Eyes in the back of your head: the mysterious sensory organ of lizards

By Kelly Davis (Scripps College) and Alex Mauro (Claremont McKenna College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

When you were young, did your mother ever tell you that she has eyes in the back of her head? You probably thought that there is no such thing as eyes in the back of your head.  Well, it’s time to reevaluate that perception because there are reptiles that can prove you wrong. Indeed, many species of lizards and the tuatara have just that—an eye on the top of their heads known as a parietal eye (Figure 1).  However, this eye can’t “see” in the same way that we generally think an eye does. In fact, its function remains enigmatic, although researchers have been trying to uncover the true purpose of this photosensory organ for many years. Theories on the subject abound, but no conclusive evidence exists to confirm any of them. In 2010, Antonieta Labra and colleagues set out to further investigate these ideas and figure out the exact function of this eye. The group conducted various comparative studies that investigated the possible functions of the organ in relation to climate, thermoregulation, and thermophysiology in an attempt to conclusively determine the function of this mysterious sensory organ.

As of yet, little is known about the parietal eye regarding its function and evolutionary history, but there is a general consensus regarding its basic physiology. The parietal eye is known to be part of a larger sensory system, which also includes the pineal complex. This organ’s job is to help regulate circadian rhythms and seasonal cycles. The pineal complex has been retained in all vertebrates except crocodilians and a few mammals whereas the parietal eye has been lost in almost all clades except the lizards and tuataras. Notably, the parietal eye has been lost in snakes, which evolved from a common ancestor to lizards. So why do lizards still have the parietal eye? Why do they still have a sensory organ that is no longer necessary for other vertebrates, or even other reptiles?

Figure 1. Dorsal view of the head of Anolis carolinensis. The parietal eye is in the center.

As aforementioned, there is a long history of research on the parietal eye before Labra and colleagues started their investigation. For example, Foa and colleagues suggested that the parietal eye can function as a “time-compensated sun compass,” meaning it can be used to orient the animal based on the intensity of the sun’s rays. This was consistent with prior evidence that suggested the parietal eye was sensitive to polarized light. Foa and colleagues tested their hypothesis with a truly fascinating study in which they trained iguanas to swim through a maze to hidden targets at different times of the day. There were no forms of visual cues other than the sun, so the lizards were forced to use the sun as a guide. Furthermore, they conducted the trials at different times of the day to demonstrate that the lizards could account for different levels of sunlight (time-compensated).  Although the lizards were successful in finding the targets, Foa et al. concluded that more evidence was needed to support this hypothesis.

Tosini and Menaker provided another hypothesis regarding the parietal eye function and carried out a study in which they surgically removed the parietal eye from iguanas in order to test its thermoregulatory capabilities. They uncovered some useful data, namely that thermoregulation was only slightly affected by the removal of the parietal eye. However, they also found that when they removed the entire pineal complex the results were a lot more dramatic: the iguanas could no longer thermoregulate properly at all.  Thus, they left the door open for further investigation of the thermoregulation hypothesis.

Labra and colleagues’ 2010 study probed further into these and other hypotheses of thermoregulation and orientation by sampling the parietal eye sizes of individuals from the genus Liolaemus (Figure 2) and comparing these values to aspects of the lizards’ habitats. Liolaemus was chosen because it is a genus that is found in many different climates and thus could provide a lot of data on many different habitats while still maintaining a high degree of relatedness between individuals (it’s difficult to compare very different species with accuracy). Additionally, the phylogeny for this genus is fairly well established, which was constructed using Bayesian methods. Labra et. al.’s study was different than previous ones because it took a comparative approach in examining the function of the parietal eye. Instead of directly testing its function (e.g. a maze or by vivisection), parietal eye size was measured with the idea that larger eye size would indicate increased function/importance. This eye size was then compared to the individual’s altitude, environmental temperature, and territory size.  It was hypothesized if the parietal eye had thermoregulatory capabilities, it would be used more in populations that lived at higher altitudes where light levels varied more in order for the lizards to properly thermoregulate. Hence, individuals at higher altitude should have bigger parietal eyes.  It was also hypothesized that larger territory size would require more orientation abilities, meaning bigger eyes should be found in individual with larger territories. Lastly, the temperature of the lizard’s habitat was also examined to see if thermophysiological correlations could be found.

Figure 2. Liolaemus tenuis.

With these previous studies and hypotheses in mind, Labra and her colleagues took on this ambitious quest. After sampling thirty species of Liolaemus, they found several correlations, but still no conclusive answers. Parietal eye size was not significantly correlated with altitude, but there was a correlation between eye size and the minimum temperature of a population’s environment. Before moving on, it is important to emphasize the correlations that were attempting to be made. Altitude was attempted to be representative of a greater range of temperatures in the lizard’s environment whereas temperature examined the maximum, minimum, and average temperatures of the lizard’s environment. Lower minimum temperatures occur at higher altitudes so their approach appears to be slightly convoluted and more direct data is needed to make significant correlations. More data was also needed to make conclusions about parietal eye size and territory size (which meant the orientation hypothesis could not be supported). Ultimately, the most concrete evidence gathered was not between parietal eye size and environmental factors at all, but simply between eye size and species: The parietal eye size variance between species was significantly lower than would be predicted if parietal eye mutation was random. Labra’s data also suggested that there was an absence of phylogenic affects. Combined, these facts indicate that the parietal eye is constrained evolutionary because of function or because of many evolutionary changes in a limited sampling range. In a nutshell, we still do not know the exact function of the parietal eye, and there is very limited data which indicates it has a function at all. The thermoregulation hypothesis seems to be the most likely, but it still needs further investigation. However, Labra and her colleagues finished the study by suggesting that the data they did obtain on orientation was promising enough to deserve more investigation and that this seems like the most likely function of the parietal eye as of now.

