrate zoology professor said that if he had to pick a group of animals that came from outer space, it would be the echinoderms (starfish, sea urchins, and sea cucumbers). Instead of having a left and right side like we’re used to, they have five-pointed symmetry. Instead of a proper circulatory system, they pump raw seawater through their bodies, using the pressure of the water to move their feet. They can regenerate their arms and even their guts. Aliens among us, indeed.
In fact, many writers have drawn inspiration from this realm for creating alien or fantasy life forms. (Writing Outside the Human Box tackled this topic in excellent detail). Marine invertebrates offer an almost endless diversity of shapes, forms and behaviors to stir the imagination.
And if you’re writing horror, try looking up marine invertebrate parasites. Tongue-eating isopods, anyone?
But if you’re not inventing new species or traumatizing your readers, if you’re just writing a few boat scenes or a romantic walk on the beach, do you really need to know about all this biodiversity? Plenty of people who live in coastal towns never see much more than seagulls and the occasional whale. However, it’s worth remembering that today’s ocean is the product of centuries of overfishing.
Are you writing historical fiction or creating a fantasy/alternate world? Try filling the oceans brimful with turtles and fish twice the size of a person. Pack in the whales like sardines. Consider reading accounts of historical abundance, like this passage from the memoir Two Years Before the Mast, in the year 1834:
On the other hand, are you writing about the future? Consider that whales may become wholly extinct, as in the charmingly cheesy Star Trek IV: The Voyage Home. Sad to say, it’s also reasonable to speculate that we may lose all the coral reefs, sea turtles, and sea birds.
We often think of the ocean as powerful and dangerous. It is. But at the same time, many marine animals and ecosystems are fragile and endangered. To incorporate both aspects in our writing is to give to the sea what we try to give to all of our characters—depth.
The ocean covers 71% of our planet and probably leaks into at least that much of our collective psyche. You can’t dip your toe in a tidepool without getting bitten by symbolism. The depths of the sea are humanity’s unconscious; maritime weather is fickle fate; fish represent Jesus; and the white whale—well, we all know about him.
The ocean has shaped high fantasy like Ursula K. LeGuin’s Earthsea series and hard sci-fi like David Brin’s Uplift books, adventures like Verne’s 20,000 Leagues Under the Sea and thrillers like Benchley’s Jaws.
The immense popularity of these last two titles, however, has contributed to one of the most common misconceptions about the ocean among readers and writers alike.
The ocean can kill you, but probably not the way you think.
When they hear that the ocean is dangerous, most people think of great white sharks, giant squid, maybe even sea serpents. But the most dangerous thing in the ocean is actually . . . water. Because you can’t breathe it.
In the US, about 3500 people die every year by drowning, and half of these occur in what the CDC calls “natural water” settings: the ocean, lakes and rivers. Compare 1750 annual deaths by natural water to less than one by shark (some years there are no fatal shark attacks) and zero by squid (there’s never been a confirmed fatal squid attack).
That’s why real people who work or play in the ocean, like divers and sailors and surfers, take safety seriously and focus primarily on the risks of drowning and exposure. Fictional characters should do the same. The Law of Conservation of Detail may prevent you from waxing lyrical about your kayaking protagonist’s life jacket, but at least you can make sure your divers follow the buddy system.
On the flip side, if you do want to hurt or kill a character in the ocean, it’s tragically easy to find inspiration in real headlines. Operating a boat under the influence. Night swimming alone. Ignoring the dive computer’s warnings.
But I know, I know. Sometimes you just have to threaten your characters with a deadly animal. In that case, may I suggest a cone snail or a blue-ringed octopus?
Most of the animals in the ocean are not whales, dolphins, or fish.
Or even (as much as it pains me to admit it) giant squid. In fact, the ocean is Earth’s premier showcase for the sheer diversity of animal life. It’s got dancing flatworms, sea cucumbers that breathe through their anuses, sailing jellyfish, octopuses that dress up like shrimp, shrimp that can break your thumb faster than you can blink . . . I could go on.
Most of these creatures are invertebrates, animals without a backbone. And some are truly bizarre. In college, my invertebrate zoology professor said that if he had to pick a group of animals that came from outer space, it would be the echinoderms (starfish, sea urchins, and sea cucumbers). Instead of having a left and right side like we’re used to, they have five-pointed symmetry. Instead of a proper circulatory system, they pump raw seawater through their bodies, using the pressure of the water to move their feet. They can regenerate their arms and even their guts. Aliens among us, indeed.
