Respective Role of the Axialand Appendicular Systems in Relation to the Transition to Limblessness

In lower quadrupedal vertebrates locomotor efficiency seems to result from the associate movements of the axial and appendicular systems, which are totally independent in structure and embryological origin. The curvature of the trunk, produced by a standing wave, magnifies the propulsive action of the limbs. In intermediate forms, the association of an elongate trunk with limbs reduced in size brings about functional consequences which may be noticeably diverse according to the degree of trunk elongation and limb reduction. According to environmental constraints, animals search for better locomotor efficiency, which implies the maintenance or breakage of this association of both locomotor systems. In some cases, limb action on the ground is added to the axial wave action through a perfect mutual adjustment of rhythmic activity, until mechanical inefficiency of the limbs is reached by possible loss of contact with the ground. In other cases, the limbs dragged on the ground during the stance phase act against the axial action or, on the contrary, are inhibited by the axial system. A review of available data tries to contribute to an understanding of the respective roles of both systems in the transition to limblessness.     

Correlated evolution of herbivory and food chemical discrimination in iguanian and ambush foraging lizards

To efficiently locate and assess foods, animal sensory capacitiesand behavioral discriminations based on them must be appropriatefor the diet and method of hunting. In lizards, actively foraginginsectivores identify animal prey using lingually sampled chemicalcues, but ambush foragers do not. Among plant eaters derivedfrom active foragers, plant chemical discrimination is addedto prey chemical discrimination, resulting in correlated evolutionof plant diet and plant chemical discrimination. Here I presentcomparative evidence on the relationships between plant dietand food chemical discrimination in Iguania, which consistsprimarily of ambush foragers and is one of two major lizardclades, and for ambushing lizards in general. Comparative analysesconducted using phylogenetic methods show that (1) all but onespecies of omnivore studied exhibited both prey and plant chemicaldiscrimination, whereas ambush foragers exhibited neither; (2)significant correlated evolution occurred between plant dietand plant chemical discrimination in Iguania and in omnivoresand herbivores derived from ambush foragers; and (3) correlatedevolution has occurred between prey and plant chemical discriminationin Iguania and, more generally, in taxa derived from ambushforagers. These results are explained by selection on planteaters to assess the nutritional value and possible toxicityof plants and by continued consumption of some animal prey evenin herbivores combined with freedom from factors that selectagainst prey chemical discrimination in ambush foragers.     

Plausibility of inferred ancestral phenotypes and the evaluation of alternative models of limb evolution in scincid lizards

Phylogenetic approaches to inferring ancestral character states are becoming increasingly sophisticated; however, the potential remains for available methods to yield strongly supported but inaccurate ancestral state estimates. The consistency of ancestral states inferred for two or more characters affords a useful criterion for evaluating ancestral trait reconstructions. Ancestral state estimates for multiple characters that entail plausible phenotypes when considered together may reasonably be assumed to be reliable. However, the accuracy of inferred ancestral states for one or more characters may be questionable where combined reconstructions imply implausible phenotypes for a proportion of internal nodes. This criterion for assessing reconstructed ancestral states is applied here in evaluating inferences of ancestral limb morphology in the scincid lizard clade Lerista. Ancestral numbers of digits for the manus and pes inferred assuming the models that best fit the data entail ancestral digit configurations for many nodes that differ fundamentally from configurations observed among known species. However, when an alternative model is assumed for the pes, inferred ancestral digit configurations are invariably represented among observed phenotypes. This indicates that a suboptimal model for the pes (and not the model providing the best fit to the data) yields accurate ancestral state estimates.     

