Giant fiber activation of flight muscles in Drosophila: asynchrony in latency of wing depressor fibers.

In Drosophila, brain stimulation of the giant fiber pathway brings about highly stereotyped electrical responses in target muscles involved in the escape response. Both the order of muscle response and the latency of that response are predictable in wild-type flies. The neuronal circuit to the targets is well defined and has been used in the analysis of a number of mutant phenotypes, including induced anomalies in temperature-sensitive (ts) mutations such as shibire (shi). It has been assumed that the stereotyped response includes simultaneous activation of all six fibers of the wing depressor muscle, DLM, resulting in equal latencies for all fibers. We report here a small, but distinct, inherent difference in latency between two sets of DLM fibers in a proportion of two wild-type strains as well as in a strain carrying the ts mutation shi. This difference may occur on one or both sides of an individual, is stable over time, and persists when the motor axon is stimulated peripherally. These results, due to the circuit leading to the target, suggest that the difference in latency arises peripherally. In flies reared at the shi permissive temperature (22 degrees C), the difference is more common in shi than in wild-type flies; however, in shi flies reared at 18 degrees C, the prevalence resembles that of wild-type flies. This indicates a subtle expression of the shi defect even at the presumed permissive temperature of 22 degrees C. The difference in latency is similar to that induced in shi flies whose development is affected by pupal heat pulse. Thus, correct interpretation of differences in latency, e.g., in shi/wild-type mosaic flies or in flies with mutations affecting the GF pathway, requires recognition of the inherent asynchrony that can occur between DLM fibers.

Induced disruption in the connectivity of an identified neuron in the Drosophila ts mutant shibire.

Temperature-sensitive mutants permit the selective expression of mutant genotype. The Drosophila ts mutant shibire (shi) is paralytic at 30 degrees C; the probable primary effect of the mutation is disruption of membrane recycling. In studying the development of the giant fiber (GF) pathway during the pupal period, we find that shi flies exposed to heat pulse during early pupal states exhibit perturbation in the development of an identified neuron that links giant fibers to motoneurons of indirect flight muscles. Concomitantly, latency in activation of these muscles by the giant fiber pathway is significantly increased. Flies exposed to heat pulse during the late pupal period remain similar to control shi and wildtype flies in giant fiber pathway anatomy and muscle latency. Thus, the critical period of development of an identified neuron in a known motor pathway can be defined by its period of sensitivity to the shi defect. The time-dependent defect is apparently specific for cells that are at a developmental stage that is greatly dependent on membrane recycling processes. Use of this mutant will allow us to investigate the possible role of membrane recycling in development and to establish critical periods of neuronal development.

Reproduction and sexual development in a freshwater gastrotrich. 2. Kinetics and fine structure of postparthenogenic sperm formation.

Spermatogenesis and spermiogenesis in Lepidodermella squammata are confined to the postparthenogenic phase of the life cycle and coincide with developmental changes in the bilateral female gonads. Male stages are bilateral but asynchronous, in the lateral abdomen anterior to the female gonads. Maximum observed sperm production is two packets per side, or 64 sperm. Sperm formation occurs more rapidly at 27 degrees C than at 20 degrees C (p less than 0.001), requiring as little as 1 day. Two spermatogonial mitotic divisions produce a clone of four primary spermatocytes connected by bridges (stage 1). Centrioles are absent. Development occurs within a cyst. Meiotic divisions produce 16 spermatids (stage 2), each containing a dense, elongate, tapered nucleus. Cytoplasmic membranes enclose one end of the nuclear rod, excluding all other organelles. Completion of this process results in stage 3, a packet of 16 sperm associated with one dense sphere, a modified 'residual body' containing cytoplasmic debris. The residual body then disappears, leaving the sperm packet of stage 4. Each mature sperm is a dense nuclear rod with surrounding membranes, lacking acrosome, mitochondrion, centrioles, and flagellum. Function of sperm has not been demonstrated. The spermatozoa are of a reduced type not previously described.

Reproduction and sexual development in a freshwater gastrotrich. 3. Postparthenogenic development of primary oocytes and the X-body.

Six small cells are present in each of the bilateral gonads of parthenogenically reproductive Lepidodermella squammata. Early in the extended postparthenogenic phase of the life history, these cells undergo limited proliferation followed by differentiation. Primary oocytes of three types are present 0.3 days after deposition of the final parthenogenic egg: small oocytes with presynaptic nuclei; intermediate oocytes with nuclei containing synaptonemal complexes; and larger oocytes with a germinal vesicle. Oocytes persist without further development at least until day four of the postparthenogenic phase. Older isolated animals may contain and even deposit an enlarged egg, but successful progeny does not result. Oocytes are located at the anterior pole of each of the bilateral gonads, adjacent to developing male tissues producing sperm. More posterior cells in the gonad are initially undifferentiated in the postparthenogenic phase. Dorsal and central cells first show specialization for secretory activity, and by day four contain peripheral layers of RER and central accumulations of polymorphic secretion droplets. The posterior and ventral cells produce secretion droplets that aggregate into an enlarging bilobed structure called the X-body. Two or three cells in each gonad contribute secretions to the X-body, which is intracellular in a secondary syncytium formed by the contributing cells. Functions for the postparthenogenic gametes and for the X-body are not yet demonstrated.