In fact, many writers have drawn inspiration from this realm for creating alien or fantasy life forms. (Writing Outside the Human Box tackled this topic in excellent detail). Marine invertebrates offer an almost endless diversity of shapes, forms and behaviors to stir the imagination.
And if you’re writing horror, try looking up marine invertebrate parasites. Tongue-eating isopods, anyone?
But if you’re not inventing new species or traumatizing your readers, if you’re just writing a few boat scenes or a romantic walk on the beach, do you really need to know about all this biodiversity? Plenty of people who live in coastal towns never see much more than seagulls and the occasional whale. However, it’s worth remembering that today’s ocean is the product of centuries of overfishing.
The ocean we’re used to is unnaturally empty.
Are you writing historical fiction or creating a fantasy/alternate world? Try filling the oceans brimful with turtles and fish twice the size of a person. Pack in the whales like sardines. Consider reading accounts of historical abundance, like this passage from the memoir Two Years Before the Mast, in the year 1834:
We were surrounded far and near by shoals of sluggish whales and grampuses, which the fog prevented our seeing, rising slowly to the surface, or perhaps lying out at length, heaving out those lazy, deep, and long-drawn breathings which give such an impression of supineness and strength. . . . I stood leaning over the bulwarks, listening to the slow breathings of the mighty creatures—now one breaking the water just alongside, whose black body I almost fancied I could see through the fog; and again another, which I could just hear in the distance—until the low and regular swell seemed like the heaving of the ocean’s mighty bosom to the sound of its own heavy and long-drawn respirations.
On the other hand, are you writing about the future? Consider that whales may become wholly extinct, as in the charmingly cheesy Star Trek IV: The Voyage Home. Sad to say, it’s also reasonable to speculate that we may lose all the coral reefs, sea turtles, and sea birds.
We often think of the ocean as powerful and dangerous. It is. But at the same time, many marine animals and ecosystems are fragile and endangered. To incorporate both aspects in our writing is to give to the sea what we try to give to all of our characters—depth.
Monday, October 29, 2012
Why Babies Aren't Actually Parasites
[Edited 10/30/12 due to existence of intraspecific parasitism; see comments.]
PSA: Babies are not parasites.
Of course, parasites have babies, and some free-living organisms have a parasitic early life stage. But the notion I seek to discredit here is that all babies are parasites of their parents and, particularly, that the human fetus is a parasite of its mother. This misconception has become distressingly common among my peers.
It distresses me because I love babies and I love parasites, so I think it's important to understand the distinction between them. In a nutshell: a parasite reduces the fitness of its host; a baby increases the fitness of its parents.
Seems pretty straightforward, right? Yet I will concede that numerous superficial similarities between babies and parasites can lead to confusion. Parasites often live inside the body of another creature, extract their nutrition from its blood, and struggle to escape attack by its immune system. That's starting to sound an awful lot like a fetus . . .
But the host-parasite relationship is one of conflict, while the mother-baby relationship is intrinsically cooperative. Consider the immunology of the two. Host and parasite are locked in an arms race: the parasite evolves ever more complex techniques of avoidance, while the host evolves ever more complex techniques of detection and attack.
Meanwhile, mother and baby cooperate to prevent immunological conflict. The site of this cooperation is the placenta--the big blob of tissue that's genetically part of the baby and physically connects baby to mom. For a long time, scientists thought of the placenta (and by extension, the fetus) as a kind of natural organ transplant. Just as in medical organ transplants, the mother's immune system would have to be suppressed to prevent it from rejecting the foreign body.
But a fascinating review paper in 2010 suggests this is the wrong way to think about pregnancy--that, in fact, the cooperative choreography between mother and child is far more sophisticated:
All this isn't to deny the fact that a pregnant woman makes certain sacrifices. Notably, she gives up nutrition that could otherwise have gone to her own body. But in sharing nutrients with her offspring through the placenta and, later, milk production, a human mother has it relatively easy. Some species transfer nutrients more, um, directly.
Babies of one rather unusual amphibian* simply graze on their mother's skin to get the early nutrition they need. And the young of certain spiders consume their mother's entire body--parental sacrifice at its most extreme!**
Then there are the jellyfish children. In some species of narcomedusae, baby jellies hang out inside their parents, slurping food out of the adults' digestive tracts. That's not so weird--I mean, think of regurgitation in birds--but then sometimes they'll go and slurp from an unrelated adult, or even from adults of another species.
