Biochemical Soul Musings on Nature, Science, Evolution, Biology, and Education

25Mar/09Off

Adaptation of the Week – Channichthyidae Icefish Blood and Antifreeze

I owe the following example of evolutionary adaptation to the always amazing evolutionary and developmental biologist Dr. Sean B. Carroll, from his lecture "Making of the Fittest" for the Darwin College - Darwin Lecture Series, available at iTunes U (I highly recommend everyone give it a listen).

The Red Blood Cell-less Icefish © Dr Julian Gutt and Alfred Wegener Institute

The Red Blood Cell-less Icefish © Dr Julian Gutt and Alfred Wegener Institute

Imagine that you are a fish - exothermic and thus unable to regulate your own body temperature - and the contingent foibles of natural history have all conspired to leave you and your kind in the frigid oceans of the Antarctic just as they are beginning to reach the freezing point (10-14 million years ago).

You like the cold and are well adapted for it, but these temperatures are beginning to give even you - a master of the cold - the icthy chills.

Now imagine that the hands of mother nature have given you the tools to change your own genetic code, and thus your nature, allowing you to make yourself even more suited for waters that are 2 degrees celsius below the freezing point of pure water.

What would you do? Would you inject your DNA with a molecular antifreeze? That seems like a reasonable addition - one we will get to momentarily.

But if you were a genius of bioengineering would you reach out a molecular scalpel and hack away the genes that allow the production of red blood cells, hemoglobin, and myoglobin, leaving only molecular fossils behind?

Icefish Larva

Icefish Larva © Uwe Kils

It doesn't seem like a particularly well thought out plan. But then again, neither you, the fish, nor mother nature are genius bioengineers. Fortunately for life, the forces of evolution still manage to get the job done, however sloppy the end results (yes, technically the job is never done - forgive my metaphor wearing thin).

In fact, natural selection performed just such a feat somewhere around 8.5 million years ago in the ancestors of a flock of related species in the Antarctic: the Channichthyidae icefishes (also known as crocodile icefishes or white-blooded fishes).

As we all know, liquids tend to become more viscous in the cold. Just compare maple syrup before and after refrigeration. Blood viscosity would have no doubt been an issue in the ancient ice fish ancestors, or at least one that could be improved upon. Normal vertebrate blood is filled with big, round, and red blood cells coursing through the blood vessels. Now imagine lowering the temperature of the blood below the normal freezing point of water - that's bound to create some significant resistance.

But aren't erythrocytes critical for carrying oxygen? How could an organism just dispense with them completely? As many scientists know, one of the great things about really cold water is that it can be packed with oxygen. Such is the case with the waters of the Southern Ocean, which are saturated with oxygen.

Thus, it seems that at some point, the icefish ancestors developed mutations in the pathways that result in red blood cell production. Furthermore, the species eventually acquired a deletion in the key genes of red blood cells: the alpha and beta hemoglobin genes. No longer could this fish produce hemoglobin.

As is often the case with evolution through loss of gene function, the deletion wasn't perfect. Almost all vertebrates have both hemoglobin genes lying next to each other within the genome. In most Channichthyidae icefishes, the beta hemoglobin gene has been completely deleted, along with all but the truncated end of the alpha hemoglobin gene (interestingly, these fish have lost their myoglobin gene as well)1. To quote the original paper by Near et al.:

"Despite the costs associated with loss of hemoglobin and myoglobin in icefishes, the chronically cold and oxygen-saturated waters of the Southern Ocean provided an environment in which vertebrate species could flourish without oxygen-binding proteins."

The upshot of all this is that the icefish has completely clear blood lacking in any erythrocytes - and they are the only species of vertebrates to have such a trait.

Normal 2 globin genes vs. lost icefish globins - modified from Near et al 2006

Normal two hemoglobin genes vs. lost icefish hemoglobins - cropped figure from Near et al 2006

Of course, a few other supporting traits evolved as well. Their hearts are significantly larger than other fish hearts, and they pump 4 to 5 times larger volume of blood per stroke2. Their capillary beds have become much more dense as well to make sure all their tissues get adequate oxygenation. Of course, like amphibians that breathe through their skin, with the loss of red blood cells, those that were better able to absorb oxygen tended to outperform their cohorts. Thus they became scaleless as well.