Locomotion patterns in two South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus

We quantified gait and stride characteristics (velocity, frequency, stride length, stance and swing duration, and duty factor) in the bursts of locomotion of two small, intermittently moving, closely related South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus. They occur in different environments: V. rubricauda is widely distributed in open areas with various habitats and substrates, while P. tetradactylus is endemic to dunes in the semi-arid Brazilian Caatinga. Both use trot or walking trot characterised by a lateral sequence. For various substrates in a gradient of roughness (perspex, cardboard, sand, gravel), both species have low relative velocities in comparison with those reported for larger continuously moving lizards. To generate velocity, these animals increase stride frequency but decrease relative stride length. For these parameters, P. tetradactylus showed lower values than V. rubricauda. In their relative range of velocities, no significant differences in stride length and frequency were recorded for gravel. However, the slopes of a correlation between velocity and its components were lower in P. tetradactylus on cardboard, whereas on sand this was only observed for velocity and stride length. The data showed that the difference in rhythmic parameters between both species increased with the smoothness of the substrates. Moreover, P. tetradactylus shows a highly specialised locomotor strategy involving lower stride length and frequency for generating lower velocities than in V. rubricauda. This suggests the evolution of a central motor pattern generator to control slower limb movements and to produce fewer and longer pauses in intermittent locomotion.     

Linking the evolution and form of warning coloration in nature

Many animals are toxic or unpalatable and signal this to predators with warning signals (aposematism). Aposematic appearance has long been a classical system to study predator–prey interactions, communication and signalling, and animal behaviour and learning. The area has received considerable empirical and theoretical investigation. However, most research has centred on understanding the initial evolution of aposematism, despite the fact that these studies often tell us little about the form and diversity of real warning signals in nature. In contrast, less attention has been given to the mechanistic basis of aposematic markings; that is, ‘what makes an effective warning signal?’, and the efficacy of warning signals has been neglected. Furthermore, unlike other areas of adaptive coloration research (such as camouflage and mate choice), studies of warning coloration have often been slow to address predator vision and psychology. Here, we review the current understanding of warning signal form, with an aim to comprehend the diversity of warning signals in nature. We present hypotheses and suggestions for future work regarding our current understanding of several inter-related questions covering the form of warning signals and their relationship with predator vision, learning, and links to broader issues in evolutionary ecology such as mate choice and speciation.     

Four New Species of Litter Inhabiting Scytodes Spiders (Araneae,Scytodidae) from Amazonia

Four new species of spitting spiders of the genus Scytodes are described: S. panguana from Peruvian Amazonia and S. paarmanni, S. martiusi and S. janauari from central Amazonia in Brazil. All are small, short legged scytodids. The specimens were collected in the course of ecological studies in the leaf litter layer on the ground of Amazonian terra firme forests. Quatro novas espécies de Scytode s são descritas: S. panguana da Amazônia peruana e S. paarmanni, S. martiusi e S. janauari da Amazônia central do Brasil. Todas são espécies pequenas, de pernas curtas, que foram coletadas durante estudos ecológicos em liteira no chão de florestas de terra firme da Amazônia.     

Ecomorphological variation and sexual dimorphism in a recent radiation of dwarf chameleons (Bradypodion)

Natural selection tends to favour optimal phenotypes either through directional or stabilizing selection; however, phenotypic variation in natural populations is common and arises from a combination of biotic and abiotic interactions. In these instances, rare phenotypes may possess a fitness advantage over the more common phenotypes in particular environments, which can lead to adaptation and ecological speciation. A recently radiated clade of dwarf chameleons (Bradypodion) restricted to southern KwaZulu‐Natal Province, South Africa, is currently comprised of two species (Bradypodion melanocephalum and Bradypodion thamnobates), yet three other phenotypic forms exist, possibly indicating the clade is far more speciose. Very little genetic differentiation exists between these five phenotypic forms; however, all are allopatric in distribution, occupy different habitats and vary in overall size and coloration, which may indicate that these forms are adapting to their local environments and possibly undergoing ecological speciation. To test this, we collected morphometric and habitat data from each form and examined whether ecological relevant morphological differences exist between them that reflect their differential habitat use. Sexual dimorphism was detected in four of the five forms. Yet, the degree and number of dimorphic characters was different between them, with size‐adjusted male‐biased dimorphism being much more pronounced in B. thamnobates. Habitat differences also existed between sexes, with males occupying higher perches in more closed canopy (forested) habitats than females. Clear morphological distinctions were detected between four of the five forms, with the head explaining the vast majority of the variation. Chameleons occupying forested habitats tended to possess proportionally larger heads and feet but shorter limbs than those in open canopy habitats (i.e. grassland). These results show that this species complex of Bradypodion is morphologically variable for traits that are ecologically relevant for chameleons, and that the variation among the five phenotypic forms is associated with habitat type, suggesting that this species complex is in the early stages of ecological speciation. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 113–130.     