The ones that stay with their parents are certainly not parasites. But the ones that feed off other adults are in murkier territory. They're certainly acting a lot more like parasites than if they'd stayed at home.
But what if it's like a "village" scenario, in which all the adults pitch in to raise all the children? Parasitism need not enter the picture; this is simply cooperative parental care. Of course, jellies do not have complex societies, so it's a rather fanciful idea. It becomes even more fanciful if you consider the baby jellies who feed from adults of a different species. It's hard to argue that those little tykes are anything but parasites.
I like the narcomedusae because they illustrate when a baby is just a baby, and when a baby becomes a parasite.
It all boils down to the fact that parent and child have a common goal: the child's survival. Host and parasite, on the other hand, have a fundamental disagreement about the desirability of the parasite's survival.
(Of course, the baby in my belly could have taken over my brain and caused me to write this manifesto.)
* This used to be "sea urchin" because I was convinced that's what I'd heard, but I recently asked an echinoderm expert, who didn't know what I was talking about. Fortunately, I also recently picked up The Sixth Extinction, which mentioned the curious habits of caecilians.
** It's worth noting that parents sometimes eat their young, as well, if things don't seem to be working out--reclaiming the nutrients they invested in order to give reproduction another shot later.
PSA: Babies are not parasites.
 |
| This is a parasite. (Head louse, by Gilles San Martin) |
Of course, parasites have babies, and some free-living organisms have a parasitic early life stage. But the notion I seek to discredit here is that all babies are parasites of their parents and, particularly, that the human fetus is a parasite of its mother. This misconception has become distressingly common among my peers.
It distresses me because I love babies and I love parasites, so I think it's important to understand the distinction between them. In a nutshell: a parasite reduces the fitness of its host; a baby increases the fitness of its parents.
Seems pretty straightforward, right? Yet I will concede that numerous superficial similarities between babies and parasites can lead to confusion. Parasites often live inside the body of another creature, extract their nutrition from its blood, and struggle to escape attack by its immune system. That's starting to sound an awful lot like a fetus . . .
But the host-parasite relationship is one of conflict, while the mother-baby relationship is intrinsically cooperative. Consider the immunology of the two. Host and parasite are locked in an arms race: the parasite evolves ever more complex techniques of avoidance, while the host evolves ever more complex techniques of detection and attack.
Meanwhile, mother and baby cooperate to prevent immunological conflict. The site of this cooperation is the placenta--the big blob of tissue that's genetically part of the baby and physically connects baby to mom. For a long time, scientists thought of the placenta (and by extension, the fetus) as a kind of natural organ transplant. Just as in medical organ transplants, the mother's immune system would have to be suppressed to prevent it from rejecting the foreign body.
But a fascinating review paper in 2010 suggests this is the wrong way to think about pregnancy--that, in fact, the cooperative choreography between mother and child is far more sophisticated:
The trophoblast [placenta] and the maternal immune system have evolved and established a cooperative status, helping each other for the success of the pregnancy. This cooperative work involves many tasks, some of which we are just starting to unveil.True, the placenta uses at least one trick from the world of parasites--a molecule that makes it partially invisible to mom's immune system--but it also oversees an active exchange of molecules and even cells between mother and baby. The full implications of this exchange aren't yet understood, though the mother's contributions undoubtedly protect the baby from infection, and the baby's cells may also offer health benefits to the mother.
All this isn't to deny the fact that a pregnant woman makes certain sacrifices. Notably, she gives up nutrition that could otherwise have gone to her own body. But in sharing nutrients with her offspring through the placenta and, later, milk production, a human mother has it relatively easy. Some species transfer nutrients more, um, directly.
Babies of one rather unusual amphibian* simply graze on their mother's skin to get the early nutrition they need. And the young of certain spiders consume their mother's entire body--parental sacrifice at its most extreme!**
Then there are the jellyfish children. In some species of narcomedusae, baby jellies hang out inside their parents, slurping food out of the adults' digestive tracts. That's not so weird--I mean, think of regurgitation in birds--but then sometimes they'll go and slurp from an unrelated adult, or even from adults of another species.
The ones that stay with their parents are certainly not parasites. But the ones that feed off other adults are in murkier territory. They're certainly acting a lot more like parasites than if they'd stayed at home.
