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PubMed2005

Hind limb proportions and kinematics: are small primates different from other small mammals?

Similar in body size, locomotor behaviour and morphology to the last common ancestor of Primates, living small quadrupedal primates provide a convenient model for investigating the evolution of primate locomotion. In this study, the hind limb kinematics of quadrupedal walking in mouse lemurs, brown lemurs, cotton-top tamarins and squirrel monkeys are analysed using cineradiography. The scaling of hind limb length to body size and the intralimb proportions of the three-segmented hind limb are taken into consideration when kinematic similarities and differences are discussed. Hind limb kinematics of arboreal quadrupedal primates, ranging in size between 100 g and 3000 g, are size independent and resemble the hind limb kinematics of small non-cursorial mammals. A common feature seen in smaller mammals, in general, is the horizontal position of the thigh at touchdown and of the lower leg at lift-off. Thus, the maximum bone length is immediately transferred into the step length. The vertical position of the leg at the beginning of a step cycle and of the thigh at lift-off contributes the same distance to pivot height. Step length and pivot height increase proportionally with hind limb length, because intralimb proportions of the hind limb remain fairly constant. Therefore, the strong positive allometric scaling of the hind limb in arboreal quadrupedal primates affects neither the kinematics of hind limb segments nor the total angular excursion of the limb. The angular excursion of the hind limb in quadrupedal primates is equal to that of other non-cursorial mammals. Hence, hind limb excursion in larger cercopithecine primates differs from that of other large mammals due to the decreasing angular excursion as part of convergent cursorial adaptations in several phylogenetic lineages of mammals. Typical members of those phylogenetic groups are traditionally used in comparison with typical primates, and therefore the ;uniqueness' of primate locomotor characteristics is often overrated.

Age Factors↗

PubMedDate not supplied

Transposable elements as mutator genes in evolution.

Strains of the bacterium Escherichia coli harbouring genes that increase mutation rates are known to have an evolutionary advantage in chemostat competition over otherwise isogeneic strains with lower mutation rates. This advantage is frequency-dependent, the mutator strain being favoured only above a starting ratio of approximately 5 x 10(-5), and it results from the fact that the necessary beneficial mutations cannot be generated in a mutator population below a certain size. Here we consider the possibility that the mutagenic properties of transposable elements confer an advantage in the same manner as mutator genes. A previous report has shown that the transposon Tn5 increases the fitness of E. coli in chemostats, although the reason for this effect has not been established. Our results show that the transposon Tn10 also confers an advantage in chemostats. In addition, we find that (1) this advantage, like that associated with mutator genes, is frequency-dependent, (2) whenever the Tn10 strains win, a segment of Tn10, probably its IS10 sequences, has undergone transposition to a new site, (3) the new insertions converge into a site contained within a 3.2 kilobase (kb) PvuII fragment of the genome, and (4) no transpositions are detected when the Tn10 population loses. We conclude that Tn10 confers an advantage by increasing the mutation rate of the host bacterium.

Biological Evolution↗

PubMedDate not supplied

Evolution and development of brain sensory organs in molgulid ascidians.

The ascidian tadpole larva has two brain sensory organs containing melanocytes: the otolith, a gravity receptor, and the ocellus, part of a photoreceptor. One or both of these sensory organs are absent in molgulid ascidians. We show here that developmental changes leading to the loss of sensory pigment cells occur by different mechanisms in closely related molgulid species. Sensory pigment cells are formed through a bilateral determination pathway in which two or more precursor cells are specified as an equivalence group on each side of the embryo. The precursor cells subsequently converge at the midline after neurulation and undergo cell interactions that decide the fates of the otolith and ocellus. Molgula occidentalis and M. oculata, which exhibit a tadpole larva with an otolith but lacking an ocellus, have conserved the bilateral pigment cell determination pathway. Programmed cell death (PCD) is superimposed on this pathway late in development to eliminate the ocellus precursor and supernumerary pigment cells, which do not differentiate into either an otolith or ocellus. In contrast to molgulids with tadpole larvae, no pigment cell precursors are specified on either side of the M. occulta embryo, which forms a tailless (anural) larva lacking both sensory organs, suggesting that the bilateral pigment cell determination pathway has been lost. The bilateral pigment cell determination pathway and superimposed PCD can be restored in hybrids obtained by fertilizing M. occulta eggs with M. oculata sperm, indicating control by a zygotic process. We conclude that PCD plays an important role in the evolution and development of brain sensory organs in molgulid ascidians.

Animals↗

PubMed2004

Different versions of the Dayhoff rate matrix.

