The tobacco hornworm Manduca sexta exhibits dramatic changes in its body morphology and behavior as it is transformed from a larva into an adult during metamorphosis. Accompanying these changes is an extensive reorganization of this moth's central nervous system (CNS), which involves both the death and remodeling of subsets of larval neurons. We report here that the segmental ganglia of the larvae also contain a stereotyped array of identifiable neuronal stem cells (neuroblasts) that contribute over 2,000 cells to each thoracic ganglion and about 40-80 cells to each abdominal ganglion. The distribution of these neuroblasts varies in a segment specific manner. Dormant neuroblasts are found adjacent to the neuropil in late embryos and early first instar larvae. After the molt to the second instar, these cells enlarge and begin to divide. Through a series of asymmetrical divisions, each neuroblast generates a discrete nest of 10-90 progeny by the end of larval life. These progeny (the imaginal nest cells) are developmentally arrested at an early stage of differentiation and remain so until metamorphosis. At the onset of metamorphosis, a wave of cell death sweeps through the nests, the extent of the death being much greater within the abdominal nests than in the thoracic nests. The surviving imaginal nest cells then differentiate to become functional neurons that are incorporated into the adult CNS.
Two types of rhythmic foregut movements are described in fifth instar larvae of the moth, Manduca sexta. These consist of posteriorly-directed waves of peristalsis which move food toward the midgut, and synchronous constrictions of the esophageal region, which appear to retain food within the crop. We describe these movements and the muscles of the foregut that generate them.The firing patterns of a subset of these muscles, including a constrictor and dilator pair from both the esophageal and buccal regions of the foregut, are described for both types of foregut movement.The motor patterns for the foregut muscles require innervation by the frontal ganglion (FG), which lies anterior to the brain and contains about 35 neurons. Eliminating the ventral nerve cord, leaving the brain and FG intact, did not affect the muscle firing patterns in most cases. Eliminating both the brain and the ventral nerve cord, leaving only the FG to innervate the foregut, generally resulted in an increased period for both gut movements and muscle bursts. This manipulation also produced increases in burst durations for most muscles, and had variable effects on the phasing of muscle activity. Despite these changes, the foregut muscles still maintained a rhythmic firing pattern when innervated by the FG alone.Two nerves exit the FG to innervate the foregut musculature: the anteriorly-projecting frontal nerve, and the posteriorly-directed recurrent nerve. Cutting the frontal nerve immediately and irreversibly stopped all muscle activity in the buccal region, while cutting the recurrent nerve immediately stopped all muscle activity in the pharyngeal and esophageal regions. Recordings from the cut nerves leaving the FG showed that the ganglion was Abbreviations: BC, Buccal Constrictor; BC 1, buccal constrictor motoneuron 1; BC2, buccal constrictor motoneuron 2; BD, Buccal Dilator; BD1, buccal dilator motoneuron 1; EC, Esophageal Dilator; EC1, esophageal dilator motoneuron l; EC2, esophageal dilator motoneuron 2; EC3, esophageal dilator motoneuron 3; ejp, excitatory j unction potential; FG, frontal ganglion; psp, postsynaptic potential Correspondence to: C.I. Miles spontaneously active, with rhythmic activity continuing within the nerves. These observations indicate that all of the foregut muscle motoneurons are located within the FG, and the FG in isolation produces a rhythmic firing pattern in the motoneurons. We have identified several motoneurons within the FG, by cobalt backfills and/or simultaneous intracellular recordings and fills from putative motoneurons and their muscles.
It is generally believed that animals make decisions about the selection of mates, kin or food on the basis of pre-constructed recognition templates. These templates can be innate or acquired through experience. An example of an acquired template is the feeding preference exhibited by larvae of the moth, Manduca sexta. Naive hatchlings will feed and grow successfully on many different plants or artificial diets, but once they have fed on a natural host they become specialist feeders. Here we show that the induced feeding preference of M. sexta involves the formation of a template to a steroidal glycoside, indioside D, that is present in solanaceous foliage. This compound is both necessary and sufficient to maintain the induced feeding preference. The induction of host plant specificity is at least partly due to a tuning of taste receptors to indioside D. The taste receptors of larvae fed on host plants show an enhanced response to indioside D as compared with other plant compounds tested.
Individual Drosophila melanogaster, with or without heads, can be trained to lift their legs to avoid electric shock. This behavior is similar to the operant conditioning previously demonstrated in intact and headless cockroaches. More than 90% of headless wild-type flies learned to our criterion. In contrast, three mutants (dunce, cabbage, and turnip), originally selected for failure to learn in an olfactory discrimination paradigm, tended to perform poorly in this new learning situation. The difference in learning behavior between normal and mutant flies is distinguishable in individuals and may be useful for mosaic analysis.Populations of Drosophila can be trained to avoid an odor by shocking the flies in its presence (1, 2), and single-gene mutants that fail to learn this olfactory task have been isolated (3, 4). We would like to find physiological or chemical processes that are altered in these mutants and thus gain clues about how normal flies learn. First, however, we need to know whether the mutations really interfere with learning per se or merely produce poor performances in our test by causing sensory defects or general debility. One ofthe mutants, dunce, has been carefully characterized and found to have normal olfactory acuity, motor coordination, and overall activity (3). Therefore, dunce's poor performance is not due to gross peripheral or neurological derangement; however, more subtle behavioral abnormalities cannot be ruled out. The learning-deficient mutants turnip and cabbage (4) are under more suspicion in this respect because they show slight deficiencies in phototaxis, which is not a learned behavior. It would be advantageous to develop a variety of learning paradigms for Drosophila. Testing normal flies and the mutants in several learning tasks would help separate "shallow" mutants, with sensory deficits or deficiencies in a particular kind of learning, from mutants with more general learning disabilities.One widely studied example of "simple" insect learning is the operant conditioning shown by the cockroach in Horridge's paradigm (5,6). In this test the animal, with one leg free to move, is suspended over an electrolyte solution. If a voltage is applied so that the animal receives a shock when a wire on its leg extends into the solution, it often learns to keep the leg flexed. Decapitated cockroaches learn even more readily. Here we report that normal Drosophila, especially headless ones, learn very reliably to flex their legs in the Horridge paradigm. The flies can also learn to extend their legs to avoid shock. Three mutant fly stocks, originally selected for deficient olfactory discrimination learning, also do poorly in the leg-flexion and legextension tests. MATERIALS AND METHODSFly Stocks and Culture Conditions. Drosophila melanogaster of the Canton-special (C-S) wild-type strain and three mutant derivatives were used. The X-linked, ethylmethane sulfonate-induced mutants dunce' (3), dunce2 (7), cabbageps26" (4), and turnipps274 (4,8) were all originally isolated (methods of...
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