Tuesday, May 26, 2015

Modeling the first snake

A reconstruction of the ancestral crown-group snake, 
Artwork by Julius Csotonyi.
The original snake ancestor was a nocturnal, stealth-hunting predator that had tiny hind limbs with ankles and toes, according to new research. Snakes show incredible diversity, with over 3,400 living species found in a wide range of habitats, such as land, water and in trees. But little is known about where and when they evolved, and how their original ancestor looked and behaved. The original snake ancestor was a nocturnal, stealth-hunting predator that had tiny hind limbs with ankles and toes, according to research published in the open access journal BMC Evolutionary Biology.
The study, led by Yale University, USA, analyzed fossils, genes, and anatomy from 73 snake and lizard species, and suggests that snakes first evolved on land, not in the sea, which contributes to a longstanding debate. They most likely originated in the warm, forested ecosystems of the Southern Hemisphere around 128 million years ago.
Snakes show incredible diversity, with over 3,400 living species found in a wide range of habitats, such as land, water and in trees. But little is known about where and when they evolved, and how their original ancestor looked and behaved.
Lead author Allison Hsiang said: "While snake origins have been debated for a long time, this is the first time these hypotheses have been tested thoroughly using cutting-edge methods. By analyzing the genes, fossils and anatomy of 73 different snake and lizard species, both living and extinct, we've managed to generate the first comprehensive reconstruction of what the ancestral snake was like."
By identifying similarities and differences between species, the team constructed a large family tree and illustrated the major characteristics that have played out throughout snake evolutionary history.
Their results suggest that snakes originated on land, rather than in water, during the middle Early Cretaceous period (around 128.5 million years ago), and most likely came from the ancient supercontinent of Laurasia. This period coincides with the rapid appearance of many species of mammals and birds on Earth.
The ancestral snake likely possessed a pair of tiny hind limbs, and targeted soft-bodied vertebrate and invertebrate prey that were relatively large in size compared to prey targeted by lizards at the time. While the snake was not limited to eating very small animals, it had not yet developed the ability to manipulate prey much larger than itself by using constriction as a form of attack, as seen in modern Boa constrictors.
While many ancestral reptiles were most active during the daytime (diurnal), the ancestral snake is thought to have been nocturnal. Diurnal habits later returned around 50-45 million years ago with the appearance of Colubroidea -- the family of snakes that now make up over 85% of living snake species. As colder night time temperatures may have limited nocturnal activity, the researchers say that the success of Colubroidea may have been facilitated by the return of these diurnal habits.
The results suggest that the success of snakes in occupying a range of habitats over their evolutionary history is partly due to their skills as 'dispersers'. Snakes are estimated to be able to travel ranges up to 110,000 square kilometers, around 4.5 times larger than lizards. They are also able to inhabit environments that traditionally hinder the dispersal of terrestrial animals, having invaded aquatic habitats multiple times in their evolutionary history.

Citation
Hsiang AY, Field DJ, Webster TH, Behlke ADB, Davis MD, Racicot RA, Gauthier JA. 2015. The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record. BMC Evolutionary Biology, 2015; 15 (1) DOI: 10.1186/s12862-015-0358-5

Tuesday, May 19, 2015

Eating-induced changes of the Burmese python's intestines due to changes in gene expression



 The Burmese python's body undergoes massive reconstruction followed by complete deconstruction every time it eats. Within three days of eating, its organs expand up to double in size and its metabolism and digestive processes increase 10- to 44-fold. Ten days after eating, the snake's meal is digested and these changes have reversed, allowing the body to shrink and return back to its pre-meal state. In a new study published in Physiological Genomics, a team of U.S. researchers tracked in detail how this extreme makeover is controlled by changes in gene expression.

The Burmese python's extreme physiology is fascinating to study because it gives unique insight into how vertebrates control organ growth and function, the researchers wrote. Although the Burmese python's body shape is distinct from other vertebrates, including humans, its organs operate the same. This means findings from snakes can be applied to understanding the human body and potentially developing new therapies for human diseases, the researchers said.

In this study, the research team focused on the small intestine, which doubles in mass and nutrient-absorption rate during digestion. The researchers found that the expression of at least 2,000 genes changed after the snake ate. Surprisingly, most of the changes occurred soon after eating -- within six hours. Genes that changed included those involved with the intestine's structure and nutrient absorption, cell division and cell death. The patterns of gene expression matched and often preceded physiological changes in the intestine, the researchers wrote. The gene expression patterns, like the structural changes, then returned to pre-eating state within 10 days after eating, "indicating a tight association between differential gene expression and the rapid and cyclic physiological remodeling of the intestine," the researchers said.