Many phylogenetic inference methods are based on Markov models of sequence evolution. These are usually expressed in terms of a matrix (Q) of instantaneous rates of change but some models of amino acid replacement, most notably the PAM model of Dayhoff and colleagues, were originally published only in terms of time-dependent probability matrices (P(t)). Previously published methods for deriving Q have used eigen-decomposition of an approximation to P(t). We show that the commonly used value of t is too large to ensure convergence of the estimates of elements of Q. We describe two simpler alternative methods for deriving Q from information such as that published by Dayhoff and colleagues. Neither of these methods requires approximation or eigen-decomposition. We identify the methods used to derive various different versions of the Dayhoff model in current software, perform a comparison of existing and new implementations, and, to facilitate agreement among scientists using supposedly identical models, recommend that one of the new methods be used as a standard.

Animals↗

PubMed2005

Cortical complexity in cetacean brains.

Cetaceans (dolphins, whales, and porpoises) have a long, dramatically divergent evolutionary history compared with terrestrial mammals. Throughout their 55-60 million years of evolution, cetaceans acquired a compelling set of characteristics that include echolocation ability (in odontocetes), complex auditory and communicative capacities, and complex social organization. Moreover, although cetaceans have not shared a common ancestor with primates for over 90 million years, they possess a set of cognitive attributes that are strikingly convergent with those of many primates, including great apes and humans. In contrast, cetaceans have evolved a highly unusual combination of neurobiological features different from that of primates. As such, cetacean brains offer a critical opportunity to address questions about how complex behavior can be based on very different neuroanatomical and neurobiological evolutionary products. Cetacean brains and primate brains are arguably most meaningfully conceived as alternative evolutionary routes to neurobiological and cognitive complexity. In this article, we summarize data on brain size and hemisphere surface configuration in several cetacean species and present an overview of the cytoarchitectural complexity of the cerebral cortex of the bottlenose dolphin.

Animals↗

PubMed1983

Selective stabilization of muscle innervation during development: a mathematical model.

The biochemical model presented concerns a critical step of the development of skeletal muscle innervation. After invasion of the muscle by exploratory motor axons, several nerve terminals converge from different motoneurons onto each muscle fibre at a single endplate. During the following weeks the redundant innervation disappears: a single nerve ending per muscle fibre becomes stabilized. The model is based on the assumption that the numbers of motoneurons and of muscle fibres remain constant during this evolution and that the selective stabilization of the adult connectivity results from the competition of the active nerve terminals for a postsynaptic retrograde factor mu. At the peak of the multiple innervation, the synthesis of mu by the muscle fiber stops, possibly as a consequence of muscle electrical and/or mechanical activity. The stock of mu becomes limited; a retrograde trans-synaptic diffusion of mu from the muscle to the nerve endings takes place. Within each nerve ending, mu enters into a chemical autocatalytic reaction which results in the production of a presynaptic stabilization factor s. The nerve impulses reaching the nerve terminal initiate this reaction. Any given nerve terminal become stabilized when the concentration of s reaches a threshold value. The mathematical analysis of the model shows that there exists a unique solution which is physically acceptable. Its application and computer simulation predict that only one nerve terminal becomes stabilized per muscle fibre. The model accounts for the experimental observations that the reduction in size of the motor units is not necessarily accompanied by a reduction in the variability of their size. The model also accounts for the acceleration or delay in regression which follows modifications of the chronic activity of the nerve endings and for the variability of the pattern of innervation observed in isogenic organisms. Plausible biochemical hypotheses concerning the factors engaged in the "selective stabilization" of the nerve-endings are discussed.

Animals↗

PubMed2000

Two-dimensional incommensurately modulated structure of (Sr0.13Ca0. 87)2CoSi2O7 crystals.

The incommensurate structure of (Sr(0.13)Ca(0.87))(2)CoSi(2)O(7) at room temperature has been determined from single-crystal X-ray diffraction data. The compound has a non-centrosymmetric tetragonal basic cell of a = 7.8743 (4) and c = 5.0417 (2) A with the space group P4;2(1)m. The refinements of the basic structure converged to R = 0.038 for 757 main reflections. The two-dimensional incommensurate structure is characterized by the wavevectors q(1) = 0.286 (3)(a* + b*) and q(2) = 0.286 (3)(-a* + b*), where a*, b* are the reciprocal lattice vectors of the basic structure. With the (3 + 2)-dimensional superspace group P(p4mg)(P4;2(1)(m)), the refinements converged to R = 0.071 for 1697 observed reflections (757 main and 940 satellite reflections). The structure is described in terms of displacement of the atoms, rotation, distortion of CoO(4) and SiO(4) tetrahedra, and the partial ordering of the Sr and Ca atoms accompanied with the modulation. Correlated evolution of these features throughout the crystal gives rise to various oxygen coordination around Ca/Sr. Comparison of the derived modulated structure to that of Ca(2)CoSi(2)O(7) clarified that the partial substitution of Ca by large alkaline-earth atoms such as Sr should decrease the distortion of the polyhedra around the cations.

