Monday, April 30, 2012

The Effects of Androgens on Sexual Differentiation in the Spotted Hyena

The Spotted Hyena exhibits some interesting characteristics when it comes to the evolution of appendages. Namely, both the male and the females of the species possess elongated sexual organs that resemble a “male-type external genitalia”. The uro-genital development of this “male-type external genitalia” occurs prior to Gonadal differentiation. Despite the morphological similarities, there are differences between the genitalia of the female and male spotted Hyena. For instance, the male of the species has an enclosed urethra that extends the entire length of the penis and is surrounded by erectile tissue incapable of expansion. On the other hand, the female spotted hyena has “a large pleated urogenital sinus is surrounded by unconstrained connective tissue” (Glickman 405). In addition to this, the penis of the male spotted hyena is thinner than the female clitoris. Researchers have used these unique characteristics as an opportunity to study the effect of androgens (specifically Androstenedione) on sexual differentiation when administered to pregnant spotted hyenas in an attempt to affect the embryo. The results are somewhat surprising. According to the study, the injection of androgens in pregnant spotted hyenas did little to stop or hinder the growth of “male-type external genitalia” leading researchers to believe the growth of the appendage is entirely genetic and not determined by hormones.  Although the injection of androgens did not stop the growth of an external clitoris in the female, it did increase the elasticity of the appendage allowing for a significant decrease in failed first births. This prompted the researches to search for a reason why the female spotted hyena did not evolve with higher levels of Androstenedione. They concluded that although first births are more likely to fail with a decrease in Androstendione, there is more benefit in the increased social aggressiveness exhibited by the female spotted hyena with lower level of the Androgen. This goes to show that reproduction is not all that matters in the sexual differentiation of a species.

*An image which depicts the “male-type genitalia” of both the male and female spotted hyena. photo taken from (Glickman 2005)

Glickman, Stephen E. et. Al. “Sexual Differentiation in Three Unconventional Mammals: Spotted Hyenas, Elephants and Tammar Wallabies” Hormones and Behavior (2005)

Friday, April 20, 2012

Fish Fingers

Back in July of 2011, Scientists at the University of Chicago have been able to transplant genes coding for limb and digit development from fish into mouse embryos. What is really interesting, is that when these genes are capable of activating the development of limbs in the mouse embryo, specifically the development of wrist and proximal digits in Zebrafish. This has lead the team of researchers to conclude that the mechanism for limb development is conserved, even though these two animals are separated by almost 400 million years of evolution. 

Evolutionary biology has taught us that mammals evolved from reptiles, which in turn evolved from fish. 
This suggests that fish, before any mammals with their fingers were present on the earth, possessed the capacity, even though inhibited, to code for limbs and digits. 

Picture of fish genes activating the development of the distal limb in a mouse embryo. 

This research was inspired by the finding of a "missing link" between fish and four legged tetrapods, called the Tiktaalik. Even though the Taktaalik had fins, they also had the skeletal structure of limbed land animals. 

It was the land-dwelling-esque skeleton structure of the Tiktaalik that urged researchers to investigate the physical and genetic homologies between fish and limbed animals. 

In the past scientists did not believe that genes from fish could activate limb development in mice. A previous study had been done in the past with pufferfish genes leading to no success. This lead many scientists to believe that limbs and their genes were specific to tetrapods. We now know otherwise. 

So is the opposite true too? Can mouse limb and digit gene switches (CsB) activate development in fish. Indeed it can! CsB's from zebrafish have been shown to be able to activate the development of the distal ends of zebrafish fins. The article by Science Daily does not go into specifics about this development, but based on the activation of wrist and digit development in zebrafish from mice genes, I would guess that since the development is in the ends of the zebrafish fins, it is the development of  digit-like structures. 

So what are the implications? In the future, scientists will explore the bodies and genes of fish and tetrapods to see how gene expression changed to produce the anatomy of a fully land dwelling animal. 


Thursday, April 19, 2012

Vesigial Organs Recap

I thought it would be a good to critique and reflect upon some of the earlier posts and commentary I have made throughout the semester about vestigial organs.

There is mounting evidence and proposed theories out there advocating different explanations for how certain vestiges have evolved and what their function actually is. However, one important question to consider is how pertinent are these arguments to the broader picture of assessing evolutionary changes?

