The crocodylian family includes crocodiles, gharials, alligators and caimans – all tough-skinned reptiles with a characteristic armor of scales. Although their outer body surface is formed of a hard keratinized epidermis, these reptiles have a distinctly sensitive surface owing to the presence of integumentary sensory organs (ISOs). Our understanding of the evolution of these organs in crocodylians is still incomplete. Michel Milinkovitch and Nicolas Di-Poi from the University of Geneva, Switzerland sought to analyze and compare the structure, innervation, development and sensory functions of the ISOs of the Nile crocodile and the spectacled caiman, as published in a recent article in EvoDevo. Milinkovitch told us more about the surprising findings they uncovered that adds to the story of the evolution of crocodylian ISOs.
What led to your research into crocodile sensory systems?
One major research topic in my laboratory is to understand the physical and biological parameters responsible for the development of skin appendages (hair, spines, scales, feathers) as well as skin colour patterns in vertebrates. While we were investigating how scales develop in various lineages of reptiles, we were surprised by the irregular shapes of the scales on the face and jaws of crocodylians (true crocodiles, gharials, alligators, and caimans), and by the lack of symmetry of the scale pattern between the left and right side of the head. This is very different from the situation in snakes and lizards where head scales form a predictable symmetrical pattern. We then decided to investigate this issue in detail and found that head scales of crocodylians develop very differently from all other known skin appendages in vertebrates.
What is so unique about the development crocodylian head scales compared to other vertebrate skin appendages?
All vertebrate keratinized skin appendages were thought to form following a ‘developmental unit’ paradigm; each feather, each hair, and each scale differentiates and grows in the embryo from a genetically-controlled developmental unit or primordium. We confirmed this paradigm was true for the body scales of crocodylians. However, we discovered (Science 339, 78-81) that head scales in crocodylians emerge from a very different and stochastic process – physical cracking of the developing skin in a stress field. This phenomenon might not involve any specific genetic instruction besides those associated with cell proliferation and general physical parameters such as skin stiffness and thickness.
During that study, we realized that the face and jaws of the developing crocodile embryo were covered by many small primordia, that do not correspond to scales but to minute sensors initially described in adult crocodylians as integumentary sensory organs (ISO). ISOs can detect surface pressure waves allowing the animal to swiftly orient, even in total darkness, towards the surface-disturbing prey. ISOs are distributed on virtually all scales (cranial and post-cranial) in gharials and true crocodiles, while they are absent from post-cranial scales in alligators and caimans. After discovering the very different processes by which head scales and body scales develop, we were convinced that head and body ISOs in true crocodiles would also develop differently and would have different sensory abilities. We therefore decided to perform morphological, molecular, and electrophysiological analyses of ISOs in the Nile crocodile and the Spectacled Caiman – that study was again a source of surprise!
What were the main results of your study? What surprised you about your findings?
First, we show that Nile crocodile ISOs all exhibit similar morphologies and modes of development, despite forming at different stages across the body and head. Second, we show that the innervation pattern of ISOs is conserved in both Nile crocodiles and spectacled caimans. However, sensory neurons in vertebrates respond to specific stimuli, with a variety of specialized receptors providing information to the central nervous system on mechanical, thermal and chemical/pH variations. These stimuli are detected by specific transduction channels. We therefore used molecular techniques to examine, in crocodylian ISOs, the expression patterns of different conserved families of transduction channels. To our great surprise, we found clear expression of not-only mechano-receptors but also of polymodal chemo- and thermo-receptors.
Importantly, all transduction channels are specifically expressed in developing ISOs during embryogenesis and nowhere else in the skin. Our molecular results strongly suggest that each ISO exhibits a combined sensitivity to mechanical, thermal and chemical stimuli. We tested this by performing electrophysiological recordings from individual ISOs in juvenile Nile crocodiles and spectacled caimans in response to different local stimuli that naturally occur in their environment. We confirmed that ISOs are able to detect mechanical stimuli and we demonstrate that they additionally respond to warm and cold temperatures, acidic pH, alkaline pH, and TRPV chemical agonists.
This shows that ISOs are remarkable multi-sensorial micro-organs with no equivalent in the sensory systems of other vertebrate lineages. It makes us look differently at crocodiles. They are certainly not insensitive monsters! Their ISOs provide them not only with exquisite mechano-sensitivity (of the same magnitude as our own fingertips) but also with the ability to detect cold, heat, and chemicals in the environment – despite their impressive armored skin.
What specific evolutionary pressures are crocodylians under in comparison to other vertebrates that caused them to develop ISOs?
We think that the dramatic development of skin toughness in the crocodile lineage would make a diffused sensory system, present in all other extant vertebrates, inefficient. The re-organization of the skin sensory system into an array of multi-sensory micro-organs is probably one of the best solutions to that conundrum.
What are the evolutionary reasons for the different ISO distributions in different taxonomic groups within crocodylia?
It is difficult to know if post-cranial ISOs were lost by alligator and caiman lineages or gained in true crocodiles and gharials. All 23 extant species of crocodylians occupy a range of semi-aquatic environments in temperate and tropical regions of North and South America, Africa, Asia, and Australia. Only true crocodiles occur in estuary habitats and exhibit adaptations to salty water such as lingual salt-secreting glands. Multi-sensory ISOs are likely to be advantageous when facing environmental variations. In addition to lingual salt glands, the presence of post-cranial chemosensitive ISOs in true crocodiles is therefore likely to confer them with an advantage over alligators and caimans in adapting to environments that differ in temperature and chemical composition. It is possible that these sensory organs have contributed in part to the wider geographical distribution of extant true crocodiles, in contrast to the more restricted distributions of alligators and caimans.
You focused on species from the alligator and crocodile family. Do you think gharials also share similar ISOs based on their position in phylogenetic trees?
Clearly, gharials exhibit ISOs on both cranial and post-cranial scales. They also have salt glands despite exclusively inhabiting freshwater habitats. It is unknown if these glands would allow them to adapt to salty water or if they constitute a surviving trait that hints at their sister-relationship with true crocodiles.
Why do you think research into reptilian evolution is so important?
Classical models, such as the fruit fly and mouse, are fantastic tools for deciphering the exquisite mechanisms of evolution and development. However, one should not forget that this handful of species gives us a biased and incomplete view of the mechanisms that generated the millions of phenotypes observed in nature. Reptiles are significantly more diverse than mammals and exhibit fantastic adaptations and sensory abilities worthy of the interest of biologists from multiple disciplines. For example, we recently demonstrated that snakes evolved their spectacular chemosensing abilities through the expansion of a family of odorant receptors different to those developed by mammals (Genome Biology & Evolution 5: 389-401).
In addition, some phenomenon particularly visible in one lineage can be unexpectedly relevant for understanding related phenomena in other species. Our analyses of crocodile head scale development suggests that physical parameters, such as mechanical tension, play important roles in the local modification of cell proliferation rates which, in turn, cause the developing tissue to bend, bulge, or heal after cracking. This is potentially relevant for multiple developmental phenomena in all species, from gastrulation to the development of wrinkles in aging human skin.