References

Foa, A., Basaglia, F., Beltrami, G., Carnacina, M., Moretto, E., & Bertolucci, C. 2009. Orientation of lizards in a morris water-maze: Roles of the sun compass and the parietal eye. Journal of Experimental Biology, 212(18): 2918-2924.

Labra, A., Voje, K. L., Seligmann, H., & Hansen, T. F.  2010. Evolution of the third eye: A phylogenetic comparative study of parietal-eye size as an ecophysiological adaptation in liolaemus lizards.  Biological Journal of the Linnean Society, 101(4), 870-883.

Tosini, G., & Menaker, M. 1998. Multioscillatory circadian organization in a vertebrate, iguana iguana. Journal of Neuroscience, 18(3), 1105-1114.

The Echolocation Gene

By Sophie Wang (Pomona College) and Arthur Levine (Pitzer College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

Prestin and the convergent evolution of high-frequency hearing

Hearing, one of the most important senses, is a specialized form of mechanosensation that allows an organism to perceive and respond to sound waves in air and in water. Many animals have the ability to hear, but the sensory input is not utilized in the same ways for all. Some animals have evolved the specialized ability to use sound to produce images of both themselves and other objects within a setting, much as humans use vision.

Thisability is called echolocation. The neural mechanisms for echolocation are not all understood, but echolocation is basically a process that involves the emission of high or low frequency calls into the environment combined with the perception of the reverberations of those calls off objects in space (prey, predators, walls, water, trees, boats). Organisms that have independently evolved sophisticated echolocation include bats and toothed whales. Both clades have been shown to have developed such sensitive echolocation as to make it indispensable for orientation and food foraging (Ying et al 2010). Given that bats and toothed whales are very distantly related one wonders if the echolocation systems may function under fundamentally different mechanisms.

In a recent paper published in Current Biology, Li et al. tackle this intriguing example of convergent evolution from a molecular perspective. Before moving on, though, we should touch a little on evolutionary biology, and biology in general. The central reductionist view of biological behavior is that all behaviors can be attributed to isolatable biological mechanisms of physiology, genetics, protein molecules, and enzymes. Therefore, it is a valuable endeavor when studying a behavior in biology to attempt to gain an integrative understanding of the mechanisms. The central dogma of molecular biology states that a sequence of DNA leads to a sequence of RNA which leads tocertain proteins which lead to structuring and function allowing for complex mechanisms that can lead to certain behaviors. In addition, biology can be viewed on a macro and generational time scale, which is especially pertinent when looking at evolution over time. For example, we can use phylogenetic trees based on morphological and molecular data for species to determine their relationship to each other in evolutionary time. We can overlay what we learn about comparative molecular biology with the macro-perspective to learn about the relationships between the evolution of behaviors, via the evolution of molecules over time in various species.

In their Current Biology paper, Li et al. studied a specific protein called prestin which is found in the cochleas of most mammals, including humans. The prestin protein is present in the outer hair cells of the cochlea, which are important in amplification of sound but not in actual transmission of soundwave-triggered signals to the brain.

The authors looked specifically at the intriguing relationship between the prestin sequences of two echolocating mammals: bats and toothed whales. Bats and whales, even though both mammals, are not exactly close relatives, but both groups rely heavily on complex echolocation in their lives. How then, if at all, are the prestin sequences of the two groups related? Does prestin play a role in echolocation?

In order to answer these questions, the authors first determined the relationship between the prestin genes of various species. They began by taking the genetic sequences for prestin of 26 different mammal species and constructing a protein tree showing their relationships to one another. They then compared this tree with a well-supported, more general phylogeny constructed from a large aggregate of information. To their surprise, the prestin tree grouped the bottlenose dolphin within the microbats (Figure 1)! (Basically, this means that the genetic sequence for prestin in the dolphin is more similar to bats than to other animals). As you can see from the general phylogeny, dolphins are much more truly closely related to a number of species (including cows) than they are to bats (Figure 2), which makes the closeness of the prestin relationship between bats and dolphins that much more intriguing!

Figure 1: Prestin protein tree showing that the prestin sequences of the echolocating dolphins (in green) are much more closely related to the sequences of the echolocating bats (in black) than to the sequences of non-echolocating whales (in blue) and cows.

Figure 2: General phylogeny of mammals showing distant relationship between whales and bats. Whales are outlined in blue and bats are outlined in red.

