How lizards turn into snakes: a phylogenetic analysis of body-form evolution in anguid lizards.

One of the most striking morphological transformations in vertebrate evolution is the transition from a lizardlike body form to an elongate, limbless (snakelike) body form. Despite its dramatic nature, this transition has occurred repeatedly among closely related species (especially in squamate reptiles), making it an excellent system for studying macroevolutionary transformations in body plan. In this paper, we examine the evolution of body form in the lizard family Anguidae, a clade in which multiple independent losses of limbs have occurred. We combine a molecular phylogeny for 27 species, our morphometric data, and phylogenetic comparative methods to provide the first statistical phylogenetic tests of several long-standing hypotheses for the evolution of snakelike body form. Our results confirm the hypothesized relationships between body elongation and limb reduction and between limb reduction and digit reduction. However, we find no support for the hypothesized sequence going from body elongation to limb reduction to digit loss, and we show that a burrowing lifestyle is not a necessary correlate of limb loss. We also show that similar degrees of overall body elongation are achieved in two different ways in anguids, that these different modes of elongation are associated with different habitat preferences, and that this dichotomy in body plan and ecology is widespread in limb-reduced squamates. Finally, a recent developmental study has proposed that the transition from lizardlike to snakelike body form involves changes in the expression domains of midbody Hox genes, changes that would link elongation and limb loss and might cause sudden transformations in body form. Our results reject this developmental model and suggest that this transition involves gradual changes occurring over relatively long time scales. Corresponding Editor: T. Garland Jr.

Why does a trait evolve repeatedly within a clade? When examining the evolution of a trait, evolutionary biologists typically focus on the selective advantages it may confer and the genetic and developmental mechanisms that allow it to vary. Although these factors may be necessary to explain why a trait evolves in a particular instance, they may not be sufficient to explain phylogenetic patterns of repeated evolution or conservatism. Instead, other factors may also be important, such as biogeography and competitive interactions. In squamate reptiles (lizards and snakes) a dramatic transition in body form has occurred repeatedly, from a fully limbed, lizardlike body form to a limb-reduced, elongate, snakelike body form. We analyze this trait in a phylogenetic and biogeographic context to address why this transition occurred so frequently. We included 261 species for which morphometric data and molecular phylogenetic information were available. Among the included species, snakelike body form has evolved about 25 times. Most lineages of snakelike squamates belong to one of two "ecomorphs," either short-tailed burrowers or long-tailed surface dwellers. The repeated origins of snakelike squamates appear to be associated with the in situ evolution of these two ecomorphs on different continental regions (including multiple origins of the burrowing morph within most continents), with very little dispersal of most limb-reduced lineages between continental regions. Overall, the number of repeated origins of snakelike morphology seems to depend on large-scale biogeographic patterns and community ecology, in addition to more traditional explanations (e.g., selection, development).

Why does a trait evolve repeatedly within a clade? When examining the evolution of a trait, evolutionary biologists typically focus on the selective advantages it may confer and the genetic and developmental mechanisms that allow it to vary. Although these factors may be necessary to explain why a trait evolves in a particular instance, they may not be sufficient to explain phylogenetic patterns of repeated evolution or conservatism. Instead, other factors may also be important, such as biogeography and competitive interactions. In squamate reptiles (lizards and snakes) a dramatic transition in body form has occurred repeatedly, from a fully limbed, lizardlike body form to a limbreduced, elongate, snakelike body form. We analyze this trait in a phylogenetic and biogeographic context to address why this transition occurred so frequently. We included 261 species for which morphometric data and molecular phylogenetic information were available. Among the included species, snakelike body form has evolved about 25 times. Most lineages of snakelike squamates belong to one of two ecomorphs, either short‐tailed burrowers or long‐tailed surface dwellers. The repeated origins of snakelike squamates appear to be associated with the in situ evolution of these two ecomorphs on different continental regions (including multiple origins of the burrowing morph within most continents), with very little dispersal of most limb‐reduced lineages between continental regions. Overall, the number of repeated origins of snakelike morphology seems to depend on large‐scale biogeographic patterns and community ecology, in addition to more traditional explanations (e.g., selection, development).

