Double

Today is the next to last day of my Photography 101 course, and the theme is “double.” Because I’ve been excited about photographing aquatic insects recently, I’ve got another photo from my aquatic setup for you:

lestid gills

Those are gills of a juvenile southern spreadwing damselfly.  A lot of people don’t know that dragonflies and damselflies actually spend most of their lives underwater as nymphs.  In fact, some dragonflies spend up to three years as nymphs, and then 4-5 weeks as adults on land.

As nymphs, damselflies get everything they need from their aquatic habitat, a pond in this case.  They eat small animals as prey, use cattails and other vegetation as shelter, and they get the oxygen they need from dissolved oxygen in the water.  The gills of damselflies help them breathe by improving their ability to absorb oxygen through their exoskeletons.  The gills massively expand the surface area of their exoskeleton, essentially adding another quarter or third of an exoskeleton to their bodies through which they can breathe.  The gills also improve their swimming, the way wearing flippers while snorkeling can help people swim.

However, damselflies lose gills all the time too.  The one in the photo above only had two of the three it should have when I saw it in the water.  Damselfly nymphs will sometimes fight each other and lose a gills.  Sometimes a predator will try to eat a nymph and get a mouthful of loose gills while the damselfly swims away.  While the gills do improve the lives of the damselflies and one missing its gills has a harder time getting oxygen or avoiding predators, they can survive with no gills at all.

I love the way damselfly gills look!  Another fascinating textured surface, compliments of insects.

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth.

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Friday 5: Bugs with Bubbles

For today’s Friday 5, I’m going to share something near and dear to my heart: aquatic insects that carry bubbles of air with them underwater.  These bubbles are important in the respiration of many aquatic insects and have some cool properties (e.g., they can act like gills!).  I can spend hours watching aquatic insects breathing, so I’m going to share some of the love with you all today!  Let’s start with a couple of simple, very standard types of bubbles.  This beetle is a predaceous diving beetle:

Thermonectus basillaris

Predaceous diving beetle, Thermonectus basillaris

Now it’s a little hard to see the bubble here (it’s just barely visible at the back end), but that’s because this beetle holds its bubble under its wings.  It acts like a SCUBA tank: the beetle uses up the oxygen and then has to go back to the surface to get another bubble.  However, if the beetle exposes that bubble to the water by squeezing a little part of it out the back end (like in the image I posted on Wednesday), this beetle can take advantage of some nifty tricks of physics and turn that bubble into a gill.  Without going into too much detail (read the post linked at the top of the page for details!), oxygen can flow into the bubble from the water and extend the length of time the beetle can remain underwater significantly, but only if the bubble is exposed to the water.  It is thus very common to see predaceous diving beetles of many species swimming around with big bubbles protruding from their posteriors.

Other beetles carry their bubbles on the outside of their bodies, such as in this water scavenger beetle:

Tropisternus lateralis

Water scavenger beetle, Tropisternus lateralis

Aquatic insects with bubbles on the outside of their bodies expose their bubbles to the water all the time and can often remain underwater for extended periods. The bubble won’t last forever though, even when it’s constantly exposed to the water, so this beetle and most other insects with belly bubbles still have to go to the surface to get a refill every now and again.  Unlike the predaceous diving beetle above that goes to the surface butt first, this beetle pops up to the surface and exposes the top of its head and thorax. I can only presume that there are some cool air channels that allow the air at the surface to flow around the side of the beetle and into the air space under the body.  Might have to look into that more closely someday!

Beetles aren’t the only insects with this style of bubble either!  This is a water boatman:

Water boatman

Water boatman

As you can see, it’s got a very similar bubble to the water scavenger beetle above.  It also exposes it’s thorax at the surface when it needs to refill.  However, water boatmen have a really interesting behavior associated with their bubbles. Because oxygen moves incredibly slowly in still water and takes ages to get from the surface to the locations where insects are living, insects such as water boatmen that hang out at the bottom of ponds are exposed to a rather low oxygen environment.  That also means that the bubble’s gill-like properties are diminished because once the oxygen close to the bubble is absorbed, it takes a while for more oxygen to reach it.  Water boatmen solve this problem by using their huge, oar-like hind legs to stir the water around their bubbles.  This creates turbulence in the water, pushing the oxygen poor water away from the bubble and bringing new, comparatively oxygen rich water into contact with it.  Awesome behavior!