So how could these prestin sequences have become so similar? In order to determine this, the authors looked at a number of the possibilities that may have caused this to occur only by coincidence in two echolocating groups, including horizontal gene transfer, DNA contamination, and a host of other genetic mechanisms. However, they found that none of these were likely, meaning that the only remaining explanation was a convergence of the sequences resulting from selection of mutations that are beneficial in echolocation. In simpler terms, they found that there was a connection between the convergence of the prestin gene and the deployment of echolocation.

Li et al. went on to further test this by comparing the different parts of the prestin code that are altered in the different species they studied. Again, they saw that some of the same areas were mutated across the echolocating species. Next, the researchers looked at where these altered sites were located within the complete and folded prestin protein. Prestin is an integral membrane protein, and they found that most of the mutations occurred in the extracellular parts of the protein. These parts of the protein are critical in changing the conformation of prestin, and having quick enough conformational changes is likely very important in being able to process the extremely high returning frequencies of echolocation. So we see that prestin is related to echolocation not through the production of sound (as whales and bats do this very differently), but through the collection of sound!

There is little controversy on the story being told by the authors of this paper, though the sample is relatively small (26 species). Their work is very exciting not because it contradicts other work, but because they were able to document an amazing example of convergent evolution at the molecular level (Shen et al 2011; Liu et al 2010; and Rossiter et al 2011).  This protein becomes significantly more interesting because it exists in lots of species, but may have independently conferred on some the ability to have sensitive echolocation. Other studies have shown convergent evolutions of the prestin protein in other bats and confirm the authors suggestion that prestin analysis groups echolocating bats and whales (Ying et al 2010).

Understanding more deeply the molecular underpinnings of the profound system of echolocation is important for learning more about the evolution of such an interesting mechanism. Studying why the molecule prestin is similar in species that are not actually so similar could give great insight into rules for molecular conservation in evolution and function of the protein in a sensory organ. In addition, understanding prestin and its molecular structure and how said structure evolved could give more insight into how it confers its function within the organism. Finally, learning more about the structure and function of this protein and how it may influence other species who normally do not produce it (mice, rats, monkeys etc) may allow us to determine if it has clinical capabilities in recovering hearing deficits in humans, as it is implicated in some hearing loss (Liu 2003). Believe it or not, some humans have gained (some) ability to echolocate after sight loss. Might these rare humans also express a similar form of prestin to bats and whales?

References

Li Y, Liu Z, Shi P, Zhang J. The hearing gene Prestin unites echolocating bats and whales. Current Biology Vol 20 No 2, 2010.

Shen B, Avila-Flores R, Liu Y, et al. J Mol Evol. Prestin shows divergent evolution between constant frequency echolocating bats. 2011 Oct;73(3-4):109-15. doi: 10.1007/s00239-011-9460-5. Epub 2011 Sep 24.

Liu Y, Cotton JA, Shen B, et al. Convergent sequence evolution between echolocating bats and dolphins. Current Biology 2010 Jan 26;20(2):R53-4. doi: 10.1016/j.cub.2009.11.058.

Rossiter SJ, Zhang S and Liu T. Prestin and high frequency hearing in mammals. Commun. Integr. Biol. 2011 Mar-Apr; 4(2): 236–239. doi: 10.4161/cib.4.2.14647

Xue ZL, Ouyang XM, Xia XJ, et al. Prestin, a cochlear motor protein, is defective in non-syndromic hearing loss. Hum. Mol. Genet. (2003) 12 (10):1155-1162.doi: 10.1093/hmg/ddg127

Were Your Ancestors Ticklish?

By Acacia Hori (Pitzer College) and Martha Kresz Bierut (Scripps College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

For hundreds of years, we have assumed that the tickle is a sensation that only occurs in humans.  Recently, scientists have begun to question whether that is indeed the case, and their experiments have shown that tickling produces behavior analogous to human laughter in some non-human species.  These researchers have suggested that laughter induced by tickling is a common trait amongst hominids, which includes chimpanzees, apes, gorillas, orangutans, and humans (Ross et al. 2009).

Tickling-induced laughter is not limited only to species within the family Hominidae.  In a paper published in the journal Behavioural Brain Research in 2000, Jaak Panksepp and Jeffrey Burgdorf at Washington State University and Northwestern University suggested that rat laughter can be elicited by tickling also.  When tickled by an experimenter, rats produce chirps at the same frequency as the chirps they produce during pleasurable social interactions, such as play.  In their experiment, Panksepp and Burgdorf found that tickled rats made this noise significantly more frequently than non-tickled rats.  The experimenters measured this sound, which is beyond the range of human hearing, by slowing the chirps down with a heterodyne bat detector.  This suggests that tickle sensation and laughter are traits that occur more widely among the animal kingdom than we previously thought.

Could there be an evolutionary connection between the tickle-induced laughter of different species? Is tickling homologous or homoplastic?

Panksepp and Burgdorf, along with other colleagues, have identified a genetic basis for tickle-induced laughter in rats by selectively breeding for and against the laughter response to tickling.  In their experiment, it took four generations of breeding for the offspring to “laugh significantly more” and nine generations for the offspring to “laugh significantly less” when compared to randomly bred counterparts (Panksepp et al. 2001). This means that laughter elicited by tickling is an example of what we call a “heritable trait,” or a trait that can be passed down from generation to generation through DNA.  Furthermore, this suggests that tickle-induced laughter has an evolutionary basis and cannot be classified entirely as learned behavior.  It also opens up the possibility that ticklish animals have a common ticklish ancestor.

