The Eyes Have It: Evolutionary Development and DNA

Today’s guest blogger is Neal Immega. He has a Ph.D. in Paleontology and is a Master Docent here at HMNS. In his post below – originally printed in the Museum’s volunteer newsletter – Neal discusses Evolution Development and DNA.

Popular media crime shows, like CSI: Crime Scene Investigation, show amazing applications of DNA technology. For example, a person can be traced to a specific location by means of cells he left on a door knob.

A new science called “Evo-Devo,” shorthand for Evolutionary Development, can tell us even more amazing information. Evo-Devo techniques probe deeply into the structures of DNA to look at how DNA actually codes for the growth of body parts, telling us more about the animal kingdom than we ever dreamed possible. It shows genetic similarities between very different organisms and lets us understand how two organisms, like mice and men, can have DNA that is 85% similar and, yet, code for very different organisms.

We all know the basics of DNA molecules, where the genetic code is stored by a very long sequence of four proteins strung together in various arrangements. That is the easy part! What we need to
worry about is how these genes blueprint a living being. Geneticists, like Sean Carroll (whose popular books are listed in the references box), have discovered that the DNA code is made up of some large master programs that control things, such as eyes, and lots of very small programs (they call them switches) that control what kind of eye will be displayed.

Normal Fruit Fly
Image courtesy of The Exploratorium

Let’s confine ourselves to understanding and experimenting on simple life forms, such as fruit flies. To figure out which specific piece of DNA causes some feature to appear in a developing embryo, geneticists experimentally inactivate a segment of DNA, transplant the complete strand (including the inactivated segment) back into the egg, fertilize that egg, and then see what turns up missing. If that missing part is not vital for survival, the egg might even grow into an adult fly. Compare the drawings of a normal fly with the one below it where the master program for eyes has been deleted.

Eyeless Fruit Fly 
Image courtesy of The Exploratorium

Such experiments have found that the master program for making eyes can cause an eye to grow on a fly’s leg, body, antenna, or inside the body, depending on where it is placed on the DNA strand. Check out the drawing showing the results of moving the master program for legs to the site of the antenna. Note that the extra legs are fully formed but lack the neuron connections to the brain and so are not functional. (In the references box is a link to an electron microscope image of a real fruit fly that shows a mutation in which eyes replace antennae.)

Various mollusks (like clams, snails, and octopuses) grow eyes that vary in complexity from very simple sensitive pits to complex eyes that would compete well with human eyes. The EXACT SAME eye master program from a fruit fly can replace the eye master program for a squid, and it will grow a perfectly functional squid eye. You might be tempted to say that fruit flies and squids are cousins.

Fruit fly with extra legs
replacing the antennae 
Image courtesy of The Exploratorium

That is an amazing statement, but to take it even further, the same experiment with a mouse eye master program will grow fly eyes on flies and squid eyes on squids. They only differ by the small switch segments. These experiments establish a link between vertebrates and invertebrates that paleontologists are unlikely to find in the rock record. This also helps explain the amazing degree of structural similarity between mice and men—although many of the master programs are similar, the really critical parts of the DNA are the small switches that control the details.

Mollusks have just one master program that is controlled by different switches. Pectens, for example, have the most complex vision arrangement of any animal with three different types of eyes on its body. The DNA can be experimentally adjusted to grow any of these eyes anywhere on the body. Random mutations could thus cause novel arrangements, and survival would judge their fitness—evolution in action.

The switch concept explains how mice, chimps, and humans can have a similar number of genes. The switches control the result of the master programs. You can pick up any modern textbook and read that men and chimps have nearly identical genes. It is the switches that make us different and that provide the evolutionary means for dramatic changes, good and bad.

The fossil record is full of cases where a dramatic new species just appears. Paleontologists have often wondered if this was caused by a missing rock interval, by migration, or by rapid evolution. The concept of rapid evolution has often been discounted because it seemed to violate the incremental nature of evolution. We now can see how rapid evolution may just be a single point mutation in a switch. There are numerous biological examples where altering one protein is lethal, as in Tay-Sachs disease, or altering another might bear strongly on survival, as in changing
the color of hair from white to black.

Geneticists can now explain things in a way that profoundly affects how we think about evolution. Biologists and paleontologists have always wondered if evolution had to generate complex structures like eyes from scratch for each phylum. The reuse of master programs from very simple life forms through complex ones means that evolution can build on what went on
before. Critics of evolution often claim that eyes are too complex to have evolved. (The “half-an-eye-is-nogood” argument is derived from the first sentence of the Darwin quote in the box below.) Now, with Evo-Devo tools, we can see commonalities between the genetics of simple life forms and complex life forms– between clams and people.

The possibilities just became more complex.

REFERENCES:
Wyoming Dinosaur Center: http://www.wyodino.org/

Sean B. Carroll:
Endless Forms Most Beautiful: The New Science of Evo-Devo, (paperback) 2006
The Making of the Fittest: DNA and the Ultimate Forensic Record of Evolution,
(paperback) 2007
Remarkable Creatures: Epic Adventures in the Search for the Origins of Species, 2009

Lynn Helena Caporale:
Darwin in the Genome: Molecular Strategies in Biological Evolution, 2002

SEM (scanning electron microscope) photograph of eyes replacing antenna in a fruit fly by Naoum
Salame. http://1tv4.sl.pt

Fly Eye Genetics:
http://www.pbs.org/wgbh/evolution/library/04/4/text_pop/l_044_01.html
Renowned scientist Dr. Walter Gehring discusses master control genes and the evolution 
of the eye.