Here’s another belly bubble, this time on a creeping water bug nymph:

Creeping water bug, Pelocoris sp

Creeping water bug, Pelocoris sp

Just another belly bubble you might be thinking, but hear me out.  A lot of aquatic bugs hold air stores under their wings.  Unfortunately for the nymphs (= the immatures), they don’t have wings, so they are missing the neat little compartments for air storage their elders have.  Many species store air in belly bubbles instead.  That means that, in several groups of aquatic bugs, the entire respiratory system moves from the bottom of the bug to the top when they undergo their final molt into adults.  Now that’s just cool!

And finally, we come to this gorgeous, tiny beetle:

Crawling water beetle, Peltodytes sp

Crawling water beetle, Peltodytes sp

That’s a crawling water beetle, and it holds air under its wings like a lot of other beetles.  What makes this beetle special is its hind legs.  If you’ve ever identified beetles using the entomology textbook An Introduction to the Study of Insects (originally by Borer and DeLong), one of the first couplets you come to mentions expanded hind coxae that are fused to the metasternum.  If that didn’t make any sense to you, this means that the portion of the legs where they attach to the body has been modified into a large flattened plate that is fused to the body.  The rest of the leg sticks out from under the plate.  These beetles use the space between that plate and the abdomen as a backup air store!  They pack some little air bubbles in there that are thought to supplement the main bubble held under the wings, and they’re right out there where they’re exposed to the water.  With a name like crawling water beetle, it should be obvious that these beetles are not strong swimmers, so they like to stay underwater as long as they can.  Carrying little leg bubbles likely gives them a valuable respiratory boost.

So there you have it!  A bevy of bubbles for your enjoyment.  Next time you see an aquatic insect, I encourage you to look for a silvery sheen on the body.  That’s a good indication that you’re looking at an air store, and you’re one step closer to understanding how that species breathes!  I don’t know about you, but I find that terribly exciting.  :)

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

Insects and Plants Use the Same Strategy for Breathing Underwater

Exposing the air store

A giant water bug, Abedus heberti, breathing using a physical gill

You all know that I have a soft spot in my heart for all things related to aquatic  insect respiration.  I’ve written several blog posts about the topic in the past.  I was thus very excited to come across a new paper a month ago, a commentary on physical gills in aquatic invertebrates and plants by Ole Pedersen and Timothy Colmer.  It was the first time I’d ever considered the possibility that plants might have hit upon the same means of underwater respiration as insects.  Mind blown!  So, I’d like to share the paper with you all too, just in case any of you find it as fascinating as I do.  (One can dream, right?)

Freshwater insects, spiders, and plants all have one thing in common: they are adapted for life on land and depend on respiratory systems that were intended for use in air.  Oxygen is much less abundant in water than in air and moves very slowly through water, so any organism built for living on land that wants to move to an aquatic habitat has to adapt to the available oxygen of their new watery home.  Insects have evolved a variety of means of compensating for the relatively low oxygen levels in water, many of which I highlighted in another blog post.  These include snorkels (such as those on giant water bugs and water scorpions), scuba tank style air stores (also in the giant water bugs, among other true bugs and many aquatic beetles), physical gills (many true bugs and diving bell spiders), plastrons (in a very limited number of aquatic insects), and gills (damselflies, mayflies, hellgrammites, etc).  According to Pedersen and Colmer, nearly all of these animals must return to the surface at some time to refresh their air supply because the respiratory needs of the animal is greater than the ability of the respiratory surface to supply oxygen.

However, gas films such as physical gills and plastrons significantly increase the length of time an organism can remain submerged.  These air films are so important that Pedersen and Colmer suggest that many insects that live in riparian areas or around ponds have body surfaces capable of trapping air films too.  These may prevent drowning if a terrestrial riparian insect becomes submerged, either accidentally or by choice.  Air films are clearly important to a variety of aquatic and riparian insects and spiders.

cattails and algae

Cattails and algae help clean the water

But they’re also important to plants!  The authors discuss how many wetland plants have surfaces that repel water and create gas films around the surface of submerged leaves.  These gas films work the same way they do in insects – absorbing oxygen from the water and improving the respiration of the organism in water.  Plants don’t have the necessary structures to create permanent plastrons, but a plant that is submerged (during flooding, for example) can often survive two weeks or more completely submerged thanks to a little film of air that surrounds it.