But what if it's like a "village" scenario, in which all the adults pitch in to raise all the children? Parasitism need not enter the picture; this is simply cooperative parental care. Of course, jellies do not have complex societies, so it's a rather fanciful idea. It becomes even more fanciful if you consider the baby jellies who feed from adults of a different species. It's hard to argue that those little tykes are anything but parasites.
I like the narcomedusae because they illustrate when a baby is just a baby, and when a baby becomes a parasite.
It all boils down to the fact that parent and child have a common goal: the child's survival. Host and parasite, on the other hand, have a fundamental disagreement about the desirability of the parasite's survival.
(Of course, the baby in my belly could have taken over my brain and caused me to write this manifesto.)
 |
| This is not a parasite. (Pregnant author, by Anton Staaf) |
* This used to be "sea urchin" because I was convinced that's what I'd heard, but I recently asked an echinoderm expert, who didn't know what I was talking about. Fortunately, I also recently picked up The Sixth Extinction, which mentioned the curious habits of caecilians.
** It's worth noting that parents sometimes eat their young, as well, if things don't seem to be working out--reclaiming the nutrients they invested in order to give reproduction another shot later.
Wednesday, March 21, 2012
Science Ruins Science Fiction Again
Last month, Russian researchers struck frozen science gold--an ancient lake, buried deep under the Antarctic ice sheet. Given that Lake Vostok had been isolated for probably millions of years, the Russians were under a lot of pressure (just like the lake! because it's under a really heavy ice sheet, get it?) to protect this unspoilt environment from contamination.
But what about the possibility of contaminating ourselves with stuff from the lake? As my brother pointed out,
As soon as you move into another organism, you're a symbiont. Symbionts can be beneficial or harmful; the harmful kind are called parasites. So, bacteria that live in people and make them sick are technically a kind of parasite--though people often say "parasites and bacteria" the way they used to say "animals and fish." (Yes, fish are technically and in all other ways animals.)
Now, all symbiotic relationships are products of co-evolution. The parasite evolves to survive inside the host, while the host evolves to reduce the harm done by the parasite. (There are a lot of strategies for that, by the way--from making initial infection more difficult to quarantining, expelling or killing the parasite). As the host environment becomes more hostile, the parasite evolves clever coping mechanisms, and so on.
Because of the specificity of most parasite-host relationships, it's highly improbable that a parasite could survive for millions of years without its host*. And if it did survive, it would probably do so by evolving into such a different form that it couldn't re-infect its old host.
That's why I'm pretty confident there aren't any nasty little parasitic bacteria in Lake Vostok, waiting to pounce on us.
Okay (said my brother) but why couldn't a non-parasitic Vostokian bacterium initiate a pandemic as soon as it was exposed to people? Every relationship has to start somewhere, right?
Sure, a free-living bacterium that had never encountered humans before could theoretically find its way into an unsuspecting scientist (poor Dr. Lukin!), survive long enough to reproduce, and start a new symbiotic relationship. But the environments of Lake Vostok and the human body are radically different. A bacterium (or any other critter) is much more likely to move inside an organism if that organism's internal decor is similar to the environment it's already adapted to. The 37 °C of the human body would almost certainly kill bacteria adapted to the -3 °C of Lake Vostok.
~Tangential Musing On Evolutionary Timescales~
Even if a brand new bacterium entered a human and survived, we'd probably never know about it. As a general rule, it takes a long time for symbioses to evolve, and it's very hard to study them when they're just getting started.
Imagine a cafe full of college freshman--there's probably a lot of flirting, but none of it may ever turn into a relationship. Tracking all the potential interactions, most of which will be dead ends, would be a huge challenge. Now consider that plenty of pairs of college freshman are likely to hit it off with each other, but most biological interactions that could become symbioses are nipped in the bud when one organism kills the other.
How long would it take to evolve the sort of traits that make for a proper pandemic? I don't know, but I wonder if anyone's done any theoretical modeling of this . . .
~End Tangent~
All the really scary epidemics in human history have come about through jumps between similar environments.
Human to human is the most obvious--Europeans bringing syphilis to the New World, for example. We often use the term "first contact" to refer to the meeting of colonizers with natives, which is a bit misleading, since we also use that term in science fiction to refer to the meeting of humans and aliens. The former is fraught with peril of disease; the latter, not so much.
Humans around the globe belong to the same species and are similar enough to fall prey to the same parasites. But in most speculative cases, humans and aliens belong not only to different species, but to entirely different evolutionary histories, perhaps going back to the origins of life itself. The idea of a parasite, carefully co-evolved with its host, being able to jump across such a gap as that--well, it strains my imaginer.