According to the researchers, this is the first study to link the extreme and rapid eating-induced changes of the Burmese python's intestines directly to changes in gene expression, and also the first to show how quickly gene expression changed. The study also found that some of the morphing genes in the python's intestine, notably those in a signaling pathway called WNT, were genes that were involved in intestinal and other cancers. This suggests that "the python intestine may represent a valuable model for studying the interactions of metabolism with the regulation of cell division/death and WNT signaling relevant to cancer," the researchers said.

Citation
Andrew AL, Card DC, Ruggiero RP, Schield DR, Adams RH, Pollock DD, Secor SM, Castoe TA. (2015) Rapid changes in gene expression direct rapid shifts in intestinal form and function in the Burmese python after feeding. Physiological Genomics, 47 (5): 147 DOI: 10.1152/physiolgenomics.00131.2014


Saturday, May 16, 2015

tail length in snakes associated with gravity


An arboreal eyelash viper (Bothriechis schlegelii)
 resting on a branch in Costa Rica. Photograph by 
Coleman M. Sheehy III. 
Gravity is a pervasive force that can severely affect blood circulation in terrestrial animals, and these effects can be particularly pronounced in tall or long organisms such as giraffes and snakes. Upright postures create vertical gradients of gravitational pressures within circulatory vessels that increase with depth. In terrestrial animals, this pressure potentially induces blood pooling and edema in the lower-most tissues and decreases blood volume reaching the head and vital organs.

Since their evolutionary origins about 100 million years ago, snakes have diversified into a wide variety of aquatic, burrowing, terrestrial, and arboreal habitats where they experience various levels of gravitational stress on blood circulation. At the extremes, these stresses range from low to none in fully aquatic species living in essentially “weightless” environments, to relatively high in climbing species, especially arboreal forms specialized for climbing trees. As a result, arboreal snakes exhibit many adaptations for countering the effects of gravity on blood circulation, including relatively tight tissue compartments in the tail. However, patterns of tail length in relation to arboreal habitats and gravity have not been previously studied.

We obtained length data for 226 snake species representing almost all snake families to test the hypothesis that arboreal snakes have longer tails than do non-climbing species. We found that average tail length increased and average body length decreased with increasing use of arboreal habitats and that arboreal snake species had average tail lengths 3–4 times longer than those of non-climbing species. Snakes with longer tails have a higher percentage of elongate blood vessels contained within the relatively tight skin of the tail, which counters blood pooling experienced during climbing. Total body length appears to be constrained in arboreal species, and total body length in adult female arboreal snakes appears to be an evolutionary tradeoff that favors longer tail lengths over maximum production of offspring as arboreal habitat-use increases. Our findings provide evidence that long tails of arboreal snakes function, at least in part, as an adaptation to counter cardiovascular stresses on blood circulation imposed by gravity.


Citation

Sheehy, C. M., Albert, J. S., & Lillywhite, H. B. (2015). The evolution of tail length in snakes associated with different gravitational environments. Functional Ecology. Early On-line.

Friday, May 1, 2015

Geckos evolved daytime activity multiple times

A diurnal Phelsuma and a nocturnal Cyrtodactylus
Geckos are the only clade of lizards that are mostly nocturnal; 72% of the 1552 described species are active at night. Geckos possess numerous adaptations to low light and low temperatures, suggesting nocturnal activity evolved early in their evolution. These adaptations include the evolution of vocalization and acoustic communication, olfactory specialization, enhanced capability for sustained locomotion at low temperatures, shifts in diet and foraging mode, and the absence of the parietal foramen and pineal eye. Geckos have acute vision and many adaptations for seeing in low light including: large eyes, pupils capable of an extreme degree of constriction and dilation, retinas without foveae, short visual focal length, multifocal color vision, and rod-like photoreceptor cells in the retina that lack oil droplets. However, not all gecko species are nocturnal; more than 430 are diurnal. Many of these diurnal lineages have their own adaptations to living in warm, photopic environments including round pupils, UV-filtering crystallin lens proteins, smaller eyes, partial to complete foveae, cone-like photoreceptor cells in the retina and a return to higher energetic costs of locomotion. Geckos are thought to be ancestrally nocturnal and diurnality evolved multiple times. However, this hypothesis has never been tested in a phylogenetic framework.

