TY - JOUR
AB - In zebrafish, as in other vertebrates, an initially singular eye held within the neural plate has to split during morphogenesis to allow the development of two separated eyes. It has been suggested that anterior progression of midline tissue within the neural plate is involved in the bilateralization of the eye held. Mutations in the recently identified silberblick (slb) gene cause an incomplete separation of the eyes. During gastrulation and early somitogenesis, the ventral midline of the central nervous system (CNS) together with the underlying axial mesendoderm is shortened and broadened in slb embryos. While in wild-type embryos the ventral CNS midline extends to the anterior limit of the neural plate at the end of gastrulation, there is a gap between the anterior tip of the ventral CNS midline and the anterior edge of the neural plate in slb. To investigate the cause for the shortening of the ventral CNS midline in slb we determined the fate of labeled ventral CNS midline cells in wild-type and slb embryos at different stages of development. In slb, anterior migration of ventral CNS midline cells is impaired, which indicates that migration of these cells is needed for elongation of the ventral CNS midline. The anterior shortening of the ventral CNS midline in slb leads to medial instead of bilateral induction of optic stalks followed by a partial fusion of the eyes at later developmental stages. The analysis of the sIb phenotype indicates that anterior migration of midline cells within the neural plate is required for proper induction and subsequent bilateralization of an initially singular eye field. These findings may therefore provide a starting point in elucidating the role of neural plate morphogenesis in positioning of the eyes. (C) 1997 Academic Press.
AU - Heisenberg, Carl-Philipp
AU - NÃ¼sslein-Volhard, Christiane N
ID - 4201
IS - 1
JF - Developmental Biology
TI - The function of silberblick in the positioning of the eye anlage in the zebrafish embryo
VL - 184
ER -
TY - GEN
AB - We evaluate Sewall Wright's three-phase 'shifting balance' theory of evolution, examining both the theoretical issues and the relevant data from nature and the laboratory. We conclude that while phases I and II of Wright's theory (the movement of populations from one 'adaptive peak' to another via drift and selection) can occur under some conditions, genetic drift is often unnecessary for movement between peaks. Phase III of the shifting balance, in which adaptations spread from particular populations to the entire species, faces two major theoretical obstacles: (1) unlike adaptations favored by simple directional selection, adaptations whose fixation requires some genetic drift are often prevented from spreading by barriers to gene flow; and (2) it is difficult to assemble complex adaptations whose constituent parts arise via peak shifts in different demes. Our review of the data from nature shows that although there is some evidence for individual phases of the shifting balance process, there are few empirical observations explained better by Wright's three-phase mechanism than by simple mass selection. Similarly, artificial selection experiments fail to show that selection in subdivided populations produces greater response than does mass selection in large populations. The complexity of the shifting balance process and the difficulty of establishing that adaptive valleys have been crossed by genetic drift make it impossible to test Wright's claim that adaptations commonly originate by this process. In view of these problems, it seems unreasonable to consider the shifting balance process as an important explanation for the evolution of adaptations.
AU - Coyne, Jerry A
AU - Nicholas Barton
AU - Turelli, Michael
ID - 4287
IS - 3
T2 - Evolution; International Journal of Organic Evolution
TI - Perspective: A critique of Sewall Wright's shifting balance theory of evolutionight's shifting balance theory of evolution
VL - 51
ER -
TY - CONF
AB - Rectangular hybrid automata model digital control programs of analog plant environments. We study rectangular hybrid automata where the plant state evolves continuously in real-numbered time, and the controller samples the plant state and changes the control state discretely, only at the integer points in time. We prove that rectangular hybrid automata have finite bisimilarity quotients when all control transitions happen at integer times, even if the constraints on the derivatives of the variables vary between control states. This is sharply in contrast with the conventional model where control transitions may happen at any real time, and already the reachability problem is undecidable. Based on the finite bisimilarity quotients, we give an exponential algorithm for the symbolic sampling-controller synthesis of rectangular automata. We show our algorithm to be optimal by proving the problem to be EXPTIME-hard. We also show that rectangular automata form a maximal class of systems for which the sampling-controller synthesis problem can be solved algorithmically.
AU - Thomas Henzinger
AU - Kopke, Peter W
ID - 4441
TI - Discrete-time control for rectangular hybrid automata
VL - 1256
ER -
TY - CONF
AB - The simulation preorder for labeled transition systems is defined locally as a game that relates states with their immediate successor states. Liveness assumptions about transition systems are typically modeled using fairness constraints. Existing notions of simulation for fair transition systems, however, are not local, and as a result, many appealing properties of the simulation preorder are lost. We extend the local definition of simulation to account for fairness: system S fairly simulates system I iff in the simulation game, there is a strategy that matches with each fair computation of I a fair computation of S. Our definition enjoys a fully abstract semantics and has a logical characterization: S fairly simulates I iff every fair computation tree embedded in the unrolling of I can be embedded also in the unrolling of S or, equivalently, iff every Fair-AFMC formula satisfied by I is satisfied also by S (AFMC is the universal fragment of the alternation-free -calculus). The locality of the definition leads us to a polynomial-time algorithm for checking fair simulation for finite-state systems with weak and strong fairness constraints. Finally, fair simulation implies fair trace-containment, and is therefore useful as an efficientlycomputable local criterion for proving linear-time abstraction hierarchies.
AU - Thomas Henzinger
AU - Kupferman, Orna
AU - Rajamani, Sriram K
ID - 4496
TI - Fair simulation
VL - 1243
ER -
TY - CONF
AB - A hybrid system is a dynamical system whose behavior exhibits both discrete and continuous change. A hybrid automaton is a mathematical model for hybrid systems, which combines, in a single formalism, automaton transitions for capturing discrete change with differential equations for capturing continuous change. In this survey, we demonstrate symbolic algorithms for the verification of and controller synthesis for linear hybrid automata, a subclass of hybrid automata that can be analyzed automatically
AU - Alur, Rajeev
AU - Thomas Henzinger
AU - Wong-Toi, Howard
ID - 4605
TI - Symbolic analysis of hybrid systems
ER -