But what about zoonoses? Aren't those examples of parasites jumping suddenly from one host species to another? Well, yes and no. Many parasites have co-evolved with both human and animal hosts, and require both to survive. Malaria is carried by mosquitoes, but can't complete its life cycle without humans. Other zoonotic parasites, like Toxoplasma, are stuck in an evolutionary dead end if they accidentally infect a human--they can survive but not reproduce.
The zoonoses that truly "jump" from species to species, successfully infecting and propagating through their new host, always move between similar environments. Ebola can only infect primates. Even versatile diseases like West Nile virus are restricted to vertebrates--a tiny fraction of the world's animal diversity. There's no way you're going to "catch" colony collapse disorder from a bee, or bitter crab disease from a crab.
So, let me sum up.
Likely sources of pandemics: "first contact" between groups of humans that have been isolated from each other; places where humans and other vertebrates live in close, unsanitary quarters.
Unlikely sources of pandemics: Lake Vostok, Mars.
* Modern humans (Homo sapiens) weren't even around when Lake Vostok was last connected to the rest of the world, but there were definitely early hominids.
But what about the possibility of contaminating ourselves with stuff from the lake? As my brother pointed out,
While this is undoubtedly an exciting moment for science, all I can think of is a science fiction story in which a bacterium discovered in a place like this causes a worldwide pandemic.To which I replied: okay, fun concept, but totally unrealistic. Then we got to talking about parasitism and co-evolution and . . . well, let's start at the beginning.
As soon as you move into another organism, you're a symbiont. Symbionts can be beneficial or harmful; the harmful kind are called parasites. So, bacteria that live in people and make them sick are technically a kind of parasite--though people often say "parasites and bacteria" the way they used to say "animals and fish." (Yes, fish are technically and in all other ways animals.)
Now, all symbiotic relationships are products of co-evolution. The parasite evolves to survive inside the host, while the host evolves to reduce the harm done by the parasite. (There are a lot of strategies for that, by the way--from making initial infection more difficult to quarantining, expelling or killing the parasite). As the host environment becomes more hostile, the parasite evolves clever coping mechanisms, and so on.
Because of the specificity of most parasite-host relationships, it's highly improbable that a parasite could survive for millions of years without its host*. And if it did survive, it would probably do so by evolving into such a different form that it couldn't re-infect its old host.
That's why I'm pretty confident there aren't any nasty little parasitic bacteria in Lake Vostok, waiting to pounce on us.
Okay (said my brother) but why couldn't a non-parasitic Vostokian bacterium initiate a pandemic as soon as it was exposed to people? Every relationship has to start somewhere, right?
Sure, a free-living bacterium that had never encountered humans before could theoretically find its way into an unsuspecting scientist (poor Dr. Lukin!), survive long enough to reproduce, and start a new symbiotic relationship. But the environments of Lake Vostok and the human body are radically different. A bacterium (or any other critter) is much more likely to move inside an organism if that organism's internal decor is similar to the environment it's already adapted to. The 37 °C of the human body would almost certainly kill bacteria adapted to the -3 °C of Lake Vostok.
~Tangential Musing On Evolutionary Timescales~
Even if a brand new bacterium entered a human and survived, we'd probably never know about it. As a general rule, it takes a long time for symbioses to evolve, and it's very hard to study them when they're just getting started.
Imagine a cafe full of college freshman--there's probably a lot of flirting, but none of it may ever turn into a relationship. Tracking all the potential interactions, most of which will be dead ends, would be a huge challenge. Now consider that plenty of pairs of college freshman are likely to hit it off with each other, but most biological interactions that could become symbioses are nipped in the bud when one organism kills the other.
How long would it take to evolve the sort of traits that make for a proper pandemic? I don't know, but I wonder if anyone's done any theoretical modeling of this . . .
~End Tangent~
All the really scary epidemics in human history have come about through jumps between similar environments.
Human to human is the most obvious--Europeans bringing syphilis to the New World, for example. We often use the term "first contact" to refer to the meeting of colonizers with natives, which is a bit misleading, since we also use that term in science fiction to refer to the meeting of humans and aliens. The former is fraught with peril of disease; the latter, not so much.
Humans around the globe belong to the same species and are similar enough to fall prey to the same parasites. But in most speculative cases, humans and aliens belong not only to different species, but to entirely different evolutionary histories, perhaps going back to the origins of life itself. The idea of a parasite, carefully co-evolved with its host, being able to jump across such a gap as that--well, it strains my imaginer.