AimThe relationship between changes in body form (limb reduction and body elongation) and geographical range size was investigated across 68 species ofLerista, a species‐rich clade of Australian scincid lizards that exhibits extensive interspecific variability in both body form and range size.LocationLeristaoccurs across the entire Australian mainland, with diversity concentrated in arid and semi‐arid regions.MethodsGeographical range size was estimated directly fromc. 14,000 museum specimens using bioclimatic modelling inMaxEnt. Body form was quantified using principal components analysis of morphometric variables. Comparative analyses testing for a correlation between these two variables used a full Bayesian approach that accounts for uncertainties in trait optimization as well as in tree topology and branch lengths.ResultsA serpentine body form (elongated with reduced limbs) was significantly associated with smaller geographical range size, in both phylogenetically corrected and uncorrected analyses – but only if species from single localities (whose ranges could not be modelled using the above methods) were excluded.Main conclusionsThese results suggest a general predictive relationship between body form and geographical range size in lizards: elongate, limb‐reduced lizards tend to exhibit more restricted geographical ranges that may reflect reduced dispersal ability and may also predispose them to greater vulnerability of extinction.

An important challenge in evolutionary biology is to understand how major changes in body form arise. The dramatic transition from a lizard-like to snake-like body form in squamate reptiles offers an exciting system for such research because this change is replicated dozens of times. Here, we use morphometric data for 258 species and a time-calibrated phylogeny to explore rates and patterns of body-form evolution across squamates. We also demonstrate how time-calibrated phylogenies may be used to make inferences about the time frame over which major morphological transitions occur. Using the morphometric data, we find that the transition from lizard-like to snake-like body form involves concerted evolution of limb reduction, digit loss, and body elongation. These correlations are similar across squamate clades, despite very different ecologies and >180 million years (My) of divergence. Using the time-calibrated phylogeny and ancestral reconstructions, we find that the dramatic transition between these body forms can occur in 20 My or less, but that seemingly intermediate morphologies can also persist for tens of millions of years. Finally, although loss of digits is common, we find statistically significant support for at least six examples of the re-evolution of lost digits in the forelimb and hind limb.

Embryonic development in Anadia bogotensis and body plan evolution in Gymnophthalmoidea(Squamata). Gymnophthalmoidea lizards inhabit in Central and South America fromlowlands to highland Andes. This clade presents species with different body plans; lacertiformspecies exhibit short and robust bodies and well-developed limbs whereas serpentiform specieshave elongate bodies and limb reduction. Between these two extreme body plans, we foundintermediate forms such as Anadia bogotensis, with a less elongated body and limbs fully developedbut not very robust. We describe the embryonic development of Anadia bogotensis,and compare it with serpentiform species of Gymnophthalmidae, and lacertiform species ofTeiidae and Alopoglossidae. We found differences in the development time in somites andlimbs. With respect to lacertiform species, there is an accelerated development of somites inserpentiform gymnophthalmids, which is involved in body elongation. Besides, hypomorphosisis the heterochronic perturbation involved in limb reduction. Therefore, there are differences inthe development time of different structures, which are related to the evolution of body plansin Gymnophthalmoidea.