I have compiled some key recaps of statements made by experts to capture the view of incompleteness of the vestigial organ argument. 
  1. It is not possible to actually prove the uselessness of a given organ.Evolutionary zoologist S. R. Scadding:
    The ‘vestigial organ’ argument uses as a premise the assertion that the organ in question has no function. There is no way this negative assertion can be arrived at scientifically. That is, one can not prove that something does not exist (in this case a certain function), since of course if it does not exist one cannot observe it, and therefore one can say nothing about it scientifically.  Since it is not possible to unambiguously identify useless structures, and since the structure of the argument used is not scientifically valid, I conclude that ‘vestigial organs’ provide no special evidence for the theory of evolution.
  2. Some organs, although apparently functionless, are not derived from evolutionary ancestors in which the structures originally had a function. 
In analysis and reflection of Darwin's idea of the mammalian genitalia: These structures . . . clearly reflect the embryonic development of a sexually dimorphic organism which begins its development in a sexually indifferent condition with structures characteristic of both sexes. They do not reflect phylogenetic development. No one supposes males evolved from females or vice versa.
  1. Organs exhibiting reduced function or no function provide poor evidence for a process supposed to generate organs with new functions
    The existence of degenerate structures does not show that microbe-to-man evolution has taken place.

    Considered sources:
    Bergman, Jerry, and George Howe. 1990. “Vestigial Organs” Are Fully Functional. Terre Haute, IN: Creation Research Society Books.

    Darwin, Charles. 1958. Origin of Species. (reprint of 6th edition). New York: Mentor.

    Scadding, S. R. 1981 (May). Do ‘Vestigial Organs’ Provide Evidence for Evolution? Evolutionary Theory, Vol. 5, pp. 173-176. 

Wednesday, April 18, 2012

Eyespots and Innate Avoidance of Them

            Eyespots are defined as light-sensitive, pigmented spots on the bodies of organisms. Often found in invertebrates, these rudimentary “eyes” allow the organism to sense their environment and are thought to be the ancestor of the mammalian eye. A false eyespot (also known as an eyespot, too) is a rounded, eyelike marking on the bodies of organisms to mimic an eye.
False eyespots are often found on insects, such as butterflies and moths, but not just on their adult forms, but their larvae, too. Both serve the function to discourage a hungry predator by fooling that predator into thinking the insect is actually another predator. This is a unique form of camouflage, called mimicry, because the prey is not just trying to blend in, but trying to scare off the predator. This form of mimicry is known as Batesian mimicry, in which an edible species evolves to physically resemble an inedible one (whether it be a predator or something colored, i.e. king snake mimicking a coral snake). These predators include birds, snakes, lizards, and some mammals that are usually small and are prey for other, eye-containing predators. Biologists are quite sure that these eyespots are used to discourage predation, because they’re often only disturbed when the larva is disturbed or detected by a potential predator. And, logically, the larvae that can scare off a predator will be able to live and potentially make it to adulthood and reproduce. Those without the eyespots were more likely not to scare off their attackers, and their genes were potentially lost, thus, selection acted in favor of eyespots because it increased fitness of the insects. Oftentimes, on the larva, the eyespots are on the tail for better maneuvering if they are discovered by a predator on the leaves.
There has been an additional theory proposed, however, that the predators’ avoidance of the eyespots are innate – hard-wired, genetic-based behavior the result of natural selection the same as the evolution of eyespots – rather than learned by each individual predator via trial and error. Janzen et al. say this is the case because if these mid-level predators didn’t immediately avoid eyes of their predators (i.e. owls, snakes, large animals, etc.), that mid-level predator was dead; no chance to pass on genes. So, the ones who survived must have some innate response of flight when that stimulus appears. Thus, the caterpillars and other insects may have evolved in great numbers in response to this realization about their predators.
So the question is, if this is co-evolution, is the relationship between caterpillars with eyespots and birds (for example) still evolving because much of insect class has eyespots and it has worked for millions of years.


Evolution of Mammalian Tails and the Coccyx

Most of us know what a tail is until we try to define it. From a biologist’s and anatomist’s point of view, a tail is a bony extension from the sacrum and coccyx at posterior end of an organism’s body. The sacrum and coccyx, however, are a part of the vertebrae, so what about invertebrates? Do they have tails? Well, no, not really. They have tail-like appendages because there is no bone in them (no matter how hard you think a scorpion’s “tail” is).
            So, what prompted the evolution of the tail? How come mainly vertebrates have tails? Was it acquired and then died out?