Journal Article↗

PubMed2003

Empirical evidence for an optimal body size in snakes.

The concept of optimal size has been invoked to explain patterns in body size of terrestrial mammals. However, the generality of this phenomenon has not been tested with similarly complete data from other taxonomic groups. In this study we describe three statistical patterns of body size in snakes, all of which indicate an optimal length of 1.0 m. First, a distribution of largest body lengths of 618 snake species had a single mode at 1.0 m. Second, we found a positive relationship between the size of the largest member of an island snake assemblage and island area and a negative relationship between the size of the smallest member of an island snake assemblage and island area. Best-fit lines through these data cross at a point corresponding to 1.0 m in body length, the presumed optimal size for a one-species island. Third, mainland snake species smaller than 1.0 m become larger on islands whereas those larger than 1.0 m become smaller on islands. The observation that all three analyses converge on a common body size is concordant with patterns observed in mammals and partial analyses of four other disparate animal clades. Because snakes differ so strikingly from mammals (ectotherms, gape-limited predators, elongate body shape) the concordant patterns of these two groups provide strong evidence for the evolution of an optimal body size within independent monophyletic groups. However, snakes differ from other taxonomic groups that have been studied in exhibiting a body size distribution that is not obviously skewed in either direction. We suggest that idiosyncratic features of the natural history of ectotherms allow relatively unconstrained distributions of body size whereas physiological limitations of endotherms constrain distributions of body size to a right skew.

Animals↗

PubMed1996

[Strabismus in Cameroon].

PURPOSE: We undertook a study on strabismus in order to determine the characteristics of this disease in Cameroon. MATERIAL AND METHODS: A prospective study was carried out in the ophthalmological unit of the Douala General Hospital from November 1991 to October 1995. All patients were examined and followed regularly by the same team. RESULTS: We found 137 strabismus, which represent 1.22% of all patients with 51 esotropias (37.2%) and 86 exotropias (62.8%). The mean age was 13.61 years at the first consultation. 82.4% of convergent strabismus appeared before the age of one year against 67.4% of divergent strabismus. Moreover, amblyopia was found in 68.6% and 59.3% of cases respectively. CONCLUSION: As compared to European series, this study shows notably the great frequency of divergent squint in black africans and the fact that spontaneous evolution of the latter can lead to amblyopia in a large number of cases.

Adolescent↗

PubMed2005

Evolutionarily stable strategies and short-term selection in Mendelian populations re-visited.

This note concerns a one locus, two allele, random mating diploid population, subject to frequency-dependent viability selection. It is already known that in such a population, any evolutionarily stable strategies (ESS), if only accessible by the genotype-to-phenotype mapping, is the phenotypic image of a stable genetic equilibrium (Eshel, I. 1982. Evolutionarily stable strategies and viability selection in Mendelian populations. Theor. Popul. Biol. 22(2), 204-217; Cressman et al. 1996. Evolutionary stability in strategic models of single-locus frequency-dependent viability selection. J. Math. Biol. 34, 707-733). The opposite is not true. We find necessary and sufficient parametric conditions for global convergence to the ESS, but we also demonstrate conditions under which, although a unique, genetically accessible ESS exists, there is another, "non-phenotypic" genetically stable equilibrium.

Animals↗

PubMed1993

Use of evolutionary limitations of HIV-1 multidrug resistance to optimize therapy.