This idea is further supported by evidence that humans and other mammals share brain structures and neural mechanisms involved in tickle-induced-laughter, including the reticular nuclei of the thalamus, hypothalamus, and midbrain (Panksepp and Burgdorf 2000).  These homologous structures point toward a common ancestral source of the tickling-laughter mechanism.

In addition to comparing their brain structures, researchers are looking at brain chemistry to determine how related human and rodent laughters may be.  They have found that a common neurotransmitter, glutamate, is necessary to trigger a laughter response to tickling.  They tested this by administering different drugs in low dosages to block specific neurotransmitter receptors in the brain.  The drug that worked the best to eliminate the chirping was MK-801, an antagonist of the NMDA receptor, which is sensitive to glutamate.  They confirmed this result by administering glutamate directly into the rat hypothalamus and observing the laughter that resulted (Burgdorf 2008).

While nothing is certain yet about whether these high frequency chirps are homologous to human social-emotional response systems, scientists are certain that through more research, these rat chirps may reveal more about the physiology of laughter, joy, and perhaps even positive emotional consciousness in the brain.

Why would the tickle be a useful evolutionary trait?

There are a few hypotheses out there that address this question.  Some think that tickle induced laughter goes back in brain evolution to a time when social interaction was mediated by the production and sensation of simple acoustic signals.  Additional work implies that tickling may be an important social mechanism for many mammalian species related to joyfulness and play, but “the vocal component of this state may have diminished through negative selection in the young of many other species” (Panksepp and Burgdorf 2010).  This means that many mammals may enjoy play and even tickling, but the vocal cues comparable to human laughter may be very different or absent.  The negative selection of the vocal reaction to joy may have occurred in mammals that experience predation, as the noises could have attracted predators.  As far as the tickle being a useful evolutionary trait, it creates a “joyful form of affective consciousness within the human brain” (Panksepp and Burgdorf 2010). The act of tickling releases endorphins and hormones that are beneficial to the mental state of humans, and its possible presence in other species points toward a similar effect.

Why does any of this matter?

The quest for the origin of laughter may seem rather inconsequential, but researchers believe that if laughter is homologous across species, its occurrence could be useful in future research in the field of medicine by providing new ways to approach phenomena such as joy and depression.  In his 1998 article, Panksepp postulates that, “depressed individuals laugh and play less than normal; the elucidation of neurochemistry that promotes chirping and playfulness in rodents may help guide development of new types of antidepressants” (Panksepp 1998).  This is just one example of the many ways in which a homologous animal model for joyful laughter could be useful, and a few others may include the study of human emotion in a broader sense. In summary, Panksepp acknowledges that what we know now is not concrete, but it is certainly intriguing, “We suspect that brain circuits of human laughter and the neural underpinning of rodent chirping do interconnect with brain areas that mediate positive social feelings, but the locations of those areas remain unknown. In sum, although we would be surprised if rats have a sense of humor, they certainly do appear to have a sense of fun.”

References:

Burgdorf J, Panksepp J, Moskal JR. Frequency-modulated 50kHz ultrasonic vocalizations a tool for uncovering the molecular substrates of positive affect. Neurosci Biobehav Rev. 2010 Dec 7.

Panksepp J, Burgdorf J. Laughing rats? Playful tickling arouses high frequency ultrasonic chirping in young rodents. In: Hameroff S, Chalmers C, Kazniak A, editors. Toward a Science of Consciousness III. Cambridge, Mass: MIT Press, 1999:231–44.

Panksepp J, Burgdorf J, Gordon N. Towards a genetics of joy: breeding rats for ‘laughter’’. In: Kaszniak A, editor. Emotions, qualia, and consciousness. Singapore: World Scientific; 2001. p. 124 – 36.

Davila Ross, Marina, Michael J Owren, and Elke Zimmermann. “Reconstructing the Evolution of Laughter in Great Apes and Humans.” Current Biology 19.13 (2009): 1106-111.

Why are Wobbegongs such good Predators?

By Morgan Halley (Scripps College) and Kendall Kritzik (Scripps College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

There are a number of different sensory receptors found in animals. They range from photoreceptors, chemoreceptors, mechanoreceptors, electroreceptors and magnetoreceptors. The mechanosensory lateral line (MLL) is a common feature that is found in all species of fish and most aquatic amphibians (Figure 1).

Figure 2. The lateral line system.

The MLL is a line of mechanoreceptors located very close to the skin surface, and can detect water movements in relation to the skin surface; however it can only detect movements that are within a few centimeters from the skin. It is thought that the MLL is responsible for many behaviors seen in fish and amphibians, including prey detection.

