Worm Breeder's Gazette 14(3): 29 (June 1, 1996)

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.

let-60 ras is required in the sex myoblasts to control precise positioning

Meera Sundaram, John Yochem, Min Han

Dept. of MCD Biology, Univ. of CO, Boulder CO 80309-0347

        The two sex myoblasts (SMs) in the hermaphrodite are born during
the L1 larval stage in the posterior region of the animal, one on the
left side and one on the right.  The SMs then migrate approximately 65
mm anteriorly during the L2 stage to occupy very precise final positions
flanking the gonad in the mid-body region, near the site of the future
vulva.  Two separable components of SM guidance have been described:  a
gonad-independent mechanism sufficient for the initial anterior
migration to the mid-body region, and a gonad-dependent mechanism
required for precise final positioning (Thomas et al., 1990).  We have
demonstrated a role for a Ras-mediated signal transduction pathway in
controlling SM migration.  Loss-of-function mutations in let-60 ras,
ksr-1, lin-45 raf, let-537/mek-2 or sur-1/mpk-1 cause the SMs to adopt a
broadened range of final positions that resembles what is seen in
gonad-ablated wild-type animals, while constitutively active let-60
ras(G13E) transgenes allow fairly precise SM positioning to occur in the
absence of the gonad.  A recent mosaic analysis of let-60 ras has
demonstrated that let-60 ras is required within the SMs to control
proper positioning.  Based on these results, we propose a model in which
gonadal signals normally stimulate let-60 ras activity in the SMs,
thereby making the SMs competent to sense or respond to positional cues
in the mid-body environment. 

        Several observations suggest that let-60 ras  could have
additional roles in SM guidance as well:  1) Several unusual but
apparently reduction-of-function let-60 alleles [WBG 13(1), 13(3)] 
cause a striking posterior displacement of the SMs similar to that
caused by mutations in egl-15 FGFR (DeVore et al., 1995); 2) The SMs
sometimes adopt abnormal dorsal postions in let-60 ras and other Ras
pathway mutants; and 3) Mosaic animals lacking let-60 ras in the SMs
appear to have a somewhat different SM distribution than mosaic animals
lacking let-60 ras in both the SMs and gonad.  We are currently pursuing
these observations.

DeVore, D. L., Horvitz, H. R. and Stern, M. J. (1995). Cell 83, 611-620.

Thomas, J. H., Stern, M. J. and Horvitz, H. R. (1990). Cell 62,