Worm Breeder's Gazette 15(5): 42 (February 1, 1999)

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.

sax-3 is Involved in the Ventral Localization of the Sex Myoblasts in the Absence of the Gonad-Dependent Attraction

Catherine S. Branda, Michael J. Stern

Genetics Department, Yale University, New Haven, CT 06520

The hermaphrodite sex myoblasts (SMs) are a pair of mesodermal cells which are born in bilaterally symmetrical positions in the posterior of the animal and later migrate anteriorly along the ventral muscle quadrants to final positions flanking the precise center of the developing gonad. Both gonad-dependent and gonad-independent guidance cues are involved in the proper localization of the SMs. A gonad-dependent attraction is required to precisely position the SMs. In the absence of the gonad, the SMs still migrate anteriorly along the ventral muscle quadrants in response to a gonad-independent mechanism to a broad range of positions spanning the center of the animal. Here we report that the sax-3(ky123) mutation can result in the dorsal mislocalization of the SMs in the absence of gonad-dependent guidance cues.

Mutations in sax-3 have previously been reported to disrupt both the asymmetry of the ventral nerve cord as well as ventral axon outgrowth (J. A. Zallen, B.A. Yi, and Bargmann, C. (1998) Cell 92: 217-27). sax-3 encodes a homologue of the Drosophila robo transmembrane receptor, which maintains the correct lateral positioning of axons with respect to the floor plate by a repulsive mechanism. sax-3 mutants have a similar phenotype in that axons are often found on the wrong side of the midline having crossed it several times. In addition, the ventral outgrowth of the HSN axon as well as some interneuron axons is often defective in sax-3 mutants.

In studying the role of sax-3 in SM migration, we have observed that this gene can influence the dorsal/ventral distribution of the SMs but does not appear to affect their anterior/posterior positioning. In sax-3 single mutants, the SMs are found correctly positioned (Fig. 1A). Since the gonadal attraction can override other cues that influence the anterior/posterior as well as the dorsal/ventral positioning of the SMs, the gonad was ablated in sax-3 mutants to eliminate this mechanism. When the gonad is ablated, the SMs are found in dorsal locations approximately 50% of the time (Fig. 1B). Elimination of EGL-17 FGF, the putative gonad-dependent attractant, also results in the dorsal mislocalization of the SMs (Fig. 1C).

To determine when the SMs move dorsally in sax-3 gonad-ablated animals, their birth and migration was continuously observed on one side of 4 different animals. Two of these four SMs were found to move dorsally just after birth and then migrate anteriorly along the dorsal muscle quadrant. Our results suggest that a sax-3-dependent process is involved in maintaining the SMs in ventral positions and that gonad-independent guidance cues are present on both the dorsal and ventral surfaces of the animal. Since some male SM descendants migrate ventrally, it will be interesting to determine if sax-3 plays an important role in these migrations.