Ultrastructure of the reproductive system of the black swamp snake (Seminatrix pygaea). VI. Anterior testicular ducts and their nomenclature

In this study, the anterior testicular ducts of the North American natricine snake Seminatrix pygaea are described using light and electron microscopy. From the seminiferous tubules, the rete testis passes into the epididymal sheath, a structure along the medial border of the testis heavily invested with collagen fibers. The rete testis consists of simple, nonciliated cuboidal epithelium (principal cells). The intratesticular ducts of the rete testis are narrow (50–70 μm) at their junction with the seminiferous tubules, widen (80–100 μm) as they extend extratesticularly, and divide into smaller branches as they anastomose with the next tubules, the ductuli efferentes. The ductuli efferentes are lined by simple cuboidal epithelium but possess nonciliated principal cells as well as ciliated cells. These are the only ducts in the male reproductive system with ciliated cells. The ductuli efferentes are narrow (25–45 μm), divide into numerous branches, and are highly convoluted. The ductus epididymis is the largest duct in diameter (240–330 μm), and the diameter widens and the epithelium thins posteriorly. The ductus epididymis is lined by nonciliated, columnar principal cells and basal cells. No regional differences in the ductus epididymis are apparent. Ultrastructural evidence suggests that all of the nonciliated principal cells in each of the anterior testicular ducts function in both absorption and secretion. Absorption occurs via small endocytic vesicles, some of which appear coated. Secretion is by a constitutive pathway in which small vesicles and a flocculent material are released via a merocrine process or through the formation of apocrine blebs. The secretory product is a glycoprotein. Overall, the characteristics of the anterior testicular ducts of this snake are concordant with those of other amniotes, and the traditional names used for snakes are changed to conform with those used for other sauropsids and mammals. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Complete mitochondrial genomes of three lizard species and the systematic position of the Lacertidae (Squamata)*

Complete mitochondrial (mt) genomes were sequenced from representatives of three lacertid lizards: Podarcis siculus, Podarcis muralis and Phoenicolacerta kulzeri. In all three genomes the arrangement of the 22 tRNAs, the two rRNAs and the 13 protein‐coding genes conforms to the common vertebrate arrangement. The phylogenetic position of Lacertidae within the order Squamata was determined through sequence analyses based on large sections of complete mt genomes. The number of nucleotide sites used for tree construction was 9234 when outgroup taxa were included, and 10 499 when only Squamata were compared. The phylogenetic analyses confirmed the sister group relationship between Lacertidae and Amphisbaenia as previously proposed on the basis of molecular data. Additionally, Bayesian analysis revealed a well supported clade comprising (Gekkonidae (Lacertidae + Amphisbaenia)), which is not in accordance with the traditional morphological view and most of the previous molecular studies. It confirms, however, the close relationship between Gekkonidae and Amphisbaenia as revealed in a recent study based on complete mt genomes from a smaller number of taxa. Intra‐ and intergeneric sequence comparisons of six commonly used marker genes showed rather high levels of divergence within the Lacertidae. In the intrageneric comparison the control region proved to be considerably more conserved than the protein coding genes.     

Cambrian stem-group ambulacrarians and the nature of the ancestral deuterostome

1. Download : Download high-res image (305KB)
2. Download : Download full-size image

     

Reconstitution of ancestral green visual pigments of zebrafish and molecular mechanism of their spectral differentiation

