Abstract
Netrin and Slit signaling systems play opposing roles during the positioning of longitudinal tracts along the midline in the ventral nerve cord of Drosophila embryo. It has been hypothesized that a gradient of Slit from the midline interacts with three different Robo receptors to specify the axon tract positioning. However, no such gradient has been detected. Moreover, overexpression of Slit at the midline has no effect on the positioning of these lateral tracts. In this article, we show that Slit is present outside of the midline along the longitudinal and commissural tracts. Sli from the midline, in a Robo-independent manner, is initially taken up by the commissural axon tracts when they cross the midline and is transported along the commissural tracts into the longitudinal connectives. These results are not consistent with a Sli gradient model. We also find that sli mRNA is maternally deposited and embryos that are genetically null for sli can have weaker guidance defects. Moreover, in robo or robo3 mutants, embryos with normal axon tracts are found and such robo embryos reach pupal stages and die, while robo3 mutant embryos develop into normal individuals and produce eggs. Interestingly, embryos from robo3 homozygous individuals fail to develop but have axon tracts ranging from normal to various defects: robo3 phenotype, robo phenotype, and slit-like phenotype, suggesting a more complex functional role for these genes than what has been proposed. Finally, our previous results indicated that netrin phenotype is epistatic to sli or robo phenotypes. However, it seems likely that this previously reported epistatic relationship might be due to the partial penetrance of the sli, robo, robo3 (or robo2) phenotypes. Our results argue that double mutant epistasis is most definitive only if the penetrance of the phenotypes of the mutants involved is complete.
Highlights
Netrin and Slit signaling systems play opposing roles during the positioning of longitudinal tracts along the midline in the ventral nerve cord of Drosophila embryo
Slit is present outside of the midline in the commissural and longitudinal tracts: While previous work showed that Sli is expressed in the midline glial cells, we found that the commissures and longitudinal connectives have low levels of Sli (Figure 1b)
When embryos were examined with this antibody, the commissural tracts and the connectives were positive for Sli (Figure 1, e and f)
Summary
Netrin and Slit signaling systems play opposing roles during the positioning of longitudinal tracts along the midline in the ventral nerve cord of Drosophila embryo. It has been postulated that a gradient of Sli emanating from the midline interacts with Robo receptors in a combinatorial manner to specify the lateral positioning of axon tracts in the longitudinal pathways (Rajagopalan et al 2000; Simpson et al 2000). There is an additional signaling system emanating from the midline in Drosophila, which is the Netrin (Net) and its receptor Frazzled (Fra) system This NetFra system appears to function as a midline attractant for the commissural tracts (Harris et al 1996; Kolodziej et al 1996; Mitchell et al 1996). While there is no evidence of a gradient of Sli emanating from the midline, here we show that there is presence of Sli in the commissural and longitudinal
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