The Elasmobranchii are a subgroup of Chondrichthyes, the cartilaginous fish that includes rays and sharks (Palmer, ed., 1999). The MLL of elasmobranchs are composed of four organs: the vesicles of Savi, spiracular organs, canal neuromasts, and pit organs. Studies of the MLL in elasmobranchs have focused on the canal morphology and topography for rays with very few studies that look at the canal systems in sharks. Because of the large variation between sharks and rays, it is not known if findings of ray MLL systems can be applied to sharks. There have also been very few studies that look at pit organ morphology and distribution.

The wobbegong sharks (Orectolobidae) are part of a unique group of Elasmobranchii that differ from other sharks in terms of shape and ecology. They have a compressed body and live on the seafloor, which is often seen in rays (Figure 2). Wobbegongs also employ sit-and-wait ambush feeding that is very rare, not only among sharks, but among elasmobranchs in general. Taking all these differences into account one should expect fundamentally different sensory adaptations in wobbegongs, and that’s what Theiss and her colleagues went after in their study on the MLL.

Figure 2. The Spotted Wobbegong Shark, Orectolobus maculatus.

Theiss et al. examined the morphology and spatial arrangement of the MLL system in two species of wobbegong shark, the spotted wobbegong Orectolobus maculatus and the ornate wobbegong Orectolobus ornatus.  These two species spend the majority of their time on the sea floor, ready to ambush fish and cephalopods both during the day and at night. The MLL morphology and distribution was hypothesized to be specialized in wobbegong sharks due to their unusual feeding strategy. The morphology of canal neuromasts was described along with the location of the lateral line canals and the distribution and number of canal pores in both species (Figure 3). The pit organ distribution and numbers were only reported for O. ornatus, because they have been previously reported for O. maculatus (Peach 2003). The MLL topography was then paired with biological and ecological functions.

Figure 3. Schematic drawing of the fine structure of the lateral line system.

In order to study the MLL, four members of O. maculatus and O. ornatus were examined for lateral line pore and canal distribution, neuromast and pit organ morphology, and pit organ distribution (O. orectus). The animals were euthanized, and the heads removed. The skin was removed from the dorsal and ventral sides of the head, and the MLL canals were stained using 0.05% Methylene Blue. In order to see the smaller pores, a dissecting microscope was used.

Pit organs could be identified by the two enlarged denticles (tiny scales) on either side of the organ. The head outline and locations of the canals, pores, and pit organs (O. orectus) were traced onto transparencies and scanned into a computer. The canal pores and pit organs were counted for each individual, and averaged based on pore type. These numbers were analyzed using two-tailed t tests. Pore and canal maps were made using Adobe Illustrator CS3 (Adobe) and a digital drawing tablet.

Skin samples from each individual were dissected from each lateral line system with the exception of the nasal and prenasal canals. The samples were bisected through the middle and examined with a scanning electron microscope in order to view the canal neuromasts. Samples containing dorsolateral, spiracular, and mandibular pit organs were also removed from three individuals of each species. The samples were decalcified and the denticles removed. The samples were mounted and examined for width, length, hair cell kinocilia and stereovilli length, and microvilli length using a scanning electron microscope (Figure 3).

It is known that, in elasmobranchs, pored MLL canals detect information on external water acceleration, as neuromasts are directly in contact with the external water environment (Figure 3). Theiss and her colleagues found that, in both species of wobbegong sharks, the pored MLL canals were located predominantly on the top of the head. This distribution is nearly identical to that of the Japanese wobbegong shark Orectolobus japonicas. Behaviorally, they speculated that this arrangement facilitates ambush predation; wobbegongs feed on fish and cephalopods at night, and so the dorsal (top) arrangement of mechanosensory canals and pores would allow them to easily detect and accurately strike at prey swimming in front and above them. Because of this mechanosensory system, the sharks would not have to rely on limited vision during the night.

Interestingly, non-pored canals do not detect external water acceleration directly; neuromasts respond instead to internal fluid velocity that is caused by skin movement. Theiss and her team found that the canals behind the eye and just before the nose of both wobbegong species are non-pored canals. This canal type is known to exist ventrally in stingrays and aid them in capturing benthic (bottom-dwelling) prey (Wueringer and Tibbetts 2008). Although wobbegongs are themselves a benthic species and feed on non-benthic prey, the authors proposed that the dorsal position of their non-pored canals optimizes their tactile sensation while feeding in a similar manner; if the shark bumps into prey on these surfaces while swimming or while striking (perhaps they identified the prey item via small water currents which they would register with dorsal pored canals), the receptors will immediately recognize the prey’s location and enable more accurate striking.

The discovery of non-pored canals in the two wobbegong species studied contradicted findings in a study of the Japanese wobbegong, in which the same canals are pored. This shows that even among species of the same clade, there are structural differences. It is therefore important to sample across a broad phylogenetic spectrum. Additionally, the hyomandibular canal (running parallel to the jaw) in the two species studied was located dorsally on the head, rather than ventrally (as commonly seen in sharks) or ventral-dorsally (as seen in batoids; Wueringer and Tibbetts 2008). This change may be a result of the sharks’ compressed head morphology. Another notable difference in the wobbegong sharks is the non-continuous distribution of canals with neuromast sensory tissue; in other elasmobranch species, sensory tissue is either continuous or near-continuous. The authors speculated that this may indicate a decrease in sensitivity in the wobbegong sharks, although it may be possible that neuromast tissue was damaged or hard to visualize in the prepared specimens. As such, further examination is needed to confirm this observation.