As if these adaptive feats weren't cool enough (pun intended), the antarctic icefishes have evolved their own antifreeze as well3,4. What's amazing about this antifreeze (an Antifreeze Glycoprotein - or "AFGP") is that it represents one clear cut case in which a gene with a specific function has evolved into a separate gene used for a completely different function in a novel way. In the case of the icefish, the ancestral gene was a trypsinogen (a pancreatic digestive enzyme), which has been mutated and co-opted to be secreted and distributed throughout the body to act as an antifreeze. Specifically (for you biologists out there), the 5' secretory signal and 3' UTR sequences of trypsinogen were tacked onto an amplified nine nucleotide sequence from within the trypsinogen precursor to create the novel antifreeze peptide.

So here we have in the icefish's adaptation to the cold, at least one case of de novo creation of a novel gene with a new function from an old gene, as well as the loss of two other genes that have left genomic fossils behind to whither in the weathers of time.

It may not be the cleanest or best engineered solution to the problem of living in an Antarctic Hell (or perhaps Heaven from the perspective of the fish), but this messiness of evolution is precisely what makes it so incredibly beautiful.

References

  1. Near T.J., Parker S.K.,  Detrich H.W. A genomic fossil reveals key steps in hemoglobin loss by the Antarctic icefishes. Molecular Biology and Evolution, v.23, 2006, p. 2008 - 2016.
  2. William C. Aird. Endothelial biomedicine. Edition: illustrated. Published by Cambridge University Press, 2007
  3. Chen L., DeVries A.L., Cheng C-H. C. Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid?fish. PNAS, April 15, 1997 vol. 94 no. 8 3811-3816
  4. Chen L., DeVries A.L., Cheng C-H. C. Convergent evolution of antifreeze glycoproteins in Antarctic notothenioid fish and Arctic?cod. PNAS, April 15, 1997 vol. 94 no. 8 3817-3822
  5. Top image © Dr Julian Gutt and Alfred Wegener Institute
  6. Icefish larval image by Uwe Kils

Previous Adaptations of the Week:

  1. Timber Rattlesnake Camoflage
  2. The Aye-Aye’s Freaky Finger (I’ve Been Cursed by an Aye-Aye!)
  3. Flatfish Eyes & Recapitulation Theory
  4. Bird/Crocodile Symbiosis?
  5. Insect Dorsal Ocelli
19Mar/09Off

Great Darwin Beard Challenge – Week 4 – The Mugshots

Alright, so Kevin at Deep-Sea News got a little busy this past week "laying down the hardwood." He claims this involved flooring installation...

Thus I have taken on the reigns of presenting this week's Great Darwin Beard Challenge images.

For those of you new here (and I know there are several due to my Science Blogging: The Future of Science Communication & Why You Should be a Part of it), check out the links at the bottom for previous installments. The short of it: from Darwin's birthday in February to the anniversary of the Origin of Species in October, we are competing for the title of "Most Darwinesque Beard."

Each week, we generally have some theme for the images, mainly just to keep ourselves entertained and distract us from the itchiness and rejections from our significant others.

Kevin's instructions this week were to take "mugshots. Try to look as criminally insane as possible."

Great Darwin Beard Challenge - Week 4

Great Darwin Beard Challenge - Week 4 (click for larger)

Participants: Andrew, the Southern Fried Scientist of Southern Fried Science, Kevin of Deep-Sea News and The Other 95%, Me, David "WhySharksMatter" also of Southern Fried Science, and David2 marine graduate student without a blog.

Personally, I think that I win the "criminally insane" look. David1 definitely has the "mentally challenged" look going for him. Andrew just looks guilty and perhaps drugged. Kevin and David2 both have the "yeah - I did it - whatcha gonna do about it" look.

Next week will be hosted by David at Southern Fried Science. Thereafter, we will be moving to biweekly updates of the contest. Technically, we are in Week 5 right now - these images are from last week.

Great Darwin Beard Challenge History:

18Mar/09Off

Developmental Biologists Online

Remarkable Creatures

Remarkable Creatures: Epic Adventures in the Search for the Origin of Species

Just a couple of quick notes to my fellow developmental biologists out there:

First, due to my recent post, Science Blogging: The Future of Science Communication & Why You Should be a Part of it, I was reminded through my comments at Larry Moran's reaction post at Sandwalk that I haven't met very many developmental biologist bloggers out there.

In fact, there is only one dedicated developmental bio blogger I've found: the superb Hoxful Monsters by Nagraj Sambrani. His blog is written for scientists - and if you care about the nitty gritty details of development and evo-devo, his is a blog you should not miss. (Yes I know PZ of Pharyngula is a developmental biologist and posts on the subject as well - but I think he has "evolved" well beyond being developmental-centered - feel free to disagree)

But there must be at least a few more out there, right? If there are, please let me know.