But what about zoonoses? Aren't those examples of parasites jumping suddenly from one host species to another? Well, yes and no. Many parasites have co-evolved with both human and animal hosts, and require both to survive. Malaria is carried by mosquitoes, but can't complete its life cycle without humans. Other zoonotic parasites, like Toxoplasma, are stuck in an evolutionary dead end if they accidentally infect a human--they can survive but not reproduce.
The zoonoses that truly "jump" from species to species, successfully infecting and propagating through their new host, always move between similar environments. Ebola can only infect primates. Even versatile diseases like West Nile virus are restricted to vertebrates--a tiny fraction of the world's animal diversity. There's no way you're going to "catch" colony collapse disorder from a bee, or bitter crab disease from a crab.
So, let me sum up.
Likely sources of pandemics: "first contact" between groups of humans that have been isolated from each other; places where humans and other vertebrates live in close, unsanitary quarters.
Unlikely sources of pandemics: Lake Vostok, Mars.
* Modern humans (Homo sapiens) weren't even around when Lake Vostok was last connected to the rest of the world, but there were definitely early hominids.
Monday, May 2, 2005
Cohabitation and symbiosis
. . . both literally mean "living together", only they come from different roots.
co: Middle English, together; from Latin com, together, with; from Indo-European kom, beside, near, by, with
habit: from Latin habitare, to dwell; frequentative of habere, to have; from Indo-European ghabh, to give or receive
sym: from from Greek sun, together, from Indo-European ksun, with
biosis: from Greek bios, life, from Indo-European root gwei, to live
Of course, they have entirely different connotations. Imagine telling your parents that you and your significant other have decided to enter into a symbiotic relationship. Calling a relationship symbiotic has the additional effect of making most people think of it as mutually beneficial.
In a strict sense, symbiosis doesn't mean that at all. Ecologists use it for its literal definition, and then categorize the different kinds of symbioses based on the benefits to the participants. A symbiosis in which all parties benefit is called a mutualism. If one party benefits from the relationship and the other is indifferent, then it's a commensalism*. And if one party benefits at the expense of the other, we call it parasitism.
And today, my friends, I am here to talk to you about parasites.
Specifically, the phylum Dicyemida (or Rhombozoa, or Mesozoa... but that's a problem for the taxonomists). Dicyemids are little worm-like creatures that live exclusively in the renal appendages (read: kidneys) of cephalopods. Yeah, weird. That means they are basically swimming in squid urine. What possible benefit this could provide to either worm or squid is difficult to imagine.
The problem with all these symbioses is that, as in any relationship, it's often difficult to tell just who is getting what out of the association. Organisms are labelled mutualists, commensals, or parasites based on the perspective of the observer, which may be quite different from the participants. Certainly a number of cases are clear-cut parasites, such as malaria or tapeworms. Dicyemids, by contrast, are called parasites although we have yet to show any effect on the host cephalopods, benficial or detrimental. Not only that, but no one knows for sure how they infect their hosts in the first place, or for that matter, why they infected them in the way-back-then first place.
Dicyemids are extraordinarily simple. They're eutelic, which means each individual adult of a given species has exactly the same number of cells as all the other adults of that species. This has attracted a number of cellular biologists to study their cell lineages--tracing the history of each cell back to the zygote that gives rise to the whole organism. We're talking on the order of about twenty cells per organism here--these guys are not very big and they're not very complex. No organs or tissues of any sort disturb this perfect parasitic simplicity.
So they were originally classified as Mesozoa, which means "middle animals" and sticks them somewhere in between the Protozoa (first animals, single-celled critters) and the Metazoa (after animals, everything else). But then the geneticists got interested and did some sequencing work on dicyemids to show that they actually seem to be extremely simplified relatives of the Lophotrochozoa, a big group of metazoans which includes squid.
Parasitic simplification is just part of the larger phenomenon of symbiotic simplification. If you're living with someone else, particularly if you're living inside them, it turns out you can get them to do a lot of things for you, like digesting food and dispersing your offspring, towards which you no longer have to allocate energy. And this is why so many parasites can no longer live when separated from the hosts... I feel a tangent coming on.
The inability to reproduce independently is a derived character in parasites, while it is inherent to viruses. At least, that's the presumption. Might that be a little... presumptuous? Is it possible that the viruses we know today evolved from some RNA lifeform that could reproduce on its own, and secondarily lost the ability?**
Back to the point, which is: squid worms!