Hox genes regulate regionalization of the axial skeleton in vertebrates, and changes in their expression have been proposed to be a fundamental mechanism driving the evolution of new body forms. The origin of the snake-like body form, with its deregionalized pre-cloacal axial skeleton, has been explained as either homogenization of Hox gene expression domains, or retention of standard vertebrate Hox domains with alteration of downstream expression that suppresses development of distinct regions. Both models assume a highly regionalized ancestor, but the extent of deregionalization of the primaxial domain (vertebrae, dorsal ribs) of the skeleton in snake-like body forms has never been analysed. Here we combine geometric morphometrics and maximum-likelihood analysis to show that the pre-cloacal primaxial domain of elongate, limb-reduced lizards and snakes is not deregionalized compared with limbed taxa, and that the phylogenetic structure of primaxial morphology in reptiles does not support a loss of regionalization in the evolution of snakes. We demonstrate that morphometric regional boundaries correspond to mapped gene expression domains in snakes, suggesting that their primaxial domain is patterned by a normally functional Hox code. Comparison of primaxial osteology in fossil and modern amniotes with Hox gene distributions within Amniota indicates that a functional, sequentially expressed Hox code patterned a subtle morphological gradient along the anterior-posterior axis in stem members of amniote clades and extant lizards, including snakes. The highly regionalized skeletons of extant archosaurs and mammals result from independent evolution in the Hox code and do not represent ancestral conditions for clades with snake-like body forms. The developmental origin of snakes is best explained by decoupling of the primaxial and abaxial domains and by increases in somite number, not by changes in the function of primaxial Hox genes.

Squamates are found in a wide range of habitats and show a corresponding diversity of morphologies that can often be correlated with locomotor mode. The evolution of a snake-like body form, frequently associated with fossoriality, from a typical lacertiform morphology involves changes in the morphology of vertebrae, girdles, and limbs; the changes are mainly manifested by the reduction or loss of limbs and body elongation. In this study, we describe the axial and appendicular skeletons of six closely related gymnophthalmid species. Three of them show a lizard-like morphology, with a four-digit forelimb and a five-digit hindlimb, and the other three show a snake-like morphology associated with a burrowing habit, with reduced limbs and a longer body in comparison to the former three species. We show that vertebral morphology is similar among the six species, with the differences being accounted for by an increase in the number of vertebrae and by the structural reduction of girdles and limbs in the snake-like species. Skeletal morphology provides valuable information on locomotion type, physiology, diet, and other biological features. The burrowing morphology usually involves accentuated reduction of girdle and limb elements, reflecting an undulating type of locomotion in which the limbs play little or no role in propelling the body; in contrast, well-developed limbs and girdles indicate a greater reliance on the limbs for body propulsion. Limb reduction is frequent among vertebrates, but many different phenotypes are found in species exhibiting some kind of reduction, indicating that different mechanisms and evolutionary pressures may be involved in generating the diverse morphologies.

Heterochronic shifts in the ossification sequences of surface- and subsurface-dwelling skinks are correlated with the degree of limb reduction

Embryonic development of the fossorial gymnophthalmid lizards Nothobachia ablephara and Calyptommatus sinebrachiatus

Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Extant salamanders display three distinct life cycle strategies that range from aquatic to terrestrial (biphasic), to fully aquatic (paedomorphic) and to fully terrestrial (direct development). Life cycle variation is associated with changes in body form such as loss of digits, limb reduction or body elongation. However, the relationships among these traits and life cycle strategy remain unresolved. Here, we use a Bayesian modelling approach to test whether life cycle transitions by salamanders have influenced rates, optima and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted towards smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number, as well as reduced correlations between limb lengths. Our results support the idea that terrestrial plantigrade locomotion constrains limb evolution and, when lifted, leads to higher rates of trait diversification and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestrial life stages provide an important framework for understanding the evolutionary and developmental mechanisms behind major shifts in ecological zones as seen among early tetrapods during their transition from water to land.

The variation in limb reduction in vertebrates has generally been observed as characterizing higher-level taxa. Such structural changes are thus considered to reflect macroevolutionary processes. A statistical analysis of metric variables of some species of the African catfish family, Clariidae, suggests that fin reduction occurs at the microevolutionary level as well. In at least three species of that family intraspecific variation in the presence/absence of the pelvic fins was observed, and one species also showed similar variation for the pectoral fins. Discriminant function analysis confirmed that the variation is intraspecific, and even occurs within the same population. Sexual dimorphism could be excluded. This variation can be observed in the most anguilliform species of the clariid family, suggesting a link with body elongation (as is the case in tetrapods that show limb reduction). Pelvic fin loss appears to precede pectoral fin reduction during evolution. From the morphology it could be ascertained that the loss of the pelvic fins is coupled to the loss of the pelvic girdle, contrary to the case for the pectoral fins and girdle. Differences in functionality may explain this. Breeding results support the occurrence of intraspecific variation, as an F1 offspring showed a difference compared with the parents. For at least one species, the benefit from body elongation and limb reduction can be related to its highly specialized life style, as it lives subterraneanly in muddy soil of the Central African rainforest. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 75, 2002, 367‐377. ADDITIONAL KEYWORDS: catfish ‐ Clariidae ‐ intraspecific variation ‐ limblessness ‐ microevolution.

