A reduced-eye (re) mutant grasshopper of Melanoplus sanguinipes has been characterized by small flat compound eyes lacking facets, no lateral ocelli and only a remnant of the median ocellus. The re grasshoppers walk, jump, fly and feed in a normal manner, but do not respond to visual and auditory stimuli, suggesting they may be blind and deaf. Extracellular recordings from the ventral nerve cord of re mutants verified the lack of neural activity in response to visual and auditory inputs, yet the mutants detected mechanical and tactile stimuli. Electroretinograms implied that a visual deficit may be within the photoreceptors of the compound eye.
Histological examination of the compound eyes and ocelli indicated that the cells of the mutant compound eye incompletely differentiate. The optic lamina underlying the retina is missing, as is the outer optic chiasma. The medulla and lobula of the mutant optic lobe are present, however, the neuropil of the medulla lacks the characteristic axonal projection patterns of wild-type grasshoppers. The re grasshopper also lacks all ocellar nerves. Ocellar nerves are normally formed from processes of second order ocellar neurons (SONs), suggesting that if the mutant SONs are present within the protocerebrum, their morphology is drastically altered. Comparison of embryos and juvenile nymphs supports the suggestion that the alterations in the re visual system are the result of abnormal differentiation during development.
Even though there is clear evidence of morphological alterations in second and third order optic lobe interneurons, one higher order visual interneuron of the midbrain, the descending contralateral movement detector (DCMD), has the same morphology as the DCMD in a wildtype brain. In this instance, the complete deprivation of the primary sensory input does not appear to alter cellular development.
Characterization of a reduced-eye mutant of the grasshopper, Melanoplus sanguinipes