Wild-type reverse transcriptase has evolved for the survival of human immunodeficiency virus type 1 (HIV-1) by natural selection. In contrast, therapy relying on inhibitors of reverse transcriptase by nucleosides like zidovudine (AZT) or dideoxyinosine (ddI), and by non-nucleosides like pyridinones or nevirapine, may exert different selection pressures on this enzyme. Therefore the acquisition of resistance to reverse transcriptase inhibitors by selection of mutations in the pol gene may require compromises in enzyme function that affect viral replication. As single mutations are unlikely to confer broad resistance when combinations of reverse transcriptase inhibitors are used, multiple mutations may occur that result in further compromises. Certain drug combinations may prevent the co-existence of adequate reverse transcription function and multi-drug resistance (MDR). Unlike bacterial or eukaryotic drug resistance, retroviral drug resistance is conferred only by mutations in its own genome and is limited by genome size. Combining drugs directed against the same essential viral protein may thus prevent HIV-1 MDR, whereas the conventional approach of targeting different HIV-1 proteins for combination therapy may not, because genomes with resistance mutations in different HIV-1 genes might recombine to develop MDR. Here we show that several mutations in the HIV-1 reverse transcriptase gene that confer resistance to inhibitors of this enzyme can attenuate viral replication. We tested whether combinations of mutations giving rise to single-agent resistance might further compromise or even abolish viral replication, and if multidrug-resistant viruses could be constructed. Certain combinations of mutations conferring resistance to AZT, ddI and pyridinone are incompatible with viral replication. These results indicate that evolutionary limitations exist to restrict development of MDR. Furthermore, a therapeutic strategy exploiting these limitations by using selected multidrug regimens directed against the same target may prevent development of MDR. This approach, which we call convergent combination therapy, eliminated HIV-1 replication and virus breakthrough in vitro, and may be applicable to other viral targets. Moreover, elimination of reverse transcription by convergent combination therapy may also limit MDR.

Acquired Immunodeficiency Syndrome↗

PubMed1989

[Semeiology of accommodative strabismus].

Only the evolution under the total optical correction permits to define whether a strabismus is partially or totally accommodative. This answer can take very variable forms.

Accommodation, Ocular↗

PubMed2002

Identifiability of parameters in MCMC Bayesian inference of phylogeny.

Methods for Bayesian inference of phylogeny using DNA sequences based on Markov chain Monte Carlo (MCMC) techniques allow the incorporation of arbitrarily complex models of the DNA substitution process, and other aspects of evolution. This has increased the realism of models, potentially improving the accuracy of the methods, and is largely responsible for their recent popularity. Another consequence of the increased complexity of models in Bayesian phylogenetics is that these models have, in several cases, become overparameterized. In such cases, some parameters of the model are not identifiable; different combinations of nonidentifiable parameters lead to the same likelihood, making it impossible to decide among the potential parameter values based on the data. Overparameterized models can also slow the rate of convergence of MCMC algorithms due to large negative correlations among parameters in the posterior probability distribution. Functions of parameters can sometimes be found, in overparameterized models, that are identifiable, and inferences based on these functions are legitimate. Examples are presented of overparameterized models that have been proposed in the context of several Bayesian methods for inferring the relative ages of nodes in a phylogeny when the substitution rate evolves over time.

Algorithms↗

PubMed2002

Evolution of cerebellum-like structures.

All vertebrate brains have a cerebellum, and most of them have one or more additional structures that are histologically similar to the cerebellum. The cerebellum-like structures include the medial octavolateral nucleus in most aquatic vertebrates; the dorsal octavolateral nucleus in many aquatic vertebrates with an electrosensory system; the marginal layer of the optic tectum in ray-finned fishes; electrosensory lobes in the few groups of advanced bony fish with an electrosensory system; the rostrolateral nucleus of the thalamus in a few widely scattered groups of bony fish; and the dorsal cochlear nucleus in all mammals except monotremes. All of these structures receive topographically organized sensory input in their deep layers. Purkinje-like cells receive the sensory input near their cell bodies. These cells extend apical dendrites up into the molecular layer where they receive synaptic input from parallel fibers. The cerebellum itself can be included within this characterization by considering the climbing fiber as at least in part a conveyor of sensory information and by recalling that climbing fibers in more basal vertebrates terminate on smooth dendrites close to the soma. Physiological findings from three different systems suggest the hypothesis that cerebellum-like structures remove predictable features from the sensory inflow. Phylogenetic homology can explain the similarities across different taxa for some types of cerebellum-like structures, but similarities within other types cannot be explained in this way. Moreover, phylogenetic homology cannot explain the similarities among different types of cerebellum-like structures. Evolutionary convergence provides the best explanation for all these similarities that cannot be explained by homology. The convergence is almost surely constrained by the availability of a genetic-developmental program for creating cerebellum-like circuitry and by the need within many different systems for the type of information processing that cerebellum-like circuitry can provide.

Animals↗

PubMed2000

Effect of inversion polymorphism on the neutral nucleotide variability of linked chromosomal regions in Drosophila.