Convergence is the evolution of similar phenotypes often due to similar selective pressures or constraints limiting evolutionary options. Snake-like morphologies, characterized by elongated bodies and reduced limbs, have evolved repeatedly among vertebrates, including numerous times in squamate reptiles (lizards and snakes). It has been suggested that elongation facilitates locomotion through substrates while limb reduction typically occurs in clade-specific patterns, but this has not been tested. We compared the fit of a series of habitat- and clade-specific models for the evolution of digits, phalanges. and trunk vertebrae in lizards. We found that species inhabiting fossorial and cluttered habitats differed in numbers of vertebrae, digits, and phalanges from species in other habitats. A model with habitat-specific rates fit best for vertebral evolution, with sand swimmers, litter dwellers, and burrowers having higher rates of vertebral evolution than non-fossorial taxa. However, we found digits and phalanges evolved in a clade-specific manner, with higher rates of limb evolution in certain clades. This suggests that limb reduction in snake-like lizards is dictated by clade-specific constraints. In contrast, fossoriality appears to relax functional constraints on vertebral number, facilitating body form diversification. These results suggest that the relaxation of constraints may be an additional mechanism for convergent evolution.

One of the major regulatory challenges of animal development is to precisely coordinate in space and time the formation, specification, and patterning of cells that underlie elaboration of the basic body plan. How does the vertebrate plan for the nervous and hematopoietic systems, heart, limbs, digestive, and reproductive organs derive from seemingly similar population of cells? These systems are initially established and patterned along the anteroposterior axis (AP) by opposing signaling gradients that lead to the activation of gene regulatory networks involved in axial specification, including the Hox genes. The retinoid signaling pathway is one of the key signaling gradients coupled to the establishment of axial patterning. The nested domains of Hox gene expression, which provide a combinatorial code for axial patterning, arise in part through a differential response to retinoic acid (RA) diffusing from anabolic centers established within the embryo during development. Hence, Hox genes are important direct effectors of retinoid signaling in embryogenesis. This review focuses on describing current knowledge on the complex mechanisms and regulatory processes, which govern the response of Hox genes to RA in several tissue contexts including the nervous system during vertebrate development.

Variation in axial formulae (i.e., number and identity of vertebrae) is an important feature in the evolution of vertebrates. Vertebrae at different axial positions exhibit a region-specific morphology. Key determinants for the establishment of particular vertebral shapes are the highly conserved Hox genes. Here, we analyzed Hox gene expression in the presacral vertebral column in the Nile crocodile in order to complement and extend a previous examination in the alligator and thus establish a Hox code for the axial skeleton of crocodilians in general. The newly determined expression of HoxA-4, C-5, B-7, and B-8 all revealed a crocodilian-specific pattern. HoxA-4 and HoxC-5 characterize cervical morphologies and the latter furthermore is associated with the position of the forelimb relative to the axial skeleton. HoxB-7 and HoxB-8 map exclusively to the dorsal vertebral region. The resulting expression patterns of these two Hox genes is the first description of their exact expression in the archosaurian embryo. Our comparative analyses of the Hox code in several amniote taxa provide new evidence that evolutionary differences in the axial skeleton correspond to changes in Hox gene expression domains. We detect two general processes: (i) expansion of a Hox gene's expression domain as well as (ii) a shift of gene expression. We infer that the ancestral archosaur Hox code may have resembled that of the crocodile. In association with the evolution of morphological traits, it may have been modified to patterns that can be observed in birds.