Second, I recently started listening to Scientific American's "Science Talk" podcasts again on my long drive to work. In the February 28th episode, there's an incredibly fascinating interview with one of the premier evo-devo researchers, Dr. Sean Carroll, in which he talks about his new book, Remarkable Creatures: Epic Adventures in the Search for the Origin of Species. This is one book I will definitely be picking up with due haste.

I highly recommned the podcasts as well.

16Mar/09Off

Children Sing Science!

What's better than children singing? Children singing about science. And to take it once step better, give all the little kiddies British accents.

Apparently these videos have been around for quite some time, but I somehow missed them. Thus I'm guessing that some of you may have missed them as well.

The following are a couple of songs from David Haines Lifetime: a Science Oratoria. You can find a much larger list of songs here, as well as details on the project.

Beware: after listening this you will have "Kingdom...phylum...class and or-r-r-rder" stuck in your head all day long.

Taxonomy

Mr. Darwin, Mr. Wallace, Mr. Matthew

14Mar/09Off

Adaptation of the Week – the Insect Dorsal Ocelli

Dog Day Cicada (wikipedia commons)

Dog Day Cicada (wikipedia commons)

There's one particular event of every summer in the South that I always await with great anticipation: the emergence of the millions of annual Dog Day cicadas (Tibicen canicularis).

It's not just the event itself that I love. The cicadas are certainly wonders in themselves; but for me, they are more than just insects of the order Homoptera - they are the standard-bearers of my favorite time of year: the "dog days" of summer. It's the time of year when the sun shines the brightest, heat covers the land as lazy dogs curl in cool digs in the shade, and Sirius - the Dog Star and the brightest in the sky - makes its appearance above the Southern horizon.

Spring is nice. Fall is fairly beautiful. Winter could be thrown to the dogs and I wouldn't bat an eye. But Summer? Ahh, summer is the incubator of my soul. When I'm in it, the warmth makes my happiness grow as ideas sprout from the imaginal discs of my imagination.

It is in no small part the fact that cicadas choose late summer to burst newly reformed into the world, leaving their former larval stages behind, that they receive my respect. I like their style.

But they deserve my awe for many other reasons beyond our shared love of summer. Many of us are well aware of the cicadas' prolonged existence as grubs feeding amongst the roots of trees for years, the exact time dependent on the particular species. Many species have synchronized both their development and life-cycles to such a degree that they burst forth from the ground all at once after 13 or 17 years of sucking sap as larvae. They enjoy an incredibly short adulthood, frantically mating for a few weeks, followed by death en masse (much like the death orgies of the market squid).

The advanced life-cycle adaptations of the cicadas and the timing thereof are deserving of their own tribute. However, the focus of this article lies elsewhere in our cicadan wonders. For the cicada contains an organ prevalent among many orders of insects that many of you have likely never even heard of: the "dorsal ocelli".

There are no more other worlds to conquer! - Alexander the Cicada

"There are no more other worlds to conquer!" - Alexander the Cicada

Did I play my role well? If so, then applause, because the comedy is finished! - Cicadan Emporer Augustus

"Did I play my role well? If so, then applause, because the comedy is finished!" - Cicadan Emperor Augustus

Dorsal Ocelli

I took the images above last summer after the poor (or perhaps ecstatically happy) little cicada had already performed its life duties. Shortly after emerging and mating, cicadas slowly become lethargic, then immobile, and finally they simply die. This individual had reached the immobile stage. It was still alive when these pictures were taken, but days later it had died - remaining in the exact same location and position you see it in now.

Now, look more closely. You may notice its head is bejeweled with three orange organs. These are its dorsal ocelli (singular: ocellus).

It is during our darkest moments that we must focus to see the light. - Cicada Benson

"It is during our darkest moments that we must focus to see the light." - Cicada Benson

The ocellus is a strange and still quite mysterious organ.  It is present throughout the insect world, but only erratically. Despite their ongoing mystery, the organs have been studied fairly extensively since the 1920s and 30s. The following distribution of ocelli among the insects (for you entomologists) is from The Function of the Insect Ocellus1, by D. A. Parry in 1947:

ORTHOPTERA : always present in Acriidae and Gryllidae; sometimes present in Blattidae, Mantidae, Tettigoniidae; not present in Grylloblattidae. DERMAPTERA : absent. PLECOPTERA : two or three present. ISOPTERA: present. EMBIOPTERA: absent. PSOCOPTERA: sometimes present. ANOPLURA: absent. EPHEMEROPTERA: present. ODONATA: usually present. THYSANOPTERA: present. HEMIPTERA: great variation. Some families separated on the presence or absence of ocelli. Several families in which some genera possess ocelli and some do not. NEUROPTERA: conspicuous in some families, absent in others. MECOPTERA: some genera with ocelli, others without. TRICHOPTERA: some families with ocelli, others without. One family including six genera with ocelli and two without. LEPIDOPTERA: sometimes present. COLEOPTERA : absent except in a few species not all in the same family. STREPSIPTERA : absent. HYMENOPTERA: usually present, but sometimes absent in the Vespoidea. DIPTERA: sometimes present. APHANIPTERA: uncertain.