Darwin, 1859, The Origin of Species, http://darwin-online.org.uk/contents.html. In most editions, the quote appears on pp143-4.

Making Baby Bearded Dragons

Today’s post is written by Sibyl Keller, a volunteer recruiter and educational coordinator at HMNS. Today, she tells us about the bearded dragons that live in her office, and what happens when another one comes to visit. 

Sibyl, holding Leonardo.

So…what’s happening in the Volunteer Office other than recruiting new volunteers, interviewing new talents, filling tours, booking docents, scheduling on-going training, handling birthday parties, writing college recs, and just keeping up with the hopping pace around HMNS?  Natural science – that’s what! it’s been happening under our noses - and keeping us all intrigued and inspired by how incredible the animal world is!

It all happened when Chris and Erin adopted their first baby – Monster, a beautiful young female bearded dragon!  Draco and Leonardo are the (lizard) kings of the Volunteer Office.  Draco is a handsome beardie – a gentleman of almost ten years.  And I was fortunate to adopt Leonardo – a young chap beardie of two years this last summer.  And then Monster arrived for a visit.

It was love at first sight -Leo and Monster couldn’t keep their eyes off of each other!  And if you have never seen beardies put on their mating dance – it is an incredibly captivating event. Leo – so eager to impress his new friend – totally bearded out with a solid heavy black coloration under his chin (it is this behavior that gives the species the name “bearded dragon.”)  Between the black beard and the head bobbing with determination – Monster was truly moved! She began waving gracefully, first with one arm, then the other. 

Even with an office full of museum staff watching the mating dance, you could have heard a pin drop until Chris expressed that…this was kind of weird…He didn’t don’t know if it is a good weird or a bad weird!

Unfortunately, I don’t have a video of our beardies dancing, but I found a video of another beardie bobbing away.

So when Chris left with the HMNS Paleo Team to head to Seymour, Texas on the Fossil Dig five weeks ago – I got to babysit Monster for a couple of weeks!  So – here we are, 5 weeks later (which happens to be the gestation period for beardies) – hummmmm…

Can you see the eggs?

After mixing together a nice compost for Monster, Googling to find out what beardies like for a nest – the waiting game began!  Day after day, Monster redistributed her compost from one side of her habitat to the other.  She started practicing her digging skills in between warming her growing belly on her heat rock.  Karen (my fellow volunteer coordinator) described her well – what a keen resemblance Monster had to a Portobello mushroom!  And – what an appetite!  Crickets, juicy superworms, carrots and collard greens – of course, the crickets were presented to her with a nice coat of calcium for the mother to be.

Then last week – the discovery was made!  After a long night – it must have been, Monster had created a mountain from her compost on top of her heat rock and was playing king of the mountain just about all morning.  It wasn’t until she got a superworm treat that she would inch away from her mountain!  My work began. 

Karen Fritz, marking the eggs.

Carefully sifting through the compost a little at a time – I was in search of the mother load – a nest of beardie eggs – Monster’s first clutch.  Totally amazed that not an egg was found, I started to think that she didn’t look so much like a Portobello mushroom, there were no eggs – maybe my imagination just got the best of me.  The Princess was so lethargic, I started to worry that maybe she was sick. 

After watching her all day, I felt better when she had a healthy appetite.  I decided to start sifting some of the mulch out of her habitat – as ingesting any of this could be very harmful to her.  As I cleaned her aquarium, I lifted her large heat rock and the discovery was made!  We hit the jackpot with 24 small marshmallow-size beardie eggs!  It is truly amazing that this little lizard knew just what to do to keep her clutch warm.  I cannot even imagine how she was able to dig out the dirt under her heat rock to lay 24 eggs without crushing them!  Nature is amazing.

How did this little Princess lay 24 eggs?!  Well – from the Internet, I discovered it was not at all uncommon for a Bearded Dragon to lay up to 30 – 50 eggs in a clutch!  But – the female wouldn’t necessarily lay all of the eggs at one time.  She could choose to lay a couple of eggs one week, one or two a week later – and as the process continues – it could be months before the whole clutch was laid!  Dang – that meant I would be spending quite a time of the Christmas holiday egg-sitting in the Volunteer Office at HMNS!  Lucky for me — and I’m sure happily for her – Monster laid all 24 eggs during one evening after hours, probably while the music played and laughter was heard during holiday celebrations taking place through out the exhibit halls up above!

The eggs, with black lines to
mark their original positions.

You might notice black lines imprinted along the length of each egg from top to bottom.  The lines were introduced by Karen Fritz, my Volunteer Office co-partner in crime, who has a smooth and steady hand and a good sharpie!  I learned of this process from me earlier research.  It is extremely important to not rotate or change the position of the eggs while moving them.  After carefully uncovering the eggs, Karen marked each egg in order for us to move them in this order.  She wanted to mark them 1, 2, 3…up to 24 — until she understood we just needed a line to identify top and bottom of each egg!  If any eggs were turned upside down, it would surely damage or kill the developing embryo.  We then placed them in small deli cup containers filled with dampened vermiculite that would hold moisture throughout their time of incubation.

Eggs, in the incubator – where they
will stay until they hatch!

As the incubator is quietly protecting these little jewels for 60 to 70 days, Monster is now far away from her little 2 week vacation spent in the Volunteer Office.  She is back home with Erin and Chris – I understand with a frisky new way about her and a grand new appetite!

Monster at home