With all the geneticists and cell biologists mucking around with dicyemids, there aren't a whole lot of ecologists poking at them. And this means that we still don't know what I consider to be the really interesting aspects of their biology: how they interact with their hosts and to what end.
It seems a few experiments could at least give us some leads into this. First off, the infectious stage and method of infection need to be pinned down. That could pretty easily be done by raising octopuses in the laboratory and exposing them to various life stages of dicyemids at various points in their lives. Once the infection of hosts can be controlled, effect of infection of host fitness can be assessed. And if the dicyemids themselves can be cultured outside of the host, providing them artifically with various aspects of their normal environment could tell us how they sustain themselves in the cephalopod kidney.
Look for publications in a few years. Or decades.
* Middle English, sharing a meal; from Medieval Latin commensalis; from Latin com, with, and mensa, table.
**I'm totally making this up. I must remember to look it up and see if anyone else has had the same idea.
co: Middle English, together; from Latin com, together, with; from Indo-European kom, beside, near, by, with
habit: from Latin habitare, to dwell; frequentative of habere, to have; from Indo-European ghabh, to give or receive
sym: from from Greek sun, together, from Indo-European ksun, with
biosis: from Greek bios, life, from Indo-European root gwei, to live
Of course, they have entirely different connotations. Imagine telling your parents that you and your significant other have decided to enter into a symbiotic relationship. Calling a relationship symbiotic has the additional effect of making most people think of it as mutually beneficial.
In a strict sense, symbiosis doesn't mean that at all. Ecologists use it for its literal definition, and then categorize the different kinds of symbioses based on the benefits to the participants. A symbiosis in which all parties benefit is called a mutualism. If one party benefits from the relationship and the other is indifferent, then it's a commensalism*. And if one party benefits at the expense of the other, we call it parasitism.
And today, my friends, I am here to talk to you about parasites.
Specifically, the phylum Dicyemida (or Rhombozoa, or Mesozoa... but that's a problem for the taxonomists). Dicyemids are little worm-like creatures that live exclusively in the renal appendages (read: kidneys) of cephalopods. Yeah, weird. That means they are basically swimming in squid urine. What possible benefit this could provide to either worm or squid is difficult to imagine.
The problem with all these symbioses is that, as in any relationship, it's often difficult to tell just who is getting what out of the association. Organisms are labelled mutualists, commensals, or parasites based on the perspective of the observer, which may be quite different from the participants. Certainly a number of cases are clear-cut parasites, such as malaria or tapeworms. Dicyemids, by contrast, are called parasites although we have yet to show any effect on the host cephalopods, benficial or detrimental. Not only that, but no one knows for sure how they infect their hosts in the first place, or for that matter, why they infected them in the way-back-then first place.
Dicyemids are extraordinarily simple. They're eutelic, which means each individual adult of a given species has exactly the same number of cells as all the other adults of that species. This has attracted a number of cellular biologists to study their cell lineages--tracing the history of each cell back to the zygote that gives rise to the whole organism. We're talking on the order of about twenty cells per organism here--these guys are not very big and they're not very complex. No organs or tissues of any sort disturb this perfect parasitic simplicity.
So they were originally classified as Mesozoa, which means "middle animals" and sticks them somewhere in between the Protozoa (first animals, single-celled critters) and the Metazoa (after animals, everything else). But then the geneticists got interested and did some sequencing work on dicyemids to show that they actually seem to be extremely simplified relatives of the Lophotrochozoa, a big group of metazoans which includes squid.
Parasitic simplification is just part of the larger phenomenon of symbiotic simplification. If you're living with someone else, particularly if you're living inside them, it turns out you can get them to do a lot of things for you, like digesting food and dispersing your offspring, towards which you no longer have to allocate energy. And this is why so many parasites can no longer live when separated from the hosts... I feel a tangent coming on.
The inability to reproduce independently is a derived character in parasites, while it is inherent to viruses. At least, that's the presumption. Might that be a little... presumptuous? Is it possible that the viruses we know today evolved from some RNA lifeform that could reproduce on its own, and secondarily lost the ability?**
Back to the point, which is: squid worms!
With all the geneticists and cell biologists mucking around with dicyemids, there aren't a whole lot of ecologists poking at them. And this means that we still don't know what I consider to be the really interesting aspects of their biology: how they interact with their hosts and to what end.