            Tails are first and foremost functional. For kangaroos, it’s used in maintaining balance and steering. Birds’ tails are also used for steering, mostly. Some birds (i.e. widowbirds and peacocks use their tails in mating displays). In boa constrictors, the tail (which is most of its body) is used to strangle other animals. In most marine life (and snakes), tails are used for locomotion. In many monkeys, tails are used as another appendage, using it to grab objects out of reach. In livestock, other than shooing insects, there is debate about the tail function, mainly because horses can have their tails shorn to no adverse effects on their existence. Then again, the horses with shorn tails are in captivity instead of in the wild. There is thus strong argument that tails evolved due to do environmental demands, and the similarities on the different continents is due to similar environmental demands. Additionally, tails are thought to be leftover from the reptilian ancestors of mammals and continued their function.
            One type of tail is the prehensile tail, or one that can grasp or hold objects, found in mainly New World monkeys and all snakes (a prehensile body). A tail is partly prehensile if cannot be used to pick up objects, but rather anchor an animal usually in a tree or in climbing, seen in the prehensile-tailed porcupine (New World animal) and opossums. Why is it only found in the New World? Some scientists argue that the forest is more dense in the New World, and so ability to maneuver quickly through this dense foliage was granted to those who had an extra, functional appendage. Additionally the absence of gliding vertebrates in the New World forests, seen in the less dense forests elsewhere promote this dense-forest explanation. Additionally, there was isolation. When Pangaea split, Australia wasn’t the only place isolated. The Americas were essentially a large island, and even with the Bering Straits being iced over, the prehensile tails of primates in South America were still able to evolve without too much outside interference. Tails evolved differently to suit environmental needs, but all were the same in that they were a bony outgrowth of the spinal column, making it necessary to have vertebrates to have a true tail (why invertebrates do not have a true tail). 
The coccyx in humans is thought to be the remnants of a tail we once had, due to its location and fusion, yet the “tails” that some are born with do not contain bone, so are not true tails. The coccyx in other vertebrates is the beginning of tails, where the coccyx is not fused, unlike the wax it is fused in us. The coccyx could’ve evolved for other reasons, and is not be useless as many vestigial traits are deemed. The coccyx serves as an anchor for ligaments, and muscles like gluteus maximus and levator ani muscles, which converge from the ring-like arrangement of the pelvic (hip) bones. This forms a bowl-shaped muscular floor of the pelvis called the pelvic diaphragm or pelvic floor. This structure is necessary for keeping lower abdominal organs in our abdomen as well as activities such as defecation, urination, sitting down, and ambulation (walking).  There is debate about our tailbone being the remnants of a tail, but, in the meantime, we can look at it as a necessary structure for bipedalism, because it anchors the muscles needed for us walk upright. Therefore, it most likely contributed more to our bipedalism than anything else. The question is, if the coccyx is indeed the remnants of a tail, did we give up a tail in order to walk upright? The last bipedal animals with tails were most likely the dinosaurs and it is believed our mammalian ancestor walked an all fours.

Lemelin, P. Comparative and Functional Myology of the Prehensile Tail in New World Monkeys.
German, Rebecca. The Functional Morphology of Caudal Vertebrae in New World Monkeys.

Tuesday, April 17, 2012

High Five? Why Not High (# Other than 5)?

Why do humans only have 5 digits on each hand and foot? How did having 5 or fewer digits per hand become "the norm" in most modern-day jawed vertebrates? (Hox genes probably came to mind, but I won't delve into that discussion here...)

Scientists believe that the condition of having no more than 5 fingers or toes most likely evolved prior to the divergence of amphibians and amniotes. Fossil evidence from 360 million years ago reveals that before this evolutionary split, there existed tetrapods that possessed limbs bearing arrays of 6 to 8 digits. Reduction from these polydactylous (polus= many, daktulos= finger) patterns to the common arrangement of 5 or fewer digits coincided with the evolution of sophisticated wrist and ankle joints -- sophisticated in the sense of greater number of bones present and complex articulations.

It is believed that animals bearing 6 or more digits had simple limb skeletons, comparable to that of modern-day cetaceans (e.g., dolphins and whales). The reasons for digit number reduction may be related to the functional demands of simple "walking limbs." For example, a whale fin does not provide a platform for an efficient "push-off," whereas walking limbs allow some rotation relative to the limb bones as the body moves forward.

All modern day tetrapods possess limbs characterized by 5 or fewer digits, with 5 being the more prevalent number. The interesting question here is: is there good reason that 5 digits (rather than 4) was functionally and biomechanically preferable to the common ancestor of modern tetrapods?

No, there was not.

But why not? Well, many tetrapod species have reduced their number of digits further. An interesting observation is that while digit numbers can be reduced, they rarely increase. This is a prime example of the wisdom in evolution that it is much easier to lose than to regain something. There are actually no truly six digit limb examples for us to study. Furthermore, while individuals of many species - mice, chickens, dogs, cats, and humans - carry mutations that result in polydactyly, having more than 5 digits has never been adopted as a norm throughout the evolutionary history of the common ancestor of tetrapods to modern tetrapods.