Some problems that arose during this study had to do with the technical difficulty of finding the pores of the lateral line canals. The authors noted that the pores have extremely small diameters and could be easily missed – particularly in the nasal region which has dense connective tissue. Additionally, the fixation process in this study damaged the fragile neuromast structures and prevented a thorough examination of their morphology; no remnants of the cupula (Figure 3) were present for analysis. It would be advantageous to fine-tune the fixation procedures so that these structures are not damaged, as characteristics of the neuromast (such as hair cell orientation) could provide useful information on directional detection of water flow. Most importantly, Theiss et. al speculated that the MLL of wobbegongs is an adaption to benthic lifestyle; however, the study design examined only two species, both of which were wobbegong sharks. It would be beneficial to confirm this speculation with studies examining the MLL of a wide variety of sharks and rays (Garland and Adolph 1994). Finally, further behavioral studies should be done to confirm the specific advantages of pored and non-pored canals in these two species.

References:

Garland, T., Jr., and Adolph, S.C. (1994). Why not to do two-species comparative studies: limitations on inferring adaptation. Physiological Zoology 67:797-828.

Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 26. ISBN 1-84028-152-9.

Peach, M.B. (2003). Inter- and intraspecific variation in the distribution and number of pit organs (free neuromasts) of sharks and rays. Journal of Morphology, 256: 89-102.

Wueringer, B.E. and Tibbetts, I.R. (2008). Comparison of the lateral line and ampullary systems   of two species of shovelnose ray. Rev Fish Biol Fisheries, 18:47-64.

Shedding Light on Non-Visual Photoreceptors

By LeeAnn Louie (Scripps College) and Vanessa Ho (Pomona College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

Photoreceptors are the receptors for visual information and thus, unsurprisingly, are present in eyes. Specifically, the two types of photoreceptors, the rods and cones, are buried on the retina at the back of the eye. As light enters the eye through the pupil, the iris controls the size of the pupil to protect the eye from too much excess brightness. The light is then focused onto the retina where the photoreceptors convert the light signals into electrical ones interpretable by the brain. As important as the initial steps in the light’s pathway to the retina are, the photoreceptors are what enable us to see a range of light and colors. So their location deep within the eye seems logical. However, it turns out that there are a few examples of the occurrence of functional photoreceptors outside from proper eyes. The pineal organ in some vertebrataes is one such example. Another example is provided by Backfisch et al. 2013. In successfully labeling r-opsin (a marker for photoreceptors) in the annelid worm Platynereis dumereilii, Backfisch and his team found additional photoreceptors in the ventral nerve cord and segmental dorsal appendages, and not just in the eye-cups as previously expected. The researchers conducted many experiments related to r-opsin expression in the trunk but most surprisingly, upon decapitation, the trunk of Platynereis still demonstrated photoavoidance despite the fact the eyes had been removed.

Platynereis is a marine annelid worm that is a key species for studying eye and brain development in basal metazoans. Thus the ability to see the expression of key components in brains and eyes makes Backfisch’s experiment useful for further research in Platynereis. They performed transgenesis to co-express EGFP, an enhanced version of the green fluorescent protein (GFP), in regulatory regions of r-opsin, allowing for visibility of all photoreceptors in the annelid.  While they did find photoreceptors in the eyespots and correlated eyelets in the brain as anticipated, they also found further expression of photoreceptors on other parts of the body than the head. When they exposed a dim-light-adapted trunk to a light stimulus, the tail moved away from the light as far as 4 mm (a long distance for an animal of about 20-25 mm in total length!) even in the absence of a brain or eyes.

Figure 1. A bright light exposed to the tip of the tail (top box) induces a photoavoidance response away from the stimulus in Platynereis (lower box). Adapted from Backfisch et al., 2013

Figure 2. The four segments of the ventral Platynereis trunk. Adapted from Backfisch et al., 2013

Further investigations into the developmental genetics of these newly identified photoreceptors revealed that these non-cephalic (located outside of the head) photoreceptors do not develop under the same regulatory genes as those in vertebrate eyes. This is shown by the distinctly separate expression of r-opsin and pax6, a transcriptional regulator involved in eye development in chordates. Instead, these photoreceptors were found to coexpress dach, which is associated with eye development, pax2/5/8, which regulate neural differentiation, and brn3c, which is expressed in sensory neuron development. These findings suggest that the trunk photoreceptors may have different evolutionary origins from other photoreceptors expressing pax6, but may be related to cephalochordate photoreceptors, which are associated with dach, pax2/5/8, and brn3c.

While the genetics of non-cephalic photoreceptors can clarify developmental relations, the expression of r-opsin in zebrafish, a vertebrate, shows striking similarity to that of Platynereis. The correlation between brn3c and r-opsin expression in Platynereis prompted Backfisch et al. to label r-opsin orthologs in zebrafish, which revealed the presence of these photoreceptor markers in the neuromasts, mechanosensory cells of the lateral line associated with body orientation, and neurons of the peripheral nervous system. This suggests a developmental, and possibly evolutionary relationship between photoreceptors and mechanoreceptors. While expression in Platynereis shows the presence of photoreceptors outside of eyes, characteristic photoreceptor markers are found in other non-cephalic sensory cells in a vertebrate species.

