In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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4 Evolving Specialization of the Arthropod Nervous System--Erin Jarvis, Heather S. Bruce, and Nipam H. Patel
4 Evolving Specialization of the Arthropod Nervous System--Erin Jarvis, Heather S. Bruce, and Nipam H. Patel
Pages 61-74
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From page 61... ...
Although there is a remarkable degree of conservation in neural development both between individual segments in any given species and between the nervous systems of different arthro pod groups, the differences that do exist are informative for inferring general principles about the holistic evolution of body plans. This review describes developmental processes controlling neural segmentation and regionalization, highlighting segmentation mechanisms that create both ectodermal and neural segments, as well as recent studies of the role of Hox genes in generating regional specification within the central nervous system.
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From page 62... ...
Subsequent studies revealed a remarkable level of evolutionary conservation of these Hox gene transcription factors, and it appears that Hox genes play a well-conserved role in patterning regional identity along the antero-posterior axis in all bilaterian animals. Whereas Hox genes have provided developmental biologists with an outstanding example of a deeply conserved mechanism of pattern formation, changes in these genes have also been implicated in the evolutionary process that has led to the diversification of body plans both between and within animal phyla.
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From page 63... ...
(A) Hierarchy of maternal gradients and zygotic gap, pair-rule, and segment polarity genes establishes the repetition of segments, whereas the homeotic genes regionalize the body plan, making segments differ from one another.
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From page 64... ...
The anterior boundary of Ubx expression in various crustacean species corresponds to the transition point from feeding to locomotory appendages. The head appendages of Mn, MxI, and MxII are not shown, but would also be classified as feeding (jaw)
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From page 65... ...
We suggest that the Hox gene system functions in arthropods in a manner that facilitates such a coordinated transformation. Our purpose here is to review the manner in which the nervous system is patterned in arthropods, highlighting first that the same system used for ectodermal segmentation, particularly at the level of segment polarity genes, contributes to generating the segmental organization of the nervous system, and second, that Hox genes play a major role in the regionalization of the nervous system just as they do for the ectoderm.
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From page 66... ...
Each ganglion mother cell divides once, symmetrically, to produce either two postmitotic neurons or two postmitotic glial cells (Doe and Goodman, 1985; Campos-Ortega, 1995)
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From page 67... ...
Detailed studies of the function of segment polarity genes reveal that this level of the segmentation hierarchy acts to pattern the NBs in a manner similar to its role in patterning the overlying ectoderm, although in a few cases it is possible to separate the function of segment polarity genes for patterning the neuroblasts vs. the ectoderm (Chu-LaGraff and Doe, 1993; Duman-Scheel et al., 1997)
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From page 68... ...
This process results in a specific and highly reproducible arrangement of ~600 neurons and glial cells within each segment of the nervous system. Many of these neurons are uniquely identifiable on the basis of morphological criteria such as cell body position and patterns of axonal projection, as well as on the basis of molecular criteria such as patterns of transcription factor and neurotransmitter expression (and some sets of glia are also uniquely identifiable on the basis of cell body position and trancription factor expression)
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From page 69... ...
CycE localizes to one daughter cell via asymmetric division of the neuroblast, which marks it for a neural fate; the absence of CycE in the other daughter cell promotes a glial fate. In the abdomen, abd-A and Abd-B directly repress CycE, and the NB divides symmetrically to produce only glial cells (Kannan et al., 2010)
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From page 70... ...
and differentiated motor neurons. The regulation of apoptosis has become increasingly refined throughout evolution, and the key roles Hox genes play in the selective death and survival of neurons support their utility in the evolution of neuronal diversification along the anteroposterior axis (Miguel-Aliaga and Thor, 2004)
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From page 71... ...
Levels of Hox and Hox cofactor expression vary between individual Lin A motor neurons, and altering levels of Antp expression in Lin A cells results in axon targeting errors. By removing expression of the thoracic Hox genes (Scr, Antp, and Ubx)
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From page 72... ...
The Hox genes Antp, Ubx, and Pb are expressed in the leg motor neuron containing thoracic segments (T1-T3; position of leg motor neurons indicated by circles)
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From page 73... ...
For example, the segment polarity gene gsb is expressed in the posterior portion of each ectodermal segment and in the underlying neuroblasts of rows 5 and 6. As shown in Fig.
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From page 74... ...
Are homeotic-type shifts in appendage specialization during arthropod evolution accompanied by matching shifts in the nervous system that allow coordinated evolution of both appendage morphology and the neural mechanisms that control the locomotion of these appendages? For example, when a crustacean locomotory appendage is transformed to a feeding appendage, is the underlying neural pattern changed as well to ensure that the transformation is functional, not just morphological?
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