 The panda's "extra" thumb (marked rs in the right figure) looks like a thumb and acts like a thumb, but is not homologous to the human digit that makes our hands so useful and contributes so much to our dexterity.  The panda's thumb is just an enlarged carpal bone, not a true digit.

The panda may come to mind when you think of an animal with 6 digits. However, the extra finger is not truly a digit, but rather a modification of the wrist bones. The psuedo-digit is actually an enlarged carpal bone and not really an extra thumb.

Interesting side note: Hand-Foot-Fenital syndrome is a rare condition in which the urogenital tract and limbs are malformed. The mutation is an example of how a single gene can influence more than one phenotypic trait.

1. Coats, Michael. "Why do most species have five digits on their hands and feet?" Scientific American. 25 April 2005. Scientific American, Inc.
2. Tabin CJ. Why we have (only) five fingers per hand: Hox genes and the evolution of paired limbs. Development. 1992;116:289-296. 

Will we be able to regrow our limbs?

Wouldn’t it be great if humans could regenerate their limbs? If it were possible for humans to “regrow” their limbs, then amputees would have no more worries. In a short time, they would not appear to be amputees any longer. They would have their old lives back. Already, there are some animals that have this ability. Salamanders, for instance are known for their ability to regrow limbs. If we could somehow harness their ability to regenerate, the world of prosthetic limbs could be a thing of the past.

In order to begin to figure out how to make humans regenerate limbs, we have to first understand how organisms like salamanders do it. When a salamander loses a limb, a “tumorlike” group of cells called a blastema forms where the limb was removed (Humphries). In only three weeks, this group of cells can form a fully developed limb (ScienceDaily). It was originally thought that the blastema was composed of pluripotent cells (cells that can differentiate into any type), like stem cells. However, in 2009, a group of scientists discovered that this is not the case. Actually, the cells retain their original identities, muscle cells regenerate the muscles, bone cells regenerate the bones, etc. This greatly simplified the matter for scientists as it revealed that the presence of pluripotent cells was not necessary for limb regrowth. In fact, the regeneration of a limb is very similar to the healing of cuts or broken bones (Humphries).

However, this brought up the question: If salamander limb regeneration follows a similar mechanism to human wound repair, why can’t humans regenerate their limbs? Actually humans do have this ability in our earlier stages of development. Fetuses actually have the ability to regrow limbs, and young children can regrow fingertips. But why is this ability lost with adulthood? I found an interesting segment from show on the Science Channel that goes into more detail on the subject.

In case you don’t feel like watching it, basically, it has to do with the extracellular matrix. The extracellular matrix is what carries information to the cells to effectively tell them what they should be doing: growing, moving somewhere else, differentiating, etc. In salamanders, the nature of the blastema allows the extracellular matrix to keep contact with the cells on the outermost of the wound. In humans, we don’t have this blastema, so the extracellular matrix is cut off from the outermost part of the wound, preventing the message of regeneration from getting to the outermost cells. This problem was solved by a scientist who decided to try isolating the extracellular matrix from pig bladders and applying it to the site of his brother’s severed finger. Lo and behold, the fingertip was able to regenerate in a few weeks, fingernail and all, a feat that was previously unheard of in older men. This scientist is particularly optimistic regarding the possibility of extending this to include the regeneration of an entire hand.

Finger regeneration before and after from the above video
Now, here comes the evolutionary biologist’s question: why would a salamander maintain contact with between its wounds and extracellular matrix while we don’t? What evolutionary process led to that difference? Well, the only explanation I could specifically find was that because salamanders are amphibians, their cells need to retain flexibility for metamorphosing (“Why can’t we…?”). However, I wasn’t satisfied with this explanation because while this explanation addresses why juvenile salamanders retain fetus-like flexibility, it does not fully explain why adult salamanders do not lose this ability just as adult humans do. So, I was thinking about it and I think I came up with a possible explanation. Because amphibians have evolved to be both aquatic and terrestrial animals, their skin has evolved in a different way than human skin. Amphibians must make sure to have their skin constantly moist while human skin is dry most of the time. Perhaps it is this necessity that fostered the development of a way to keep the cells always surrounded by the extracellular matrix. This is of course just my own speculation, so if you have another explanation that better explains it, please let me know!

Fortunately, we have made great strides in the study of regeneration technologies in the past few years. Hopefully, it won’t be long before we’ll only see prosthetics in the history books.    

Humphries, Courtney. “A Limb Regeneration Mystery Solved” 2009.

Science Daily. “Salamanders, Regenerative Wonders” 2009.

“Why can’t we regenerate limbs like other species?” 1999.