The authors did a short study comparing the oxygen uptake by both an insect (a true bug in the genus Aphelocheirus, one of the plastron-bearing insects that only very rarely goes to the surface) and a plant (reed canary grass, Phalaris).  They found that gas films strongly improved the ability of both the insect and the plant to take up oxygen from the water and that the gas films worked in both high and low dissolved oxygen concentrations.  The authors also removed the gas films and discovered that the oxygen uptake strongly decreased.  In the end, they concluded that gas films increase the area through which organisms can absorb oxygen from the water, greatly enhancing their ability to survive underwater and the time they could remain submerged.

Sweetwater

Sweetwater Wetlands

The authors further suggest that gas films might aid in plant photosynthesis.  Plants require carbon dioxide to photosynthesize and normally it enters the plants through pores in the leaves called stomata.  In water, however, stomata are thought to close, so carbon dioxide must travel directly through the leaf’s surface, a long and slow process.  Plants with gas films have an advantage: they can both absorb carbon dioxide more readily through the gas film than without it and they likely keep their stomata open, allowing carbon dioxide to easily flow into the leaves and allow photosynthesis to take place.

shallow treatment

A giant water bug going to the surface to get more oxygen

Pedersen and Colmer concluded with a few comments about water quality and gas film respiration.  They posit that these sorts of systems only work in relatively clean water, that in polluted waters the oxygen levels are too low to support submerged plants and animals with simple gas films.  In dirty water, insects with snorkel or scuba tank like respiratory systems stand a better chance of getting the oxygen they need because they don’t depend on oxygen in the water and go to the surface for oxygen instead.

What I really like about this paper is the connection it draws between the plants and arthropods, how two very different groups of organisms have hit upon the same solution to functioning underwater.  Clearly this system wouldn’t work for all wetland organisms as animals with lungs don’t passively absorb oxygen the way plants and arthropods do, but gas films seem to work well for things that have more passive respiratory systems, regardless of the type of organism. I think that’s pretty darned cool!  Plants and arthropods are wildly different organisms and it’s simply amazing to consider that they’ve developed similar solutions to deal with living in and around water.  Yet one more example of how fantastic the natural world is!

Literature Cited:

Pedersen O, & Colmer TD (2012). Physical gills prevent drowning of many wetland insects, spiders and plants. The Journal of experimental biology, 215 (5), 705-9 PMID: 22323192

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Unless otherwise stated, all text, images, and video are copyright © C. L. Goforth

Science Sunday: Experimenting with Respiratory Behaviors of a Giant Water Bug

Abedus herberti

Abedus herberti in its standard underwater pose.

Last week I covered some of my research with giant water bugs and described how one species, Abedus herberti, breathes.  Respiration in this species is a fairly simple matter of going to the surface to collect a bubble of air that the bugs carry with them underwater, then using that air store as a source of oxygen while submerged.  By adding two simple behaviors, gaping and dynamic gaping, the bugs can dramatically increase the length of time they can remain submerged.  If you observe these behaviors and know a little something about aquatic insect respiration, it seems clear that these behaviors have some respiratory function.  However, that’s not good enough for science!  You have to provide evidence that a behavior does what you say it does.  So, today I’m going to finish the A. herberti respiration story.  First I’ll share the condition in which the bugs exhibit gaping and dynamic gaping and I’ll finish up by sharing how I know that these are indeed respiratory behaviors.

When you observe A. herberti in ideal conditions in the field, you notice that they prefer to stay within a few inches of the surface, usually holding onto the rocks along the banks of streams.  They simply climb up the rocks and stick their air straps out to collect air, then crawl back down.  This way they remain out of sight of predators, but still have easy access to the surface.  However, not all bugs are in this sort of ideal environment.  Some bugs need to dive further into the water to find a suitable perch, and they fight against the air bubble they carry the whole way down.  With these observations in mind, you could imagine that bugs that can reach the surface easily might never use gaping and dynamic gaping.  Bugs in deeper water, however, might want to stay underwater as long as possible because surfacing is hard work.  If gaping and dynamic gaping are respiratory behaviors, you might expect bugs in deeper water to exhibit them more often than bugs in shallow water.

shallow treatment

Shallow treatment

To test this idea, I did an experiment where I placed bugs in observation tanks filled with water to three different depths.  In the shallow treatment, the bugs could reach the surface easily without letting go of the substrate.  In the mid-depth treatment, the surface was just out of reach of the bugs as they did their surfacing behavior and they were forced to float to the surface.  The deep treatment tanks contained water deep enough that the bugs were well out of reach of the surface.  I placed bugs in the tanks one at a time and observed the behaviors of several bugs in each depth before comparing the treatments.