Now, in a new paper Gamble et al. (2015) performed comparative analyses using a newly generated gecko phylogeny and examined the evolution of temporal activity patterns to: test the hypothesis of an early origin of nocturnality in geckos; verify repeated subsequent transitions to diurnality; and determine whether the evolution of temporal activity patterns has influenced diversification rates. The results provide the first phylogenetic analysis of temporal activity patterns in geckos and confirm an ancient origin of nocturnality at the root of the gecko tree. Gamble et al. identify multiple transitions to diurnality at a variety of evolutionary time scales and transitions back to nocturnality occur in several predominantly diurnal clades.

The authors found several transitions occurred deep in the phylogeny, including ancestors to the Pygopodidae, the New World sphaerodactyl geckos and the Phelsuma plus Lygodactylus clade. More recent transitions occurred in Rhoptropus, within New Zealand and New Caledonian diplodactylids (Naultinus and Eurydactylodes), and within Gymnodactylus, Ptyodactylus and Mediodactylus. Both Asian Cnemaspis clades seem to include multiple transitions, although additional taxonomic sampling is needed to confirm this. They also identified several well-supported eversions to nocturnality within otherwise diurnal clades, including Sphaerodactylus, Gonatodes, Phelsuma and the Pygopodidae. Their results indicate frequent shifts in temporal activity patterns in geckos at a variety of evolutionary timescales. Determining what factors initiate shifts in individual clades was beyond the scope of the paper, but they suggest three possible causes: climate, predators and competition.

Some shifts in activity pattern may be related to thermoregulation and evading extreme temperatures and desiccation. For example, geckos in the genus Sphaerodactylus appear to overheat easily and several species that inhabit hot, xeric habitats are nocturnal, including: S. leucaster, S. thompsoni and S. ladae in southern Hispaniola; S. roosevelti in south-west Puerto Rico; and S. inaguae from the Bahamas. Similarly, some gecko species living at high altitudes, such as Mediodactylus amictopholis, are thought to have shifted to diurnal activity to facilitate thermoregulation in colder climates. However, there are numerous counter inhabiting extreme environments. Pristurus and Rhoptropus, for instance, are diurnal genera that can be active at extremely high temperatures in arid environments while Homonota darwnii and Alsophylax pipiens live in cold climates at extreme latitudes and remain nocturnal. Furthermore, nocturnal geckos seem quite capable of regulating body temperature while hidden in retreats during the day and thus switching to diurnality solely for thermoregulatory purposes may be uncommon overall.

Predation could also instigate changes in temporal activity patterns in geckos and such shifts are well documented in other vertebrate species. Most predator-induced niche shifts in geckos involve the alteration of the spatial niche. However, the hypothesis that geckos may transition to a more conspicuous, diurnal lifestyle in environments where predators are less abundant or absent, such as on islands. Lack of predators is thought to be responsible for dramatic changes in phenotype and behavior in many island species, such as the evolution of flightlessness in birds. Thus, it is reasonable that similar selective pressures could alter temporal activity in geckos.

Shifts in temporal activity patterns may also be related to competition avoidance and the exploitation of underutilized resources. Temporal resource partitioning helps competitors coexist by avoiding direct confrontation or reducing resource overlap. For example, the early shift to nocturnality in ancient geckos has been attributed to avoiding competition with diurnal lizards and exploiting the relatively open nocturnal niche. The lack of competition with other diurnal lizards, mostly iguanians, is frequently cited as promoting transitions back to diurnality in geckos. Indeed, many diurnal geckos occur in regions with a paucity of iguanian species. The success of Phelsuma and Lygodactylus in Madagascar has been attributed to the lack of arboreal iguanians, with the exception of the extremely specialized chameleons.

The scenario presented here will be useful in reinterpreting existing hypotheses of how geckos have adapted to varying thermal and light environments. These results can also inform future research of gecko ecology, physiology, morphology and vision as it relates to changes in temporal activity patterns.

Citation

Gamble T, Greenbaum E, Jackman TR, Bauer AM (2015), Into the light: diurnality has evolved multiple times in geckos. Biological Journal of the Linnean Society. doi: 10.1111/bij.12536