We previously reported that zebrafish have four tandemly duplicated green (RH2) opsin genes (RH2-1, RH2-2, RH2-3, and RH2-4). Absorption spectra vary widely among the four photopigments reconstituted with 11-cis retinal, with their peak absorption spectra (lambda(max)) being 467, 476, 488, and 505 nm, respectively. In this study, we inferred the ancestral amino acid (aa) sequences of the zebrafish RH2 opsins by likelihood-based Bayesian statistics and reconstituted the ancestral opsins by site-directed mutagenesis. The ancestral pigment (A1) to the four zebrafish RH2 pigments and that (A3) to RH2-3 and RH2-4 showed lambda(max) at 506 nm, while that (A2) to RH2-1 and RH2-2 showed a lambda(max) at 474 nm, indicating that a spectral shift had occurred toward the shorter wavelength on the evolutionary lineages A1 to A2 by 32 nm, A2 to RH2-1 by 7 nm, and A3 to RH2-3 by 18 nm. Pigment chimeras and site-directed mutagenesis revealed a large contribution (approximately 15 nm) of glutamic acid to glutamine substitution at residue 122 (E122Q) to the A1 to A2 and A3 to RH2-3 spectral shifts. However, the remaining spectral differences appeared to result from complex interactive effects of a number of aa replacements, each of which has only a minor spectral contribution (1-3 nm). The four zebrafish RH2 pigments cover nearly an entire range of lambda(max) distribution among vertebrate RH2 pigments and provide an excellent model to study spectral tuning mechanisms of RH2 in vertebrates.     

Ultrastructure of the reproductive system of the black swamp snake (Seminatrix pygaea). III. Sexual segment of the male kidney

In mature male snakes and lizards, a distal portion of the nephron is hypertrophied in relation to its appearance in females and immature males. This sexual segment of the male kidney apparently provides seminal fluid that is mixed with sperm and released into the female cloaca during copulation. In this article, we provide the first study at the ultrastructural level of seasonal variation in the sexual segment of the kidney of a squamate, the natricine snake Seminatrix pygaea. Previous workers have indicated that the sexual segment is secretory only when the testes are spermatogenically active. The sexual segment of the kidney in S. pygaea does not go through an extended period of inactivity but does show a cycle of synthesis and secretion that can be related to the spermatogenic cycle and mating activity. We show that synthesis of secretory product is initiated with the onset of spermatogenic activity in the spring and culminates with completion of spermiation in the fall. Secretion of the product, however, occurs in a premating period in March when the testes are inactive. Secretion during this premating period is probably necessary to provide time for the passage of the products down the ureter in order to mix with sperm during mating later in spring.     

The skull of the Upper Cretaceous snake Dinilysia patagonica Smith‐Woodward, 1901, and its phylogenetic position revisited

The cranial anatomy of Dinilysia patagonica, a terrestrial snake from the Upper Cretaceous of Argentina, is redescribed and illustrated, based on high‐resolution X‐ray computed tomography and better preparations made on previously known specimens, including the holotype. Previously unreported characters reinforce the intriguing mosaic nature of the skull of Dinilysia, with a suite of plesiomorphic and apomorphic characters with respect to extant snakes. Newly recognized plesiomorphies are the absence of the medial vertical flange of the nasal, lateral position of the prefrontal, lizard‐like contact between vomer and palatine, floor of the recessus scalae tympani formed by the basioccipital, posterolateral corners of the basisphenoid strongly ventrolaterally projected, and absence of a medial parietal pillar separating the telencephalon and mesencephalon, amongst others. We also reinterpreted the structures forming the otic region of Dinilysia, confirming the presence of a crista circumfenestralis, which represents an important derived ophidian synapomorphy. Both plesiomorphic and apomorphic traits of Dinilysia are treated in detail and illustrated accordingly. Results of a phylogenetic analysis support a basal position of Dinilysia, as the sister‐taxon to all extant snakes. The fossil taxa Yurlunggur, Haasiophis, Eupodophis, Pachyrhachis, and Wonambi appear as derived snakes nested within the extant clade Alethinophidia, as stem‐taxa to the crown‐clade Macrostomata. The hypothesis of a sister‐group relationship between Dinilysia and Najash rionegrina, as suggested by some authors, is rejected by the results of our analysis. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2012, 164 , 194–238.     