I learned that bugs forced to release their hold on the bottom are much more likely to use gaping and dynamic gaping than bugs that can reach the surface without letting go.  A few bugs in the shallow treatment gaped and one dynamically gaped, but the behaviors were rare.  Nearly every bug in the deep water gaped and dynamically gaped though, as did most of the mid-depth bugs.  Thus, bugs forced to float to the surface and then swim back to the bottom are more likely to express both gaping and dynamic gaping than bugs that can reach the surface easily.  Most likely, gaping and dynamic gaping require less energy than surfacing, so they do these behaviors to cut down on the number of trips to the surface they must make in deep water.

Exposing the air store

Exposing the air store during the gaping behavior

Once I established that water depth played a role in gaping and dynamic gaping, I set out to collect evidence that these behaviors were respiratory behaviors.  I did two tests.  In the first, I divided several bugs into two groups.  I taped the wings to the abdomen using waterproof tape in the first group, preventing them from dynamically gaping or gaping.  In the second group, I added the same amount of tape, but cut the strips in half so that they could do both behaviors.  I then tested both groups to see how long they could stay underwater by forcing them to stay submerged until they showed signs of stress.  I measured the length of time the bugs remained underwater before becoming stressed and the time spent gaping and dynamically gaping.  The next day, I did it all again, except I reversed the treatments.

Abedus herberti at the surface

Abedus herberti at the surface

With this experiment I learned that when denied access to the surface, nearly all bugs exhibited both gaping and dynamic gaping when free to do so.  The bugs were also able to remain underwater over three times as long when they could perform gaping and dynamic gaping than when they could not.  This suggested that there was a respiratory purpose to the behavior, that bugs were able to absorb oxygen into the air store from the water when exposed and extend the length of time they could remain submerged.  That is, they were using the air store as a physical gill.

To strengthen the evidence for a respiratory role for the behaviors even more, I did one final experiment.  I once again forced the bugs to remain submerged until they showed stress, but allowed all bugs to perform gaping and dynamic gaping freely.  I then altered the oxygen content of the water by bubbling air (adds oxygen) or nitrogen (removes oxygen) through a tank for 15 minutes before adding a bug.  I divided the bugs and put half in the oxygen treatment and half in the nitrogen treatment the first day one at a time, then reversed the treatments the following day.  Then I compared the time submerged, gaping, and dynamically between treatments.

Abedus breathing

Abedus herberti gaping

The results were clear: bugs in high oxygen water can remain submerged 10 times longer than bugs in low oxygen water.  Bugs that could freely gape and dynamically gape could only remain submerged a few minutes in the low oxygen, nitrogen treated water while bugs in the high oxygen, air treated water were able to remain submerged nearly an hour.  Clearly, the air store is acting as a physical gill.  Bugs in low oxygen water weren’t able to remain submerged longer than bugs that were prevented from gaping and dynamically gaping entirely.  In both cases, the air store was unable to absorb oxygen from the water.  Bugs in the high oxygen water, however, were able to remain underwater much longer, strong evidence that the air store does act as a physical gill and absorbs oxygen from the environment when it is exposed to water.

So, the giant water bug Abedus herberti depends on oxygen at the surface, but it can extend the length of time the air store provides oxygen underwater by using two simple behaviors: gaping and dynamic gaping.  I think it’s amazing that two seemingly insignificant behaviors, these tiny little movements, are capable of doing so much for these bugs.  This sort of thing is why I love being a biologist!  Isn’t nature marvelous?

Literature Cited:

Goforth, C. L. and Smith, R. L.  2012.  Subsurface behaviours facilitate respiration by a physical gill in an adult giant water bug, Abedus herberti.  Animal Behaviour: doi:10.1016/j.anbehav.2011.12.02.  (Published online only currently – will replace this with the print citation when the issue is released)

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

Science Sunday: How Giant Water Bugs Breathe

It’s the start of a new week and you know what that means: Science Sunday!  I thought I’d mix things up a little today by talking about some of my own research.  The subject of today’s post is this bug, a bug that should be quite familiar to my long-time regular (awesome!) readers:

giant water bug

Giant water bug, Family Belostomatidae, Abedus herberti

This is Abedus herberti, a giant water bug in the family Belostomatidae within the order Hemiptera (true bugs).  It’s a large, aquatic insect native to Arizona and northern Mexico that you’ll find in streams, often in the mountains.  It’s an interesting bug for many reasons, but it is especially well-known because the male bugs care for the eggs until they hatch (see my post about giant water bug parents for more details!) and they are wickedly efficient predators.  These traits make these bugs fascinating for entomologists like me, but they’re not what got me interested in giant water bugs originally.  I got excited about giant water bugs because of this:

Abedus herberti at surface

Abedus herberti at surface, collecting air

Respiration!  I never thought I would ever work with either insect respiration or aquatic insects (except dragonflies), but this project opened up a whole new world of possibilities to me and completely changed the direction of my research.  So, today I’m going to tell you  about the project.  It’s too long for one post, so this week I’ll give you an overview of the respiratory behaviors of Abedus herberti and next Sunday I’ll talk about the experiments I did to show that this is a respiratory behavior.

Giant water bugs are aquatic insects and, as such, have several adaptations that allow them to live in water.  I’ve talked about aquatic insect respiration before, so I’m not going to go over the respiratory adaptations again here, but note that giant water bugs depend on air to breathe.  Water bugs in the genus Lethocerus have a long respiratory tube (called a respiratory siphon) that they stick out of the water that works a lot like a human using a snorkel.  They also have a small space under their wings that holds a small amount of air so they can breathe underwater for a short time.  (Imagine using a SCUBA tank – same deal!)  Abedus herberti does things a bit differently.  First, the respiratory siphon has been reduced to short air straps:

air straps

Abedus herberti. Arrow points to the air straps.

Second, it has a much bigger space under the wings.  That means it can carry more air with it underwater and can remain submerged a lot longer.

So how does Abedus herberti breathe?  Let’s trace the behavior from the moment the bug sticks its air straps out of the water, fills the space under its wings with air, and dives into the water to settle near the bottom.  The bug then follows one of three behavioral pathways.  The simplest is this: the bug absorbs oxygen from the air bubble into the body.  When it has used up most of the oxygen, it goes to the surface to replace the bubble.  If the bug’s close enough to the surface, it simply raises its abdomen and sticks the air straps out.  If it’s in deeper water, it stretches as far as it can to try to reach the surface without letting go by raising the abdomen up, releasing the hind and middle legs, and holding on with only the front claws.  If that’s not enough, it will let go completely, float to the surface, and quickly replenish the air store before diving to the bottom again.  You can see the behavior in this rather blurry video:

That’s one behavioral pattern.  In the second pattern, the bugs add one more step: gaping.  The bugs surface, dive, and sit at the bottom, using the oxygen in the air bubble as before.  However, after about 5 minutes they expose the air bubble to the water.  To do this, they lower the abdomen, creating a space between the abdomen and the wings:

Abedus herberti

The giant water bug Abedus herberti gaping, exposing its air store to the water. The silvery part is the air bubble.

Gaping is a tiny behavior, one very small movement, but it does so much for the bug.  By exposing the air bubble to the water, the bug transforms the air bubble from a simple oxygen store into a physical gill capable of absorbing oxygen directly from the water, tripling the length of time it can remain underwater!  The bugs may gape for 20 minutes, then close the gap before returning to the surface.

The third behavioral pattern adds one important step: dynamic gaping.  This pattern starts with the bug surfacing, diving, sitting on the bottom, and gaping.  After gaping for 5 or more minutes, the bug starts doing this:

They do this motion over and over and over for up to three hours.  Gaping allows the air store to become a physical gill, but dynamic gaping makes the physical gill function as efficiently as possible by stirring the water around the bubble.  This pushes oxygen-depleted water away from the bubble and draws in oxygenated water.  The physical gill is much less efficient at absorbing oxygen from the water when the bug gapes, but does not dynamically gape.  Dynamic gaping is thus a form of ventilation that allows the bugs to remain underwater ten times longer than they can without gaping or dynamically gaping!  But even a dynamically gaping bug must eventually return to the surface (see my post on better breathing underwater to learn why), so it closes the gap between the abdomen and wings and surfaces.

The advantages of this behavior are clear: gaping allows the bugs to remain underwater 3 times longer and dynamic gaping ten times longer than they can when they do not expose the air store to the water.  But why is it important to stay underwater?  This is one reason:

egret

Egrets and other wading birds like to eat water bugs!

