Why Are There So Many Cichlid Species? On the Interplay of Speciation and Adaptive Radiation

The explosive speciation of cichlid fishes in the African great lakes has intrigued biologists for many decades. Interest was revitalized in 1996 after the publication in Science of geological data [1] indicating that the youngest lake, Lake Victoria, must have been completely dry during the most recent Ice perhaps as recently as 12,400 years ago. This implies that the approximately 500 haplochromine cichlid species must have evolved within this extremely short timespan from a single ancestral species [2]. But even with lower estimates of species number and higher estimates of the age of the species flock, the haplochromine cichlids still present one of the most dramatic examples of speciation and diversification in vertebrates

Hox Genes, Digit Identities and the Theropod/Bird Transition

Vargas and Fallon (2005) propose that Hox gene expression patterns indicate that the most anterior digit in bird wings is homologous to digit 1 rather than to digit 2 in other amniotes. This interpretation is based on the presence of Hoxd13 expression in combination with the absence of Hoxd12 expression in the second digit condensation from which this digit develops (the first condensation is transiently present). This is a pattern that is similar to that in the developing digit 1 of the chicken foot and the mouse hand and foot. They have tested this new hypothesis by analysing Hoxd12 and Hoxd13 expression patterns in two polydactylous chicken mutants, Silkie and talpid2. They conclude that the data supports the notion that the most anterior remaining digit of the bird wing is homologous to digit 1 in other amniotes either in a standard phylogenetic sense, or alternatively in a (limited) developmental sense in agreement with the Frameshift Hypothesis of Wagner and Gautier (1997, i.e. that the developmental pathway is homologous to the one that leads to a digit 1 identity in other amniotes, albeit that it occurs in the second instead of the first digit condensation). We argue that the Hoxd12 and Hoxd13 expression patterns found for these and other limb mutants do not allow distinguishing between the hypothesis of Vargas and Fallon (2005) and the alternative one, i.e. the most anterior digit in bird wings is homologous to digit 2 in other amniotes, in a phylogenetic, or developmental sense. Therefore, at the moment the data on limb mutants does not present a challenge to the hypothesis, based on other developmental data (Holmgren 55, Hinchliffe 484, Burke and Feduccia 497, Kundrat et al. 2002, Larsson and Wagner 2002, Feduccia and Nowicki 2002), that the digits of bird wings are homologous to digits 2,3,4 in amniotes. We recommend further testing of the hypothesis by comparing Hoxd expression patterns in different taxa

Why Five Fingers? Evolutionary Constraints on Digit Numbers

Evolutionary changes in the number of digits and other limb elements appear to be severely constrained, probably as a result of a low level of modularity during limb development. Reduced limb structures typically develop through a process of construction followed by destruction and amniotes have evolved many digit-like structures rather than actual extra digits. In amniotes, limb development occurs during the crucial phylotypic stage, when many inductive interactions are occurring throughout the body. As a result, changes in limb development usually engender changes in other body parts. Thus, mutations that change the number of limb bones are expected to have many pleiotropic effects, which severely reduces the chance of such mutations being successful. In amphibians with aquatic larvae, limb development occurs after the phylotypic stage and limb development is decoupled from the interactivity of the phylotypic stage. The constraint of pleiotropic effects is, therefore, expected to be weaker. This expectation agrees with the larger variability in the number of hand and foot structures in amphibians, with frogs even occasionally possessing six toes. These facts once again emphasize the importance of pleiotropic effects as constraints to evolutionary change, including their role in the conservation of body plans

Conservation of the Segmented Germband Stage: Robustness or Pleiotropy?

Gene expression patterns of the segment polarity genes in the extended and segmented germband stage are remarkably conserved among insects. To explain the conservation of these stages, two hypotheses have been proposed. One hypothesis states that the conservation reflects a high interactivity between modules, so that mutations would have several pleiotropic effects in other parts of the body, resulting in stabilizing selection against mutational variation. The other hypothesis states that the conservation is caused by robustness of the segment polarity network against mutational changes. When evaluating the empirical evidence for these hypotheses, we found strong support for pleiotropy and little evidence supporting robustness of the segment polarity network. This points to a key role for stabilizing selection in the conservation of these stages. Finally, we discuss the implications for robustness of organizers and long-term conservation in general

Dynamic Monitoring of Data Center Slices

Slicing is a move towards segmentation of resources and deployment of NFV for the purpose of enhanced services and applications on globally shared resources. The slicing approach in this paper considers Data Center slicing and the VIM on-demand model. We focus on the monitoring of Data Center slices, showing what is needed from the monitoring perspective and how the monitoring should be done. The proposed monitoring approach is validated on a platform that supports the on-demand creation of lightweight VIM instances

Real-Time Management and Control of Monitoring Elements In Dynamic Cloud Network Systems

This paper explores new scenarios where Cloud Network Service Providers take advantage of using more flexible resource management and orchestration solutions in the form of dynamic virtualised compute, network and storage resources. The main focus of this work is to analyse how those challenges will considerably impact the requirements of the monitoring process. A framework in the context of 5G is here presented to support the dynamic on-demand management, configuration and control of a monitoring subsystem which: can easily scale up / down according to the number of running entities in the system as result of the instantiation / termination of multiple services; can provide mechanisms to dynamically activate / deactivate its constituent elements on-demand according to the type of services to be monitored; and can provide mechanisms to dynamically adjust the configuration if its elements. Experimental outcomes, where a Monitoring Controller was used to adjust the measurement collection / sending rate of the probes in the monitoring subsystem on the-fly are also presented. The paper shows how this prevented the transmission of vast amounts of data when the number of virtual entities and related monitoring probes in the system scaled up to hundreds of elements

Extending Slices into Data Centers: the VIM on-demand model

This paper explores some of the mechanisms, components, and abstractions that can be utilized in order to encompass network slicing into a bigger picture for NFV delivery. In particular, we make the case for Data Center (DC) infrastructure slicing, as part of the full NFVI foundation, to ensure that the attributes prescribed to network slices are propagated into the Data Center. We show how creating a VIM (Virtual Infrastructure Manager) on-demand and dynamically allocating a new VIM for each slice, rather than having one for the whole DC, which can be beneficial for various precision scenarios. Index Terms—infrastructure slicing, VIM, network slicing

Increased prevalence of abnormal vertebral patterning in fetuses and neonates with trisomy 21

Purpose: To assess the prevalence of an abnormal number of ribs in a cohort of fetuses and neonates with trisomy 21 and compare this with a subgroup of fetuses without anomalies. Materials and methods: Radiographs of 67 deceased fetuses, neonates, and infants that were diagnosed with trisomy 21 were reviewed. Terminations of pregnancy were included. The control group was composed of 107 deceased fetuses, neonates, and infants without known chromosomal abnormalities, structural malformations, infections or placental pathology. Cases in which the number of thoracic ribs or presence of cervical ribs could not be reliably assessed were excluded. The literature concerning vertebral patterning in trisomy 21 cases and healthy subjects was reviewed. Results: Absent or rudimentary 12th thoracic ribs were found in 26/54 (48.1%) cases with trisomy 21 and cervical ribs were present in 27/47 (57.4%) cases. This prevalence was significantly higher compared to controls (28/100, 28.0%, Χ2(1) = 6.252, p = .012 and 28/97, 28.9%, Χ2(1) = 10.955, p < .001, respectively). Conclusions: Rudimentary or absent 12th thoracic ribs and cervical ribs are significantly more prevalent in deceased fetuses and infants with trisomy 21