Morphology and evolution of the snake cornea

To investigate whether the thickness of the cornea in snakes correlates with overall anatomy, habitat or daily activity pattern, we measured corneal thickness using optical coherence tomography scanning in 44 species from 14 families (214 specimens) in the collection at the Natural History Museum (Denmark). Specifically, we analyzed whether the thickness of the cornea varies among species in absolute terms and relative to morphometrics, such as body length, spectacle diameter, and spectacle thickness. Furthermore, we examined whether corneal thickness reflects adaptation to different habitats and/or daily activity patterns. The snakes were defined as arboreal (n = 8), terrestrial (n = 22), fossorial (n = 7), and aquatic (n = 7); 14 species were classified as diurnal and 30 as nocturnal. We reveal that the interspecific variation in corneal thickness is largely explained by differences in body size, but find a tendency towards thicker corneas in diurnal (313 ± 227 μm) compared to nocturnal species (205 ± 169 μm). Furthermore, arboreal snakes had the thickest corneas and fossorial snakes the thinnest. Our study shows that body length, habitat, and daily activity pattern could explain the interspecific variation in corneal morphology among snakes. This study provides a quantitative analysis of the evolution of the corneal morphology in snakes, and it presents baseline values of corneal thickness of multiple snake species. We speculate that the cornea likely plays a role in snake vision, despite the fact that results from previous studies suggest that the cornea in snakes is not relevant for vision (Sivak, Vision Research, 1977, 17, 293–298).     

Ontogenetic allometry of the digital rays of the leopard gecko (Gekkota: Eublepharidae; Eublepharis macularius)

The role of allometry in producing the variation in autopodial morphology observed among the lizards is not well understood. Allometry of metapodial and digit lengths in the manus and pes of the primitively padless gekkotan (Eublepharis macularius) is explored using maximum‐likelihood repeated‐measures ANCOVAs with body length as the covariate. Estimated variance–covariance matrices differed significantly within and between autopodia, and integration was stronger among the metapodials than the digits. The first metapodial and the first digit of each autopodium exhibit the strongest covariances with each of the remaining components in each variance–covariance matrix, suggesting that the lengths of the first rays are important for allometric integration of both manus and pes. Metapodials scale isometrically and digits negatively allometrically; both display allometric heterogeneity among themselves in both autopodia. Both autopodia exhibit changes in proportion over the ontogenetic size range, attributable to variation in scaling among the components of the rays. Allometric coefficients do not vary among pedal digits, despite differences in phalanx number, although phalanx number is associated with differences in slope in the manual digits. This is suggestive of heterogeneity in allometry among the manual phalanges, which thus may be associated with variation in phalanx length within gekkotan digits.     

酶祖先序列重建与定向进化

酶祖先序列重建是指通过计算机算法推导来自灭绝生物的祖先酶的氨基酸序列的技术.通常可分为6个步骤,依次为现代酶的核酸/氨基酸序列收集、多序列比对、系统发育树构建、祖先酶序列的计算机推测、基因克隆、酶学性质表征.该方法广泛应用于研究分子在行星时间尺度上对环境条件不断变化的适应性和进化机制.随着酶在生物催化领域中扮演越来越重...     

Simultaneous Expression of UV and Violet SWS1 Opsins Expands the Visual Palette in a Group of Freshwater Snakes

Snakes are known to express a rod visual opsin and two cone opsins, only (SWS1, LWS), a reduced palette resulting from their supposedly fossorial origins. Dipsadid snakes in the genus Helicops are highly visual predators that successfully invaded freshwater habitats from ancestral terrestrial-only habitats. Here, we report the first case of multiple SWS1 visual pigments in a vertebrate, simultaneously expressed in different photoreceptors and conferring both UV and violet sensitivity to Helicops snakes. Molecular analysis and in vitro expression confirmed the presence of two functional SWS1 opsins, likely the result of recent gene duplication. Evolutionary analyses indicate that each sws1 variant has undergone different evolutionary paths with strong purifying selection acting on the UV-sensitive copy and d_N/d_S ∼1 on the violet-sensitive copy. Site-directed mutagenesis points to the functional role of a single amino acid substitution, Phe86Val, in the large spectral shift between UV and violet opsins. In addition, higher densities of photoreceptors and SWS1 cones in the ventral retina suggest improved acuity in the upper visual field possibly correlated with visually guided behaviors. The expanded visual opsin repertoire and specialized retinal architecture are likely to improve photon uptake in underwater and terrestrial environments, and provide the neural substrate for a gain in chromatic discrimination, potentially conferring unique color vision in the UV–violet range. Our findings highlight the innovative solutions undertaken by a highly specialized lineage to tackle the challenges imposed by the invasion of novel photic environments and the extraordinary diversity of evolutionary trajectories taken by visual opsin-based perception in vertebrates.