Many things love eating large, protein filled insects, so staying hidden underwater as long as possible likely helps A. herberti avoid predators.  However, the bugs carry a lot of air with them, which makes them very buoyant. If they let go of the bottom, they float immediately to the top.  Diving is probably very hard too because they have to fight against their tendency to float to the surface.  So, if the bugs benefit from remaining underwater, but it’s hard to stay underwater, then it’s a good idea to stay underwater as long as possible.  Gaping and dynamic gaping to the rescue!  These two simple, easy behaviors greatly extend the length of time the bugs can remain submerged, but the behaviors probably also require far less energy than diving from the surface.  If so, then gaping and dynamic gaping help the bugs avoid predators, save energy by avoiding trips to the surface, and maximize the time the bugs can spend trying to capture food.

So that’s gaping and dynamic gaping!  Next week, I’ll discuss how I know that these are actually respiratory behaviors.  I hope you’ll check back for part two!

Literature Cited:

Goforth, C. L. and Smith, R. L.  2012.  Subsurface behaviours facilitate respiration by a physical gill in an adult giant water bug, Abedus herberti.  Animal Behaviour: doi:10.1016/j.anbehav.2011.12.02.  (Published online only currently – will replace this with the print citation when the issue is released)

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Unless otherwise stated, all text, images, and video are copyright © TheDragonflyWoman.com

From the Literature: The Cost of Breathing

I have a subscription to the science journal Science, but I rarely do more than shuffle through the contents every 6-8 issues.  While it’s considered one of the premier science journals, there are a lot of articles that are not applicable to what I do, so I usually choose to focus my attention on journals more closely aligned with my research.  However, I happened to look at the October 14, 2011 issue of Science and I’m happy I did!  I came across a great paper by Nick Lane that proposes a possible link between respiration and fitness, fertility, and life span within eukaryotes and explains how this might work.  I’ll talk about Lane’s hypothesis today, but I want to go a step further too.  The examples in the article are all vertebrates and the hypothesis fits well to these organisms.  But how well does it fit with insects?  I’ll discuss that at the end.

Note: if you don’t remember what the respiratory chain of cellular respiration is, you’re likely to get a little lost as you read through this.  I created a handy-dandy respiration refresher yesterday though, so you might want to take a look at that before you go any further.

Lane begins his paper by describing how respiration depends on two different sets of genes within cells, the mitochondrial genome (the genes found in the mitochondria) and the nuclear genome (the genes found in the cell nucleus).  The mitochondrial genome is passed directly from a mother to her offspring while the nuclear genome results from the combination of genes contributed by the mother and the father.  These two genomes must work together to create a functional respiratory chain, the important last step in cellular respiration.  If the mitochondrial and nuclear genomes are perfectly compatible, the respiratory chain will work nearly flawlessly and produce ATP and water as it should.  If there is a mismatch between genomes such that the respiratory chain develops incorrectly, electrons can escape the respiratory chain and react with oxygen to form free radicals, specifically reactive oxygen species (ROS).  Leaks in the respiratory chain can result in excessive ROS production, which can in turn cause damage and even programmed cell death.  It is thus essential that the mitonuclear compatibility be sufficient to form a functional respiratory chain capable of supporting an organism’s energy requirements.  In fact, offspring with severe mitonuclear mismatches will likely die during development or shortly after birth, leaving only those individuals with functional cellular respiration alive to reproduce.

hummingbird

A hummingbird has a very high metabolic rate.

But, Lane wondered, how good does this mitonuclear match really need to be for an organism to survive?  Consider an animal with a high metabolic rate such as a hummingbird or a bat.  These animals must have high mitonuclear compatibility to produce a respiratory chain capable of supporting the high energy demands of flight.  In these animals, any individual with even a minor mitochondrial mismatch should die because it cannot produce the necessary energy to survive.  However, more sedentary animals, such as rats and other rodents, can likely tolerate a higher ROS leak because their energy demands are lower.  It’s not as crucial that every electron make it to the end of the respiratory chain in these species.  If so, then offspring with mitonuclear mismatch (up to a point, the ROS threshold) may survive long enough to pass their genes on.

Let’s consider what this hypothesis, if correct, means with regard to reproductive success (i.e. fitness), fertility, and life span.  Think about the hummingbird and the rat for a moment.  If the hummingbird requires low ROS leakage, many of its offspring are likely to die during development because the ROS leakage threshold is low.  In contrast, a rat has a higher ROS leakage threshold and more of its offspring will survive relative to the hummingbird.  This means that fertility should be low in hummingbirds and high in rats, which is indeed the case.  Fitness could also be affected by ROS leakage thresholds.  Hummingbirds have fewer offspring relative to rats, but their offspring are also less likely to survive to reproduce themselves if their ROS threshold is very low. But there’s a trade-off too (there’s almost always a trade-off in biology!): a higher ROS leakage threshold means that more ROS are being formed.  Excessive free radicals can lead to cellular damage and death.  Rats, with their higher ROS leakage threshold, will have a higher ROS production as well as higher cell damage.  High free radical production is known to decrease life span, so the rat, with its high ROS production relative to the hummingbird, will have a shorter life than the hummingbird.

gerbil

A typical rodent has a low metabolic rate.

