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Characterization of a reduced-eye mutant of the grasshopper Melanoplus sanguinipes

University of British Columbia

A reduced-eye mutant grasshopper of Melanoplus sanguinipes was first isolated by Chapco (1980) and is characterized by small flat compound eyes lacking facets. The reduced-eye mutants have the same body plan (ie. size and shape) as wild type grasshoppers. Examination of the reduced-eye mutant heads by scanning electron microscopy confirms Chapco's (1980) observations that the compound eyes of the mutant lack facets. They also show that the mutant lacks lateral ocelli and has only a remnant of the medial ocellus. Behavioural observations indicate that the mutant has essentially the same motor capabilities as a wild type animal. Reduced-eye mutants are able to walk, jump, fly and feed in a normal manner. Electromyograms of hindleg extensor tibiae muscles and thoracic flight muscles show that the muscle activity patterns for walking and flying are identical in both mutants and wild types. One notable difference is the lack of escape responses to threatening visual and auditory stimuli, however, jumping can be evoked by tactile stimuli. This suggests that the reduced-eye mutant grasshopper may be blind and deaf. Extracellular recordings from the ventral nerve cord of reduced-eye mutants verify that there is no neural activity in response to visual and auditory inputs, yet the mutants do perceive tactile stimulation. Electroretinograms support the behavioural observations that the mutant is blind and indicate that the primary locus -For the sensory deficit may be within the retina of the compound eyes. However, attempts to backfill neurons in the optic lobe from the compound eye with cobalt, in addition to gross brain dissections, also uncovered the failure of the retinula cells to project into the central nervous system. Furthermore, the optic lobes are substantially reduced in size and there is no evidence of any ocellar nerves. Histological examination confirms the failure of the compound eyes to innervate the optic lobes and reveals other abnormalities within the compound eyes and optic lobes. The retina of the mutant fails to differentiate into ommatidia. The optic lamina, which underlies the retina, is missing as is the outer chiasm, which connects the lamina to the optic medulla in wild type grasshoppers. The medulla and lobula of the optic lobe are present in the mutant, however, the neuropil of the medulla lacks the characteristic axonal projection patterns of wild type grasshoppers. Comparison of 60 and 85% embryos and nymphal instars indicate that the alterations in the reduced-eye visual system are the result of abnormal differentiation in the embryo, specifically of cells in the retina and the outer optic anlage. The outer anlage develops into the lamina and medulla of the optic lobe. Cell degeneration is also evident. Even though there is clear evidence of morphological alterations in some interneurons, one higher order interneuron, the descending contralateral movement detector (DCMD), has a soma and axon of approximately the same size. In this instance, the complete deprivation of sensory input does not appear to alter cellular development. It is hoped that this reduced-eye mutant will be useful for subsequent studies of insect visual system development, as well as in the neuronal analysis of behaviours normally dependent on visual input.

Text

2010-04-20

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http://hdl.handle.net/2429/23925

Zoology

University of British Columbia

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