It seems a few experiments could at least give us some leads into this. First off, the infectious stage and method of infection need to be pinned down. That could pretty easily be done by raising octopuses in the laboratory and exposing them to various life stages of dicyemids at various points in their lives. Once the infection of hosts can be controlled, effect of infection of host fitness can be assessed. And if the dicyemids themselves can be cultured outside of the host, providing them artifically with various aspects of their normal environment could tell us how they sustain themselves in the cephalopod kidney.
Look for publications in a few years. Or decades.
* Middle English, sharing a meal; from Medieval Latin commensalis; from Latin com, with, and mensa, table.
**I'm totally making this up. I must remember to look it up and see if anyone else has had the same idea.
Tuesday, April 26, 2005
The origin of life, the universe, and everything...
…seems as good a place to start as any. Specifically, I’ll start with Life, since I’m a biologist, not a physicist or astronomer. (Even making that statement, though, seems limiting. Why pigeonhole yourself? But that’s a discussion for another day.)
Life! All of us, being more or less alive, have a fairly intuitive understanding of what life is. But people have a lot of trouble when it comes down to defining life. They can say, “This frog is alive. This amethyst crystal is not.”
(I’m sure you can find some argumentative types to come down on the side of the crystal, but anyway.)
The currently more or less accepted NASA definition of life is (abbreviated): a chemical system capable of evolution by natural selection. This is a nice compact little statement that implies a whole lot of characteristics of life, most particularly: variation, fitness consequences of variation, and heredity of variation (which in turn implies some template for the variation, i.e. RNA/DNA, and some method of reproduction). However, it also implies the existence of a population. A single individual cannot, in the usual sense, undergo evolution by nature selection, which occurs through differential reproductive success. Some rabbits are more fit than others, thus they produce more offspring, thus over time their genes undergo positive selection. One bunny rabbit, alone in the universe, is not, by this definition, “life”.
Some people are okay with this. Others are troubled. Some rationalize it with a sematic distinction which, I confess, seems a little silly to me: this poor lonely rabbit, while not constituting life, is still “alive”.
Because any definition either states or implies the presence of a number of characteristics about what is being defined, one might argue that a straightforward list of the characteristics of life would be the most accurate definition. But some people—I call them definitionists—argue that a list is not a definition, and we need a definition.
Perhaps it isn’t. But perhaps we don’t.
It’s not hard to come up with a list of properties that we associate with life. One such list might look like this:
metabolism
membranes
template for variation
reproduction
solvent-based (intriguing, no? it’s been postulated that solid-state chemistry could produce life… but its metabolism might proceed at the rate of one or two reactions per millenium)
chemical bonding
etc.
In a functional sense, such a list might be much more useful than a sentence-long definition. Why? Well, what do we want it for, anyway? Partially, of course, to satisfy our basic human drive to understand, nay, to grok. But also because we are looking for it (life) and we want to know when we find it. We are looking for evidence of it in some of the oldest rocks on Earth. We are looking for it on Mars. We want to look for it on Europa, Titan, Triton, and perhaps elsewhere.
And in that case, the easiest thing to do is make specific tests for various properties of life as we know it, and start marking off boxes on our bingo sheet of Life. Some people may feel they’ve gotten Bingo long before others are ever satisfied. But that in itself is a silly analogy, because my next question is why we should have a binary understanding of life in the first place?
As mentioned above, there are a large number of items which the vast majority of people will identify consistently as life or non-life. But there are a number of non-intuitive items as well. Take viruses. A lot of very well-educated people don’t want to call them “life” because they lack the ability to reproduce independently. Viruses reproduce by injecting their template (RNA) into a host cell, along with various enzymes that instigate the host cell’s replicating machinery to build more viruses. In the most basic sense, viruses lack the ability to reproduce.
But many who have suffered through viral infections of one form or another would be eager to testify that viruses are alive—alive and malicious. Leaving moral questions out of the assessment for the moment, however, we’ve still got a quandary. Plenty of “higher” parasites, from single-celled protozoans to macroscopic worms, are obligate parasites. Their life cycle is tied so intimately to their hosts that they will die on their own—and they certainly cannot reproduce in this state. However, because on the most basic level they do their own reproducing, that is, they possess their own replicative machinery, they are deemed alive.
But that’s not really why we call them alive. We call them alive because they’re wiggly little worms, for goodness’ sake.
(Okay, we’ll call this the Introduction to the Origin of Life. Tomorrow, I’ll get to the Origin of Life itself. The juicy stuff. I promise.)
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