So Lane’s hypothesis fits the data for terrestrial vertebrates very well and he has generated a testable idea that people will be able to experimentally support or refute.  That’s good!  I think this idea, that a cellular process helps determine fitness, fertility, and life span, is very interesting and I’m eager to see the research that results from this paper.  However, at the end of the paper Lane suggests that the ROS leak hypothesis should apply across species, presumably across eukaryotic species such as fungi, plants, and invertebrates.  He might be right, but let’s consider insects for a moment.  There’s nearly always an exception among insects, and I think this is a case where there might be a big exception.

Let’s think about flies for a moment.  Many flies have high metabolic rates, which, according to Lane’s hypothesis, should mean that their ROS leak threshold is low with a corresponding low fertility and long life.  This isn’t necessarily the case.  Some flies produce few offspring, but many of them produce a lot of offspring!  Many flies also have very short lifespans of only a few weeks from egg to dead adult.  Let’s then contrast this to the flightless giant water bug I work with (not an altogether fair comparison as they are larger than most flies and that needs to be taken into account, but this is just a thought experiment).  The water bug spends most of its life sitting in one place, a highly sedentary insect with a low metabolic rate.  If Lane’s hypothesis applies to all eukaryotes, this giant water bug should have a short life and high fitness.   Giant water bugs have huge eggs, so they don’t produce as many offspring as some species, but 1000 or more offspring in a mating season is nothing to sneeze at and they seem to fit Lane’s hypothesis.  However, these bugs tend to have comparatively long life spans, sometimes a year and a half as adults!  Last, think about social insect queens.  These are sedentary insects that sometimes have very long lifespans (sometimes decades long!).  According to Lane’s hypothesis, these insects should have short lifespans, which isn’t always the case.

I don’t want to sound like I think Lane’s hypothesis is worthless because I think it can potentially explain several patterns of metabolism, fitness, and life span that we see in nature and I honestly hope it pans out.  However, I have two predictions, one or both of which I suspect will hold true as researchers begin exploring the limits of the hypothesis:

Prediction 1.  The ROS hypothesis will not apply to all eukaryotes and instead apply mainly to warm-blooded, terrestrial vertebrates.

Prediction 2. If the hypothesis holds up to experimentation, I suspect researchers will find other forces at play in insects and other invertebrates that overshadow or alter the impact of ROS leaks on life span and fitness so that they do not follow the same patterns that we see in warm-blooded, terrestrial vertebrates.

Ultimately, I don’t think the hypothesis in its current state is going to be as widely applicable to eukaryotes as Lane suggests, but I hope it will lead to some interesting new research dealing with the link between cellular processes and the life histories of organisms.  If it does that, even if it proves to be incorrect, I think this hypothesis can contribute something very meaningful to science.  And even if it doesn’t pan out, at least it was an exciting idea that got people thinking.  I consider that a success!

Literature Cited:

Lane, N. (2011). The Costs of Breathing Science, 334 (6053), 184-185 DOI: 10.1126/science.1214012

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Science Sunday: A Respiration Refresher

I was going to discuss a paper today, but I was finding it difficult to explain without using a lot of scientific jargon.  I finally realized why: I had to describe the process of cellular respiration before I could talk about the paper for it to make sense to a wide audience.  As a result, the post kept getting longer and longer and I hadn’t even gotten the good parts.  So, I am going to split the post in half!  Today I’ll cover respiration in general and tomorrow I’ll go over the paper.  If you know the details of how respiration works, feel free to skip over this post because it’s going to be review.  If you’re like most people, however, and it’s been a while since you took high school biology or intro biology in college, then this post will help make tomorrow’s post a little more clear.  Consider this your respiration refresher!

insect respiratory system

Diagram of a simple insect tracheal system.