Recombination is a main factor determining nucleotide variability in different regions of the genome. Chromosomal inversions, which are ubiquitous in the genus Drosophila, are known to reduce and redistribute recombination, and thus their specific effect on nucleotide variation may be of major importance as an explanatory factor for levels of DNA variation. Here, we use the coalescent approach to study this effect. First, we develop analytical expressions to predict nucleotide variability in old inversion polymorphisms that have reached mutation-drift-flux equilibrium. The effects on nucleotide variability of a new arrangement appearing in the population and reaching a stable polymorphism are then studied by computer simulation. We show that inversions modulate nucleotide variability in a complex way. The establishment of an inversion polymorphism involves a partial selective sweep that eliminates part of the variability in the population. This is followed by a slow convergence to the equilibrium values. During this convergence, regions close to the breakpoints exhibit much lower variability than central regions. However, at equilibrium, regions close to the breakpoints have higher levels of variability and differentiation between arrangements than regions in the middle of the inverted segment. The implications of these findings for overall variability levels during the evolution of Drosophila species are discussed.

Animals↗

PubMed2005

Acceleration- and deceleration-phase nonlinear Rayleigh-Taylor growth at spherical interfaces.

The Layzer model for the nonlinear evolution of bubbles in the Rayleigh-Taylor instability has recently been generalized to the case of spherically imploding interfaces [D. S. Clark and M. Tabak, Phys. Rev. E 71, 055302(R) (2005)]. The spherical case is more relevant to, e.g., inertial confinement fusion or various astrophysical phenomena when the convergence is strong or the perturbation wavelength is comparable to the interface curvature. Here, the model is further extended to the case of bubble growth during the deceleration (stagnation) phase of a spherical implosion and to the growth of spikes during both the acceleration and deceleration phases. Differences in the nonlinear growth rates for both bubbles and spikes are found when compared with planar results. The model predictions are verified by comparison with numerical hydrodynamics simulations.

Journal Article↗

PubMed1997

The Cbl protooncogene product: from an enigmatic oncogene to center stage of signal transduction.

The c-cbl protooncogene was first identified as the cellular homologue of a viral oncogene v-cbl that induces pre-B lymphomas and myeloid leukemias in mice. Until recently, the biochemical basis for Cbl's transforming potential and its physiological role remained unclear. However, a convergence of biochemical studies in mammalian cells and genetic studies in C. elegans and Drosophila has now identified Cbl as a negative regulator of tyrosine kinase signaling. The N-terminal transforming region of Cbl (Cbl-N) and an adjacent RING finger domain are the elements most conserved during evolution. The Cbl-N region has now been shown to contain a novel phosphotyrosine-binding (PTB) domain that directly interacts with autophosphorylated tyrosine kinases via a D(N/D)XpY motif. A critical role of the PTB domain in Cbl function is demonstrated by the localization of a loss-of-function mutation in C. elegans Cbl homologue SLI-1 within this region. The corresponding mutation in human Cbl inactivates the PTB domain function and abrogates Cbl-mediated regulation of tyrosine kinase function. Recent studies have also identified a novel signaling pathway initiated by the interaction of mammalian Cbl proteins with the SH2 domains of Crk adaptor molecules, which results in Cbl's linkage with C3G, a guanine nucleotide exchange protein for Rap1 family of small G-proteins. Presently, Rap1 is thought to antagonize Ras function, although Rap1-specific targets have emerged recently. Thus, recent advances have firmly placed the little known protooncoprotein Cbl on the center stage of tyrosine kinase-mediated signal transduction.

Amino Acid Sequence↗

PubMed2004

Parent-offspring conflict in the evolution of vertebrate reproductive mode.

We propose and evaluate the hypothesis that parent-offspring conflict over the degree of maternal investment has been one of the main selective factors in the evolution of vertebrate reproductive mode. This hypothesis is supported by data showing that the assumptions of parent-offspring conflict theory are met for relevant taxa; the high number of independent origins of viviparity, matrotrophy (direct maternal-fetal nutrient transfer), and hemochorial placentation (direct fetal access to the maternal bloodstream); the extreme diversity in physiological and morphological aspects of viviparity and placentation, which usually cannot be ascribed adaptive significance in terms of ecological factors; and divergent and convergent patterns in the diversification of placental structure, function, and developmental genetics. This hypothesis is also supported by data demonstrating that embryos and fetuses actively manipulate their interaction with the mother, thereby garnishing increased maternal resources. Our results indicate that selection may favor adaptations of the mother, the fetus, or both in traits related to reproductive mode and that integration of physiological and morphological data with evolutionary ecological data will be required to understand the adaptive significance of interspecific variation in viviparity, matrotrophy, and placentation.

Adaptation, Physiological↗