Similar to zebrafish, photoreceptor markers are also found in mechanosensory cells in the auditory organ (Johnston’s organ) of Drosophila melanogaster. Upon screening genes involved in hearing, Senthilan and his team suggest that Drosophila r-opsin play a critical role of mechanotransduction channel gating in auditory function. Furthermore, the development of many sensory organs in Drosophila are governed by the atonal gene suggesting common evolutionary origins in which a protosensory organ eventually diversified into the many different sensory organs metazoans have today.

Backfisch et al. posits that photoreceptors and mechanorecptors have a common evolutionary origin, but Smith summarizes in his textbook that mechanoreceptors, given their presence in organisms as simple as bacteria, are the true sensory precursors. He also notes that transmembrane signalling in mechanoreceptors are present across the clades indicating a likely common evolutionary origin early on. It is unclear which hypothesis, if either, is correct. The finding that r-opsin are present in mechanosensory organs despite that these organs do not require photosensitive information presents an interesting conundrum. This area of research would certainly benefit from studies into the molecular basis of these receptors across species to better pinpoint evolutionary origins.

Though the current findings add insightful contributions to the possible developmental and evolutionary origins of photoreceptors, eyes and other sensory organs, the induction of stable transgenesis perhaps holds more research potential in Platynereis. Specifically, the present study illustrates how stable transgenesis in Platynereis can contribute to the growing field of developmental and evolutionary biology by providing an in-depth characterization of the labeled cell types under investigation. Additionally, Backfisch et al. are hopeful that other assays will also be considered when investigating the photoreceptors associated in photoavoidance or hormonal activity. This current study elucidates the “molecular fingerprint” of noncephalic photoreceptors and strives to uncover their evolutionary origins.

References:

1. Backfisch, B, et al. “Stable transgenesis in the marine annelid Platynereis dumerilli sheds new light on photoreceptor evolution.” PNAS. 110.1 2012. 193-198.
2. Senthilan, P.R. et al. “Drosophila Auditory Organ Genes and Genetic Hearing Defects.” Cell. 150.5 (2012): 1042–1054. Web. 15 Mar. 2013.
3. Smith, Christopher. Biology of Sensory Systems. 2nd edition. Wiley, 2009. 75.

Timing the Rise of a Weakly-Electric Sense

By Jesse Osborn (Scripps College) and Hillary Bruegl (Scripps College) [edited by Lars Schmitz, as part of BIOL 167 "Sensory Evolution", an upper division class at the Claremont Colleges]

The weakly electric sense of South American Gymnotiformes and the African Mormyridae is an amazing example of convergent evolution, i.e. the independent origin of similar biological traits from dissimilar ancestral states in unrelated lineages. This sense allows these fishes to not only receive electrical impulses, but also generate them. In a paper published in the journal PLoS One, Lavoue et al. examine the temporal pattern of the evolution of this fascinating sense organ in these two fish clades. Surprisingly, the timing of these separate origins is extremely similar (16-19 or 22-26 million years, depending on the calibration method) allowing for the investigation of pathways leading to evolutionarily novelty and influences of key innovations in communication on species radiation.

Electroreception is a widely distributed sense among non-teleost aquatic craniates. However among teleost fishes, it is restricted to two distantly related groups: the Gymnotiformes plus Siluriformes and the Mormyroidea plus Notopteridae. Gymnotiformes are freshwater naked-back knifefish that have an eel-like appearance. The only fin present is the anal fin, extending along most of the body. They are nocturnal and sometimes bury themselves in the substrate of deep rivers and swamps. Mormyroidea are freshwater elephantfishes and have fish-like form with an extended trunk-like mouth. The Gymnotiformes and Mormyroidea both evolved the ability to produce weak electric discharges used for electrolocation and communication in addition to having the high frequency electroreceptors that detect these discharges. The parallels between their electrical systems are extraordinary. First, they both evolved a novel myogenic electrical organ derived from skeletal muscle progenitor cells, as well as having the same sodium channel α-subunit gene duplicate. Second, the origin of high frequency electroreceptors is derived from similar lateral line precursors. Gymnotiformes and Mormyroidea also share other phenotypic similarities such as: body form, swimming behavior, reproductive behavior, nocturnal activity patterns, electric signal types, neural algorithms used to avoid jamming electrolocation and communication (Figure 1, below).

Figure 1. Morphological convergences between Mormyroidea and Gymnotiformes.

Prior to this study, no one had used a broad enough taxonomic sampling or a sufficiently large molecular data set to simultaneously and robustly estimate the ages of the origins of the electric organs in Gymnotiformes and Mormyroidea on a single tree. Lavoue et al. were able to provide comprehensive molecular evidence of phylogenetic independence but contemporaneous origins of Gymnotiformes and Mormyroidea. They saw similar amounts of time had elapsed between the origin of initial electroreception, as well as the subsequent origin of the electrogenesis organ. Additionally, they were able to place the evolution of these two groups in the temporal context of the earth and found that Gymnotiformes and Mormyroidea arose around the same time the South Atlantic Ocean formed but are unsure if their origins were before or after the complete separation of Africa and South America.