The word “respiration” describes two different but related processes in biological organisms.  At the organismal scale, it describes the process by which an animal (or plant or fungus) obtains the oxygen it requires from the environment.  For example, most vertebrates use two different organ systems to deliver oxygen to their cells, the respiratory system and the circulatory system.  These animals breathe by inhaling air into the lungs.  Oxygen then passes out of the lungs, into red blood cells, and is transported to cells via the bloodstream.  Carbon dioxide wastes then follow the opposite path and are released into the atmosphere.  Insects, as I’ve described before, use a different system.  Instead of lungs, they have a series of branching tubes (trachea and tracheoles) through which oxygen flows from the atmosphere to individual cells or small groups of cells.  CO2 then travels back out along the same tubes.  A variety of respiratory systems can thus achieve the same thing: transporting oxygen from the environment to the cells and moving respiratory waste products from the cells to the environment.

Cell structure

The structure of a simple animal cell. The flagellum is absent in most multicellular animals. (Image in public domain, available at http://en.wikipedia.org/wiki/File: Animal_cell_structure_en.svg)

The purpose of organismal respiration is to provide cells with the oxygen required for energy production in the mitochondria via cellular respiration.  If you recall from your biology classes, eukaryotic cells (i.e. cells from living organisms other than bacteria) are full of little organelles that have various duties within the cell.  The nucleus is the “brain” of the cell and contains the DNA necessary for cellular replication and several cellular processes.  The endoplasmic reticulum is the primary site of protein synthesis while the golgi apparatus packages and processes proteins.  Then there are the mitochondria, the double membraned “power plants” of the cells.  Mitochondria contain a small amount of DNA (important to note for tomorrow’s post) and are the site of cellular respiration.  Oxygen is thus delivered via organismal respiration to the mitochondria within cells so that cellular respiration can occur.

Cellular respiration is a rather complex process that converts nutrients obtained by the organism from its food (or via photosynthesis in plants) into energy and waste products (CO2 and water).  So, how does cellular respiration work?  There are four different processes that occur altogether: glycolosis, pyruvate oxidation, the citric acid cycle, and the respiratory chain.  I’ll summarize each step here, though to keep things as simple as possible, I’m not going to include all the gory details.  If you’re interested in more information, I’ve included the link to each step on Wikipedia so you can read more.

1) Glycolosis.  Glycolosis involves 10 different chemical reactions, but is ultimately important because it converts glucose into pyruvate, which is essential in the next step.  The other products of glycolosis are a few electrons (necessary later on) and 2 molecules of adenosine triphosphate (ATP), energy rich molecules that power many reactions within the cell.  This step takes place outside the mitochondria.

2) Pyruvate decarboxylation.  In this step, pyruvate is converted into acetate, which is then activated by a coenzyme.  Carbon dioxide is also produced during this step.  As CO2 is not needed for anything, it is a waste product that is eliminated from the cells via organismal respiration.  Pyruvate decarboxylation occurs in the space between the two membranes of the mitochondria, the intermembrane space.

3) Citric acid cycle.  The citric acid cycle takes the acetate formed during pyruvate oxidation and oxidizes it (strips electrons from the molecule), producing CO2 and transferring electrons to a carrier molecule (NAD+) that is involved in the next step.  This process takes place within the interior of the mitochondria.

4) Respiratory chain.  This is the step that is important for tomorrow’s post and the final step of cellular respiration.  Here, the reduced form of NAD generated in the citric acid cycle transfers electrons into a series of reactions that occur along the inner membrane of the mitchondria.  Electrons are passed from one carrier molecule within the membrane to the next.  As the they move, they cause the active transport of protons into the intermembrane space.  The protons then flow back across the membrane into the interior of the mitochondria to form ATP, the fuel that powers cellular processes.  This step of cellular respiration is also where oxygen comes into play and why organismal respiration is so important.  Oxygen molecules pick up the free electrons as they come out of the respiratory chain, grab a few protons, and transform into water, a harmless waste product.

Cellular respiration is an incredibly important process in most eukaryotic organisms (all living things other than bacteria), so it is essential that the four processes described here work properly.  For example, if a cell has a faulty respiratory chain, it can’t produce enough ATP to function and produces harmful free radicals rather than water.  Low ATP and high free radical production signal other chemical pathways that lead to programmed cell death, eliminating the faulty cell from the organism or killing a developing embryo before it fully develops.  Tomorrow, I’ll discuss a paper that proposes an intriguing new idea about how variation in respiratory chain efficiency might contribute to reproductive success, fertility, and life span.  I think it’s a fascinating paper that proposes a very interesting new idea, so I hope you’ll check back again tomorrow!

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