Worm Breeder's Gazette 12(5): 70 (February 1, 1993)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

The Contribution of the DD mns to Locomotory Behavior

W. W. Walthall

Bio!ogy, Georgia State University, Atlanta, 30302

Precise synaptic specificity underlies the establishment of functional neural circuits. John White examined the morphological development of the DD mns and found that during the course of development they undergo a complete respecification of their synaptic pattern. To determine whether behavioral correlates exist for the two different synaptic patterns the 6 DD mns were surgically removed. In their absence two different locomotory defects were exhibited and the transition period coincides with the change in synaptic pattern of the DD mns.

The six DD mns arise from a pair of embryonic precursors (ABpr/lppapp) that normally undergo two additional rounds of cell divisions. Each produces three DD mns as well as an interneuron (RIGR or L). The two precursors were destroyed using a pulsed dye laser. Confirmation of the kills was made by Nomarski observation of anesthetized Ll animals. Successfully ablated animals had only 11 mns (the cell bodies of DD 1 and DD 6 are located in ganglia and verification of their absence was not attempted). During the first 24 hours animals, which were kept at 15°, exhibited a decrease in the amount of spontaneous .movement but moved forward when prodded. The young animals were incapable of backward movement. Forward movement in older animals was also normal but backward movement was uncoordinated. The degree of uncoordination of lasered adults was somewhat variable but the pattern was consistent. In some trials backward movement was sustained but the animal would move in circles with its dorsal aspect oriented centrally, in other trials the same animal would immediately form a tight coil, again with the dorsal side facing toward the center. This defect, which became more pronounced as the animal aged correlates well with the connectivity pattern for the DD mns.

The specific role of the RIG neurons is being investigated. It is possible that - they contribute to the defect in backward movement exhibited by newly hatched animals but unlikely that they contribute to the locomotory pattern in the adults.

The dorsal and ventral processes of the DD and VD mns are closely fasciculated with one another and it seemed likely that the presence of the DD mns might play a critical role in the guidance of the late-arriving VD mns. The dorsal coiling locomotory pattern suggests that the VD mns establish functional synapses with ventral muscles in the absence of the DD mns.