In order to derive a phylogenetic tree containing chronological origins of these electric senses, the authors used complete mitochondrial genomes as the character set. They used unique and extensive taxonomic sampling including several basal teleost species and 27 species of Mormyroidea and Gymnotiformes representing all families of weakly electric teleosts. They also used a relaxed-clock Bayesian method to infer phylogenetic relationships and divergence times simultaneously, and completed two reconstructions of the phylogeny: the first using strong maximum age constraints and the second using soft maximum age constraints.

Lavoue et al. found that the nodes representing the two independent origins of electrogenesis were supported under all data subsets and analyses. This finding was consistent with Arnegard et al’s phylogenetic hypothesis that was based off of a less complete mt-seq data set (2010). For the most part, phylogenetic relationships were also consistent with previously published hypothesis. A minor inconsistency was discovered in regards to Lavoue et al’s hypothesis that Isichthys, Brienomyrus and Mormyrus were a monophyletic group (2003). Instead they found that these three genera are sister groups of all Mormyroides. This new finding is important in creating the most accurate phylogeny and temporal scale for the evolution of this group and the electric senses.

Using these two reconstructions of the phylogeny, authors also found that the mean ages of Mormyroidea and Gymnotiformes are very similar to each other with less than a 15% difference. These origins were found to occur well after the split of the two lineages from their most recent common ancestor, between 185.7 Mya and 284.1 Mya using the two reconstructions. This finding was found to be congruent with previous paleontological evidence. Since this is such a large range, future studies could be used to narrow the range and provide more accurate timing.

Figure 2. Phylogenetic chronogram of the Teleostei using strong maximum age constraints.

Additionally, the independent origins of electrogenesis were found to occur roughly the same interval of time following the origins of electroreception (Gymnotiformes -100.2 Mya and Mormyroids-93.7 Mya). They also found that in both the Mormyroidea and Gymnotiformes, there was also a similar interval of time between the appearance of passive electroreception and the appearance of myogenic electric organ allowing for electrogenesis (Figure 2). These similar time intervals are the most interesting finding as they provide an empirically valuable case of convergent evolution, making this complete and temporally calibrated phylogeny extremely useful for evolutionary biologists studying the time period required for evolution to construct a weak myo-electric organ de novo from skeletal muscle precursors. The authors suspect that whole genome duplication occurred just prior to the radiation of teleosts contributed to the origin of novel electrogenesis in these two families.

Figure 3. The fracturing of Pangaea and the formation of the South Atlantic Ocean.

Lastly, authors were able to use the temporal scale and fit the phylogeny to the paleogeographic history of the earth, allowing them to see that Gymnotiformes and Mormyroidea diverged around the same time as Gondwana, the southerly two supercontinents part of Pangaea, was almost completely fragmented apart (Figure 3). Although the divergence can’t be firmly placed as before or after the split, the authors speculate that the two shared environmental conditions during the early Late Cretaceous that may have contributed to their contemporaneous origins. Formally, the divergence has been placed near the beginning of the separation of the two continents.

Although this framework is as accurate as can be for the information given, future studies can always improve upon the accuracy of phylogenetic relationships and time calibration. The most important improvement could be made upon the exact time intervals between the independent origins of Mormyroidea and Gymnotiformes and the intervals between each family’s origins of electroreception and electrogenesis. This information can be used to further identify the evolutionary pathways used to form novel traits and locate these temporally. This study serves as an extraordinary and scientifically valuable case of convergent evolution in vertebrates.

References

Arnegard ME, Bogdanowicz SM, Hopkins CD. (2005). Multiple cases of striking genetic similarity between alternate electric fish signal morphs in sympatry. Evolution. 59: 324-343.

Lavoue S, Sullivan JP, Hopkins CD. (2003) Phylogenetic utility of the first two introns of the S7 ribosomal protein gene in African electric fishes (Mormyroidea: Teleostei) and congruence with other molecular markers. Biol J Linnean Soc. 78: 237-292.

Lavoue S, Miya M, Arnegard M, Sullivan JP, Hopkins CD, Nishida M. (2012). Comparable Ages for the Independent Origins of Electrogenesis in African and South American Weakly Electric Fishes. PLoS ONE. 7,5. 

San Diego Zoo

As part of my class on “Sensory Evolution” we went on a field trip to the San Diego Zoo. A few impressions below. Can you spot the nictitating membrane in the Komodo Dragon and the cloudy eye in the Gopher Snake (indicative of imminent moulting?). More to follow soon!

Takin

Blue-spotted Tree Monitor

Cuban Iguana

Exuma Island Iguana

Plumed Basilisk

Green Crested Lizard

Scheltopusik

Komodo Dragon

Komodo Dragon

San Diego Gopher Snake

Ethipian Mountain Viper

Steller’s Sea Eagle

Ornate Hawk-eagle

Greater Flamingo

Superb Starling