Many species it seems have found great use in the ocellus, as evidenced by its retention throughout much of the Insecta class, while others have completely disposed of it. 

But what is it?

“If the sight of the blue skies fills you with joy, if a blade of grass springing up in the fields has power to move you, if the simple things in nature have a message you understand, Rejoice, for your soul is alive.” - Eleanora Cicada

“If the sight of the blue skies fills you with joy, if a blade of grass springing up in the fields has power to move you, if the simple things in nature have a message you understand, Rejoice, for your soul is alive.” - Eleanora Cicada

Essentially, the dorsal ocellus is an eye.  But dorsal ocelli are not like the large compound eyes always present nearby. Nor are they like our own.

Early studies measuring the focal depth of various ocelli lenses all came to the conclusion that ocelli cannot focus forms on their simple retinas. It has since been shown that this is mostly true, except with some dragonflies which apparently may be able to form images with their ocelli.

What dorsal ocelli can do quite well is sense light. In fact they are much more sensitive to light intensity than the main compound eyes.

Studies in the 40s showed that ocelli nerve impulses were inhibited by light. When the ocellus was occluded, signals would then propogate down the large nerves to ganglia. Essentially, if a shadow passed over the ocellus, signals fired. And because the nerves are very large in diameter (often the largest nerve fibers), they are very fast.

It was additionally shown that light perception in the ocelli alone could not lead to reflexive movement. Thus it was suggested, and some still hold, that light perception (or shadow perception) acts to set the excitatory potential of the nervous system. Thus, if a shadow passes overhead, the nervous system would be primed to react to visual stimuli from the compound eyes.

Advances in Insect Physiology

Advances in Insect Physiology

More recent studies have shown that ocelli are intricately involved in orientiation to light (including UV), particularly to the horizon, and so are integral parts of the flight stabilization machinery, which makes sense when considering that most flying insects have ocelli.

Again, research in dragonflies indicate that the ocelli can form images with very wide fields, and can sense motion. There are other indications that ocelli may play a role in circadian entraining.

To my knowledge, no physiological research has been conducted on the cicada ocelli. Regardless, it appears that whatever the function of the ocellus, it is intricately and physically intertwined with the circuitry of vision from the compound eyes.

The ocellus represents just one more example - among myriads - of a sense that we as humans can hardly fathom. It is hard enough to imagine perceiving the world through thousands of individual ommatidia (the many eyes within a single insect compound eye). Add to that a complex system of light perception wired to the eye circuitry to aid in orientation, flight stability, or to prime the brain for visual stimuli. Such perception is impossible to even imagine.

It's clear from my limited research that science has yet to fully explain the purpose of these beautiful adaptations, despite the prevalence of their existence. It just goes to show that we have not come close to deciphering all the mysteries of life - even mysteries that have stared us in the face for a century.

So this summer, as the cicadas raise their eyes and dorsal ocelli to the summer sun for the first and last time, take a second to give them a closer look. You may just find yourself in awe of these photosensitive jewels.

References

  1. Parry D.A. (1947) The Function of the Insect Ocellus. Journal of Experimental Biology. Vol. 24. Nos. 3 & 4. pp. 211-219 (pdf)
  2. Beament J.W. L. (1966) Treherne J.E. Advances in Insect Physiology. Academic Press. (book)
  3. Berry R.P., Stange G., Warrant E.J. Form vision in the insect dorsal ocelli: An anatomical and optical analysis of the dragonfly median ocellus. Vision Research. Volume 47, Issue 10, May 2007, pp. 1394-1409.
  4. Simple eyes in Arthropods. Wikipedia.org

Previous Adaptations of the Week:

  1. Timber Rattlesnake Camoflage
  2. The Aye-Aye’s Freaky Finger (I’ve Been Cursed by an Aye-Aye!)
  3. Flatfish Eyes & Recapitulation Theory
  4. Bird/Crocodile Symbiosis?