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An analysis of the tap withdrawal response in male caenorhabditis elegans Mah, Kim Bill


The nematode, Caenorhabditis elegans, displays sexual dimorphism; males arise by the process of x-chromosome nondisjunction. The male is differentiated from the hermaphrodite by the addition of 14 sensory structures in the tail, as well as by the total complement of neurons. The male possesses 381 neurons of which at least 87 are male specific, compared with a total of 302 neurons in the hermaphrodite. The purpose of this thesis was to investigate whether the additional sensory and neural circuitry in the male resulted in a difference in the tap withdrawal response in male C. elegans compared to that of the hermaphrodite. First, the basal activity level of males was compared to that of hermaphrodites. Males exhibited significantly more spontaneous reversals than hermaphrodites', these reversals were categorized as true spontaneous reversals and self-feeling reversals. The reversal magnitude of true spontaneous reversals did not differ across sex, however the magnitude of self-feeling reversals was significantly greater in males compared to hermaphrodites. Next, the reflexive reversal response to tap was examined. Both males and hermaphrodites exhibited a graded magnitude of response to an increasing number of taps. Males, however, showed an increasing incidence of accelerations as tap number increased, while hermaphrodites showed the same number of reversals as the number of taps increased. In response to taps of increasing intensity, both males and hermaphrodites showed an increase in reversal magnitude. Response magnitudes exhibited by males were significantly greater than those of hermaphrodites. The interaction between two antagonistic reflexes was then examined. A tail-touch followed by a tap 1 s later significantly inhibited both the number and size of the reversal response in males and hermaphrodites. There were no significant sex differences at any tail-touch/tap intervals tested, although the hermaphrodites exhibited facilitation at the 20 and 30 s tail-touch/tap interval. The administration of 25 tail-touches prior to the administration of tail-touch/tap significantly decreased the amount of inhibition produced by tail-touch followed by tap 1 s later in both males and hermaphrodites. Finally, males were tested to see if they could show simple non-associative learning. Both males and hermaphrodites demonstrated habituation. The rate of habituation did not differ between the sexes, however, the asymptotic level of habituation was higher in males than in hermaphrodites. Thus, the additional sensory structures of the male did not appear to affect habituation. It was hypothesized that the mechanisms governing the rate of habituation are the same in both sexes, but that the mechanisms governing the degree of habituation may be different in the males and hermaphrodites. The rate of spontaneous recovery from habituation did not differ between the sexes, therefore it was hypothesized that the mechanisms governing recovery are the same in males and hermaphrodites. Additional analysis of handling effects revealed that males showed a smaller response if tested at 1-2 min rather than after 2-24 hr after transfer to the test plate. Hermaphrodites showed the opposite pattern of response: larger responses were observed if the worm was tested 1-2 min post-transfer than if tested 2-24 hr post-transfer. Thus, these experiments showed that the mechanisms underlying behavioural plasticity in the C. elegans males and hermaphrodites are likely to be similar, and further that the additional sensory receptors and associated neural circuitry do not result in differences in non-associative learning. However, differences in the responses of males and hermaphrodites were found: spontaneous reversal activity in the males was greater and handling resulted in differences in response magnitudes in males and hermaphrodites.

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